The Y chromosome is one of two sex
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s (
allosome
A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical ...
s) in
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
...
n
mammals, including
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s, and many other animals. The other is the
X chromosome
The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex ...
. Y is normally the
sex-determining chromosome in many
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, since it is the presence or absence of Y that determines the male or female
sex
Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes (spermatozoa, sperm, pollen), while females produce larger ones ( ova, of ...
of
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
produced in
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
. In mammals, the Y chromosome contains the
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
SRY, which triggers male development. The
DNA in the human Y chromosome is composed of about 59 million
base pairs, making it similar in size to
chromosome 19
Chromosome 19 is one of the 23 pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 19 spans more than 58.6 million base pairs, the building material of DNA. It is considered the most gene-rich chromosom ...
.
The Y chromosome is passed only from father to son. With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest-evolving parts of the
human genome
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
. The human Y chromosome carries an estimated 100–200
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s, with between 45 and 73 of these being
protein-coding. All single-copy Y-linked genes are
hemizygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
(present on only one chromosome) except in cases of
aneuploidy
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
such as
XYY syndrome
XYY syndrome, also known as Jacobs syndrome, is an aneuploid genetic condition in which a male has an extra Y chromosome. There are usually few symptoms. These may include being taller than average, acne, and an increased risk of learning disa ...
or
XXYY syndrome
XXYY syndrome is a sex chromosome anomaly in which males have 2 extra chromosomes, one X and one Y chromosome. Human cells usually contain two sex chromosomes, one from the mother and one from the father. Usually, females have two X chromosomes ( ...
.
Overview
Discovery
The Y chromosome was identified as a sex-determining chromosome by
Nettie Stevens
Nettie Maria Stevens (July 7, 1861 – May 4, 1912) was an American geneticist who discovered sex chromosomes. In 1905, soon after the rediscovery of Mendel's paper on genetics in 1900, she observed that male mealworms produced two kinds of sper ...
at
Bryn Mawr College
Bryn Mawr College ( ; Welsh: ) is a women's liberal arts college in Bryn Mawr, Pennsylvania. Founded as a Quaker institution in 1885, Bryn Mawr is one of the Seven Sister colleges, a group of elite, historically women's colleges in the United ...
in 1905 during a study of the
mealworm
Mealworms are the larval form of the yellow mealworm beetle, ''Tenebrio molitor'', a species of darkling beetle. Like all holometabolic insects, they go through four life stages: egg, larva, pupa, and adult. Larvae typically measure about or ...
''Tenebrio molitor''.
Edmund Beecher Wilson
Edmund Beecher Wilson (October 19, 1856 – March 3, 1939) was a pioneering American zoologist and geneticist. He wrote one of the most influential textbooks in modern biology, ''The Cell''.
Career
Wilson was born in Geneva, Illinois, the so ...
independently discovered the same mechanisms the same year, working with
hemiptera. Stevens proposed that chromosomes always existed in pairs and that the smaller chromosome (now labelled "Y") was the pair of the X chromosome discovered in 1890 by
Hermann Henking
Hermann Paul August Otto Henking (16 June 1858 – 28 April 1942) was a German cytologist who discovered the X chromosome in 1890 or 1891. The work was the result of a study in Leipzig of the testicles of the firebug (''Pyrrhocoris apterus''), dur ...
. She realized that the previous idea of
Clarence Erwin McClung
Clarence Erwin McClung (April 5, 1870 – January 17, 1946) was an eminent American zoologist and prairie pioneer cytologist who discovered the role of chromosomes in sex-determination.
Graduating pharmacy at the University of Kansas in 1892, ...
, that the X chromosome determines sex, was wrong and that
sex determination is, in fact, due to the presence or absence of the Y chromosome. In the early 1920s
Theophilus Painter
Theophilus Shickel Painter (August 22, 1889 – October 5, 1969) was an American zoologist best known for his work on the structure and function of chromosomes, especially the sex-determination genes X and Y in humans. He was the first to discove ...
determined that X and Y chromosomes determined sex in humans (and other mammals).
The chromosome was given the name "Y" simply to follow on from Henking's "X" alphabetically. The idea that the Y chromosome was named after its similarity in appearance to the letter "Y" is mistaken. All chromosomes normally appear as an amorphous blob under the microscope and only take on a well-defined shape during
mitosis. This shape is vaguely X-shaped for all chromosomes. It is entirely coincidental that the Y chromosome, during
mitosis, has two very short branches which can look merged under the microscope and appear as the descender of a Y-shape.
Variations
Most
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
...
n mammals have only one pair of sex chromosomes in each cell. Males have one Y chromosome and one
X chromosome
The X chromosome is one of the two sex-determining chromosomes (allosomes) in many organisms, including mammals (the other is the Y chromosome), and is found in both males and females. It is a part of the XY sex-determination system and XO sex ...
, while females have two X chromosomes. In mammals, the Y chromosome contains a gene,
SRY, which triggers embryonic development as a male. The Y chromosomes of humans and other mammals also contain other genes needed for normal sperm production.
There are exceptions, however. Among humans, some men have two Xs and a Y ("XXY", see
Klinefelter syndrome
Klinefelter syndrome (KS), also known as 47,XXY, is an aneuploid genetic condition where a male has an additional copy of the X chromosome. The primary features are infertility and small, poorly functioning testicles. Usually, symptoms are sub ...
), or one X and two Ys (see
XYY syndrome
XYY syndrome, also known as Jacobs syndrome, is an aneuploid genetic condition in which a male has an extra Y chromosome. There are usually few symptoms. These may include being taller than average, acne, and an increased risk of learning disa ...
), and
some women have three Xs or a single X instead of a double X ("X0", see
Turner syndrome
Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition in which a female is partially or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hair ...
). There are other exceptions in which
SRY is damaged (leading to an
XY female), or copied to the X (leading to an
XX male
XX male syndrome, also known as de la Chapelle syndrome, is a rare congenital intersex condition in which an individual with a 46, XX karyotype (otherwise associated with females) has phenotypically male characteristics that can vary among cases ...
).
Origins and evolution
Before Y chromosome
Many
ectotherm
An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
ic
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
have no sex chromosomes. If they have different sexes, sex is determined environmentally rather than genetically. For some of them, especially
reptiles, sex depends on the incubation temperature. Some vertebrates are
hermaphrodites
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have sep ...
, although other than a very few
ray-finned fish
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
, they are
sequential
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called th ...
(the same organism produces male or female gametes, but never both, at different points in its life), rather than
simultaneous (the same organism producing both male and female gametes at the same time).
Origin
The X and Y chromosomes are thought to have evolved from a pair of identical chromosomes,
termed
autosomes, when an ancestral animal developed an allelic variation, a so-called "sex locus" – simply possessing this
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
caused the organism to be male.
The chromosome with this allele became the Y chromosome, while the other member of the pair became the X chromosome. Over time, genes that were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome or were acquired through the process of
translocation.
Until recently, the X and Y chromosomes were thought to have diverged around 300 million years ago.
However, research published in 2010, and particularly research published in 2008 documenting the sequencing of the
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
genome,
has suggested that the XY sex-determination system would not have been present more than 166 million years ago, at the split of the
monotremes from other mammals.
This re-estimation of the age of the
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
...
n XY system is based on the finding that sequences that are on the X chromosomes of
marsupials and
eutherian
Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.
Eutherians are distinguished from noneutherians by various phenotypic tra ...
mammals are present on the autosomes of platypus and birds.
The older estimate was based on erroneous reports that the platypus X chromosomes contained these sequences.
Recombination inhibition
Recombination between the X and Y chromosomes proved harmful—it resulted in males without necessary genes formerly found on the Y chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome. As a result, genes beneficial to males accumulated near the sex-determining genes, and recombination in this region was suppressed in order to preserve this male specific region.
Over time, the Y chromosome changed in such a way as to inhibit the areas around the sex determining genes from recombining at all with the X chromosome. As a result of this process, 95% of the human Y chromosome is unable to recombine. Only the tips of the Y and X chromosomes recombine. The tips of the Y chromosome that could recombine with the X chromosome are referred to as the
pseudoautosomal region
The pseudoautosomal regions, PAR1, PAR2, are homologous sequences of nucleotides on the X and Y chromosomes.
The pseudoautosomal regions get their name because any genes within them (so far at least 29 have been found for humans) are inherited ...
. The rest of the Y chromosome is passed on to the next generation intact, allowing for its use in tracking
human evolution
Human evolution is the evolutionary process within the history of primates that led to the emergence of '' Homo sapiens'' as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development o ...
.
Degeneration
By one estimate, the human Y chromosome has lost 1,393 of its 1,438 original genes over the course of its existence, and
linear extrapolation of this 1,393-gene loss over 300 million years gives a rate of genetic loss of 4.6 genes per million years. Continued loss of genes at the rate of 4.6 genes per million years would result in a Y chromosome with no functional genes – that is the Y chromosome would lose complete function – within the next 10 million years, or half that time with the current age estimate of 160 million years.
Comparative genomic analysis reveals that many mammalian species are experiencing a similar loss of function in their heterozygous sex chromosome. Degeneration may simply be the fate of all non-recombining sex chromosomes, due to three common evolutionary forces: high
mutation rate
In genetics, the mutation rate is the frequency of new mutations in a single gene or organism over time. Mutation rates are not constant and are not limited to a single type of mutation; there are many different types of mutations. Mutation rates ...
, inefficient
selection
Selection may refer to:
Science
* Selection (biology), also called natural selection, selection in evolution
** Sex selection, in genetics
** Mate selection, in mating
** Sexual selection in humans, in human sexuality
** Human mating strateg ...
, and
genetic drift
Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance.
Genetic drift may cause gene variants to disappear completely and there ...
.
However, comparisons of the human and
chimpanzee Y chromosomes (first published in 2005) show that the human Y chromosome has not lost any genes since the divergence of humans and chimpanzees between 6–7 million years ago, and a scientific report in 2012 stated that only one gene had been lost since humans diverged from the
rhesus macaque
The rhesus macaque (''Macaca mulatta''), colloquially rhesus monkey, is a species of Old World monkey. There are between six and nine recognised subspecies that are split between two groups, the Chinese-derived and the Indian-derived. Generally ...
25 million years ago. These facts provide direct evidence that the
linear extrapolation model is flawed and suggest that the current human Y chromosome is either no longer shrinking or is shrinking at a much slower rate than the 4.6 genes per million years estimated by the linear extrapolation model.
High mutation rate
The human Y chromosome is particularly exposed to high mutation rates due to the environment in which it is housed. The Y chromosome is passed exclusively through
sperm, which undergo multiple
cell division
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
s during
gametogenesis
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic di ...
. Each cellular division provides further opportunity to accumulate base pair mutations. Additionally, sperm are stored in the highly oxidative environment of the
testis
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
, which encourages further mutation. These two conditions combined put the Y chromosome at a greater opportunity of mutation than the rest of the genome.
The increased mutation opportunity for the Y chromosome is reported by Graves as a factor 4.8.
However, her original reference obtains this number for the relative mutation rates in male and female germ lines for the lineage leading to humans.
The observation that the Y chromosome experiences little
meiotic
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately res ...
recombination and has an accelerated rate of
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
and degradative change compared to the rest of the
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
suggests an evolutionary explanation for the adaptive function of
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
with respect to the main body of genetic information. Brandeis
proposed that the basic function of meiosis (particularly meiotic recombination) is the conservation of the integrity of the genome, a proposal consistent with the idea that meiosis is an adaptation for
repairing DNA damage.
Inefficient selection
Without the ability to recombine during
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
, the Y chromosome is unable to expose individual
allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s to natural selection. Deleterious alleles are allowed to "hitchhike" with beneficial neighbors, thus propagating maladapted alleles into the next generation. Conversely, advantageous alleles may be selected against if they are surrounded by harmful alleles (background selection). Due to this inability to sort through its gene content, the Y chromosome is particularly prone to the accumulation of
"junk" DNA. Massive accumulations of retrotransposable elements are scattered throughout the Y.
The random insertion of DNA segments often disrupts encoded gene sequences and renders them nonfunctional. However, the Y chromosome has no way of weeding out these "jumping genes". Without the ability to isolate alleles, selection cannot effectively act upon them.
A clear, quantitative indication of this inefficiency is the
entropy rate
In the mathematical theory of probability, the entropy rate or source information rate of a stochastic process is, informally, the time density of the average information in a stochastic process. For stochastic processes with a countable index, the ...
of the Y chromosome. Whereas all other chromosomes in the
human genome
The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the ...
have entropy rates of 1.5–1.9 bits per nucleotide (compared to the theoretical maximum of exactly 2 for no redundancy), the Y chromosome's entropy rate is only 0.84.
[ Fig. 6, using the ]Lempel-Ziv
LZ77 and LZ78 are the two lossless data compression algorithms published in papers by Abraham Lempel and Jacob Ziv in 1977 and 1978.
They are also known as LZ1 and LZ2 respectively. These two algorithms form the basis for many variations including ...
estimators of entropy rate. This means the Y chromosome has a much lower information content relative to its overall length; it is more redundant.
Genetic drift
Even if a well adapted Y chromosome manages to maintain genetic activity by avoiding mutation accumulation, there is no guarantee it will be passed down to the next generation. The population size of the Y chromosome is inherently limited to 1/4 that of autosomes: diploid organisms contain two copies of autosomal chromosomes while only half the population contains 1 Y chromosome. Thus, genetic drift is an exceptionally strong force acting upon the Y chromosome. Through sheer random assortment, an adult male may never pass on his Y chromosome if he only has female offspring. Thus, although a male may have a well adapted Y chromosome free of excessive mutation, it may never make it into the next gene pool.
The repeat random loss of well-adapted Y chromosomes, coupled with the tendency of the Y chromosome to evolve to have more deleterious mutations rather than less for reasons described above, contributes to the species-wide degeneration of Y chromosomes through
Muller's ratchet
In evolutionary genetics, Muller's ratchet (named after Hermann Joseph Muller, by analogy with a ratchet effect) is a process through which, in the absence of recombination (especially in an asexual population), an accumulation of irreversible d ...
.
Gene conversion
As it has been already mentioned, the Y chromosome is unable to recombine during
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
like the other human chromosomes; however, in 2003, researchers from
MIT
The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
discovered a process which may slow down the process of degradation.
They found that human Y chromosome is able to "recombine" with itself, using
palindrome base pair sequences.
Such a "recombination" is called
gene conversion
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces a ...
.
In the case of the Y chromosomes, the
palindromes are not
noncoding DNA
Non-coding DNA (ncDNA) sequences are components of an organism's DNA that do not encode protein sequences. Some non-coding DNA is transcribed into functional non-coding RNA molecules (e.g. transfer RNA, microRNA, piRNA, ribosomal RNA, and r ...
; these strings of bases contain functioning genes important for male fertility. Most of the sequence pairs are greater than 99.97% identical. The extensive use of gene conversion may play a role in the ability of the Y chromosome to edit out genetic mistakes and maintain the integrity of the relatively few genes it carries. In other words, since the Y chromosome is single, it has duplicates of its genes on itself instead of having a second, homologous, chromosome. When errors occur, it can use other parts of itself as a template to correct them.
Findings were confirmed by comparing similar regions of the Y chromosome in humans to the Y chromosomes of
chimpanzees,
bonobos and
gorilla
Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...
s. The comparison demonstrated that the same phenomenon of gene conversion appeared to be at work more than 5 million years ago, when humans and the non-human primates diverged from each other.
Future evolution
According to some theories, in the terminal stages of the degeneration of the Y chromosome, other chromosomes increasingly take over genes and functions formerly associated with it and finally, within the framework of this theory, finally, the Y chromosome disappears entirely, and a new sex-determining system arises.
Several species of
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
in the sister families
Muridae and
Cricetidae
The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, muskrats, and New World rats and mice. At almost 608 species, it is the second-largest family of mammals, and h ...
have reached these stages,
in the following ways:
* The
Transcaucasian mole vole
The Transcaucasian mole vole (''Ellobius lutescens'') is a species of rodent in the family Cricetidae.
It is found in Armenia, Azerbaijan, Georgia, Iran, and Turkey.
Chromosomes
The karyotype has a low, odd, diploid number, 2n = 17,X. Transc ...
, ''Ellobius lutescens'', the
Zaisan mole vole
The Zaisan mole vole (''Ellobius tancrei''), or eastern mole vole, is a species of rodent in the family Cricetidae. It is found in central Asia.
Description
The Zaisan mole vole is highly adapted to life underground. It grows to a head and body ...
, ''Ellobius tancrei'', and the Japanese spinous country rats ''
Tokudaia osimensis
The Ryukyu spiny rat (''Tokudaia osimensis'') is a species of rodent in the family Muridae. Endemic to Amami Ōshima island in the Amami Islands of the Ryukyu archipelago of Japan, its natural habitat is subtropical moist broadleaf forest. The ...
'' and ''
Tokudaia tokunoshimensis
The Tokunoshima spiny rat (''Tokudaia tokunoshimensis'') is a rodent found only on the island of Tokunoshima in the Ryukyu Islands of Japan. Due to its small habitat, it is considered endangered. It is commonly found in the secondary and primary ...
'', have lost the Y chromosome and
SRY entirely.
''
Tokudaia
''Tokudaia'' is a genus of murine rodent native to Japan. Known as Ryūkyū spiny rats or spinous country-rats, population groups exist on several non-contiguous islands. Despite differences in name and appearance, they are the closest living rel ...
'' spp. have relocated some other genes ancestrally present on the Y chromosome to the X chromosome.
Both sexes of ''Tokudaia'' spp. and ''Ellobius lutescens'' have an XO genotype (
Turner syndrome
Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition in which a female is partially or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hair ...
),
whereas all ''Ellobius tancrei'' possess an XX genotype.
The new sex-determining system(s) for these rodents remains unclear.
* The
wood lemming
The wood lemming (''Myopus schisticolor'') is a species of rodent in the family Cricetidae. It belongs to the rodent subfamily Arvicolinae, so is a relative of the voles, lemmings, and muskrats.
It is found in the taiga biome of China, Estonia, F ...
''Myopus schisticolor'', the
Arctic lemming
The Arctic lemming (''Dicrostonyx torquatus'') is a species of rodent in the family Cricetidae.
Although generally classified as a "least concern" species, the Novaya Zemlya subspecies ''(Dicrostonyx torquatus ungulatus)'' is considered a vulner ...
, ''Dicrostonyx torquatus'', and multiple species in the grass mouse genus ''
Akodon
''Akodon'' is a genus consisting of South American grass mice. They mostly occur south of the Amazon Basin and along the Andes north to Venezuela, but are absent from much of the basin itself, the far south of the continent, and the lowlands west ...
'' have evolved fertile females who possess the genotype generally coding for males, XY, in addition to the ancestral XX female, through a variety of modifications to the X and Y chromosomes.
* In the
creeping vole
The creeping vole (''Microtus oregoni''), sometimes known as the Oregon meadow mouse, is a small rodent in the family Cricetidae. Ranging across the Pacific Northwest of North America, it is found in forests, grasslands, woodlands, and chaparral ...
, ''Microtus oregoni'', the females, with just one X chromosome each, produce X gametes only, and the males, XY, produce Y gametes, or gametes devoid of any sex chromosome, through
nondisjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division (mitosis/meiosis). There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis ...
.
Outside of the rodents, the
black muntjac, ''Muntiacus crinifrons'', evolved new X and Y chromosomes through fusions of the ancestral sex chromosomes and
autosomes.
Modern data cast doubt on this hypothesis.
This conclusion was reached by scientists who studied the Y chromosomes of rhesus monkeys. When genomically comparing the Y chromosome of rhesus monkeys and humans, scientists found very few differences, given that humans and rhesus monkeys diverged 30 million years ago.
Some organisms have lost the Y chromosome. For example, most species of Nematodes. However, in order for the complete elimination of Y to occur, it was necessary to develop an alternative way of determining sex (for example, by determining sex by the ratio of the X chromosome to autosomes), and any genes necessary for male function had to be moved to other chromosomes.
In the meantime, modern data demonstrate the complex mechanisms of Y chromosome evolution and the fact that the disappearance of the Y chromosome is not guaranteed.
1:1 sex ratio
Fisher's principle
Fisher's principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females. A. W. F. Edwards has remarked that it is "probably the mo ...
outlines why almost all species using
sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
have a
sex ratio
The sex ratio (or gender ratio) is usually defined as the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. Many species d ...
of 1:1.
W. D. Hamilton
William Donald Hamilton (1 August 1936 – 7 March 2000) was a British evolutionary biologist, recognised as one of the most significant evolutionary theorists of the 20th century.
Hamilton became known for his theoretical work expounding a ...
gave the following basic explanation in his 1967 paper on "Extraordinary sex ratios",
given the condition that males and females cost equal amounts to produce:
:# Suppose male births are less common than female.
:# A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
:# Therefore, parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
:# Therefore, the genes for male-producing tendencies spread, and male births become more common.
:# As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
:# The same reasoning holds if females are substituted for males throughout. Therefore, 1:1 is the equilibrium ratio.
Non-therian Y chromosome
Many groups of organisms in addition to therian mammals have Y chromosomes, but these Y chromosomes do not share common ancestry with therian Y chromosomes. Such groups include monotremes, ''
Drosophila
''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many speci ...
'', some other insects, some fish, some reptiles, and some plants. In ''
Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the " vinegar fly" or "pomace fly". Starting with ...
'', the Y chromosome does not trigger male development. Instead, sex is determined by the number of X chromosomes. The ''D. melanogaster'' Y chromosome does contain genes necessary for male fertility. So XXY ''D. melanogaster'' are female, and ''D. melanogaster'' with a single X (X0), are male but sterile. There are some species of Drosophila in which X0 males are both viable and fertile.
ZW chromosomes
Other organisms have mirror image sex chromosomes: where the homogeneous sex is the male, said to have two Z chromosomes, and the female is the heterogeneous sex with a Z chromosome and a
W chromosome.
For example, female birds, snakes, and butterflies have ZW sex chromosomes, and males have ZZ sex chromosomes.
Non-inverted Y chromosome
There are some species, such as the
Japanese rice fish
The Japanese rice fish (''Oryzias latipes''), also known as the medaka, is a member of genus ''Oryzias'' ( ricefish), the only genus in the subfamily Oryziinae. This small (up to about ) native of East Asia is a denizen of rice paddies, marshes, ...
, in which the XY system is still developing and cross over between the X and Y is still possible. Because the male specific region is very small and contains no essential genes, it is even possible to artificially induce XX males and YY females to no ill effect.
Multiple XY pairs
Monotremes possess four or five (
platypus
The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or mono ...
) pairs of XY sex chromosomes, each pair consisting of sex chromosomes with homologous regions. The chromosomes of neighboring pairs are partially homologous, such that a chain is formed during
mitosis.
The first X chromosome in the chain is also partially homologous with the last Y chromosome, indicating that profound rearrangements, some adding new pieces from autosomes, have occurred in history.
Platypus sex chromosomes have strong sequence similarity with the avian
Z chromosome
Z (or z) is the 26th and last letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its usual names in English are ''zed'' () and ''zee'' (), with an occas ...
, (indicating close
homology
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
* Sequence homology, biological homology between DNA, RNA, or protein sequences
*Homologous chrom ...
),
and the SRY gene so central to sex-determination in most other mammals is apparently not involved in platypus sex-determination.
Human Y chromosome
In humans, the Y chromosome spans about 58 million
base pairs (the building blocks of
DNA) and represents almost 2% of the total DNA in a male
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
. The human Y chromosome contains over 200 genes, at least 72 of which code for proteins.
Traits that are inherited via the Y chromosome are called
Y-linked
Y linkage, also known as holandric inheritance (from Ancient Greek ὅλος ''hólos'', "whole" + ἀνδρός ''andrós'', "male"), describes traits that are produced by genes located on the Y chromosome. It is a form of sex linkage.
Y&nbs ...
traits, or holandric traits (from
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...
ὅλος ''hólos'', "whole" + ἀνδρός ''andrós'', "male").
Loss of Y chromosome
Men can lose the Y chromosome in a subset of cells, which is called the
mosaic loss of chromosome Y (mLOY). This
post-zygotic mutation is strongly associated with age, affecting about 15% of men 70 years of age. Smoking is another important risk factor for LOY. It has been found that men with a higher percentage of
hematopoietic stem cells
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of ...
in blood lacking the Y chromosome (and perhaps a higher percentage of other cells lacking it) have a higher risk of certain
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
s and have a shorter life expectancy. Men with LOY (which was defined as no Y in at least 18% of their hematopoietic cells) have been found to die 5.5 years earlier on average than others. This has been interpreted as a sign that the Y chromosome plays a role going beyond sex determination and reproduction. However, it was thought that the loss of Y could also be an effect rather than a cause and/or a "neutral
karyotype related to normal
aging". In 2022, a study showed that
blood cells' loss of the Y chromosome in
a subset of cells (mLOY), reportedly affecting at least 40% of
70 years-old men to some degree, contributes to
fibrosis
Fibrosis, also known as fibrotic scarring, is a pathological wound healing in which connective tissue replaces normal parenchymal tissue to the extent that it goes unchecked, leading to considerable tissue remodelling and the formation of perma ...
,
heart risks, and mortality in a ''causal'' way. Male smokers have between 1.5 and 2 times the risk of non-respiratory cancers as female smokers. Potential countermeasures identified so far include not smoking or
stopping smoking and at least one potential drug that "may help counteract the harmful effects of the chromosome loss" is under investigation.
Structure
Cytogenetic band
Non-combining region of Y (NRY)
The human Y chromosome is normally unable to recombine with the X chromosome, except for small pieces of
pseudoautosomal region
The pseudoautosomal regions, PAR1, PAR2, are homologous sequences of nucleotides on the X and Y chromosomes.
The pseudoautosomal regions get their name because any genes within them (so far at least 29 have been found for humans) are inherited ...
s (PARs) at the
telomere
A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...
s (which comprise about 5% of the chromosome's length). These regions are relics of ancient
homology
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
* Sequence homology, biological homology between DNA, RNA, or protein sequences
*Homologous chrom ...
between the X and Y chromosomes. The bulk of the Y chromosome, which does not recombine, is called the "NRY", or non-recombining region of the Y chromosome.
Single-nucleotide polymorphism
In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently lar ...
s (SNPs) in this region are used to trace direct paternal ancestral lines.
More specifically, PAR1 is at 0.1–2.7 Mb. PAR2 is at 56.9–57.2 Mb. The non-recombining region (NRY) or male-specific region (MSY) sits between.
Sequence classes
Genes
Number of genes
The following are some of the gene count estimates of human Y chromosome. Because researchers use different approaches to
genome annotation
DNA annotation or genome annotation is the process of identifying the locations of genes and all of the coding regions in a genome and determining what those genes do. An annotation (irrespective of the context) is a note added by way of explanati ...
their predictions of the
number of genes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
on each chromosome varies (for technical details, see
gene prediction
In computational biology, gene prediction or gene finding refers to the process of identifying the regions of genomic DNA that encode genes. This includes protein-coding genes as well as RNA genes, but may also include prediction of other functiona ...
). Among various projects, the collaborative consensus coding sequence project (
CCDS) takes an extremely conservative strategy. So CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.
Gene list
In general, the human Y chromosome is extremely gene poor—it is one of the largest
gene deserts
Gene deserts are regions of the genome that are devoid of protein-coding genes. Gene deserts constitute an estimated 25% of the entire genome, leading to the recent interest in their true functions. Originally believed to contain inessential and ...
in the human genome. Disregarding
pseudoautosomal genes, genes encoded on the human Y chromosome include:
Y-chromosome-linked diseases
Diseases linked to the Y chromosome typically involve an
aneuploidy
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
, an atypical number of chromosomes.
Y chromosome microdeletion
Y chromosome microdeletion (YCM) is a family of genetic disorders caused by missing genes in the Y chromosome. Many affected men exhibit no symptoms and lead normal lives. However, YCM is also known to be present in a significant number of men with reduced fertility or reduced sperm count.
Defective Y chromosome
This results in the person presenting a female
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...
(i.e., is born with female-like genitalia) even though that person possesses an XY
karyotype. The lack of the second X results in infertility. In other words, viewed from the opposite direction, the person goes through
defeminization
In developmental biology and zoology, defeminization is an aspect of the process of sexual differentiation by which a potential female-specific structure, function, or behavior is changed by one of the processes of male development. Although the t ...
but fails to complete
masculinization
Virilization or masculinization is the biological development of adult male characteristics in young males or females. Most of the changes of virilization are produced by androgens.
Virilization is most commonly used in three medical and biology ...
.
The cause can be seen as an incomplete Y chromosome: the usual karyotype in these cases is 45X, plus a fragment of Y. This usually results in defective testicular development, such that the infant may or may not have fully formed male genitalia internally or externally. The full range of ambiguity of structure may occur, especially if
mosaicism
Mosaicism or genetic mosaicism is a condition in multicellular organisms in which a single organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized ...
is present. When the Y fragment is minimal and nonfunctional, the child is usually a girl with the features of
Turner syndrome
Turner syndrome (TS), also known as 45,X, or 45,X0, is a genetic condition in which a female is partially or completely missing an X chromosome. Signs and symptoms vary among those affected. Often, a short and webbed neck, low-set ears, low hair ...
or
mixed gonadal dysgenesis.
XXY
Klinefelter syndrome (47, XXY) is not an
aneuploidy
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46. It does not include a difference of one or more complete sets of chromosomes. A cell with any ...
of the Y chromosome, but a condition of having an extra X chromosome, which usually results in defective postnatal testicular function. The mechanism is not fully understood; it does not seem to be due to direct interference by the extra X with expression of Y genes.
XYY
47, XYY syndrome (simply known as XYY syndrome) is caused by the presence of a single extra copy of the Y chromosome in each of a male's cells. 47, XYY males have one X chromosome and two Y chromosomes, for a total of 47 chromosomes per cell. Researchers have found that an extra copy of the Y chromosome is associated with increased stature and an increased incidence of learning problems in some boys and men, but the effects are variable, often minimal, and the vast majority do not know their karyotype.
In 1965 and 1966
Patricia Jacobs and colleagues published a chromosome survey of 315 male patients at
Scotland
Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
's only special security hospital for the
developmentally disabled
Developmental disability is a diverse group of chronic conditions, comprising mental or physical impairments that arise before adulthood. Developmental disabilities cause individuals living with them many difficulties in certain areas of life, espe ...
,
finding a higher than expected number of patients to have an extra Y chromosome.
The authors of this study wondered "whether an extra Y chromosome predisposes its carriers to unusually aggressive behaviour", and this conjecture "framed the next fifteen years of research on the human Y chromosome".
Through studies over the next decade, this conjecture was shown to be incorrect: the elevated crime rate of XYY males is due to lower median intelligence and not increased aggression,
and increased height was the only characteristic that could be reliably associated with XYY males.
The "criminal karyotype" concept is therefore inaccurate.
Rare
The following Y-chromosome-linked diseases are rare, but notable because of their elucidating of the nature of the Y chromosome.
=More than two Y chromosomes
=
Greater degrees of Y chromosome polysomy (having more than one extra copy of the Y chromosome in every cell, e.g., XYYY) are considerably more rare. The extra genetic material in these cases can lead to skeletal abnormalities, dental abnormalities, decreased IQ, delayed development, and respiratory issues, but the severity features of these conditions are variable.
=XX male syndrome
=
XX male syndrome
XX male syndrome, also known as de la Chapelle syndrome, is a rare congenital intersex condition in which an individual with a 46, XX karyotype (otherwise associated with females) has phenotypically male characteristics that can vary among case ...
occurs when there has been a
recombination in the formation of the male
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s, causing the
SRY portion of the Y chromosome to move to the X chromosome. When such an X chromosome contributes to the child, the development will lead to a male, because of the SRY gene.
Genetic genealogy
In human
genetic genealogy (the application of
genetics
Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...
to
traditional genealogy), use of the information contained in the Y chromosome is of particular interest because, unlike other chromosomes, the Y chromosome is passed exclusively from father to son, on the patrilineal line.
Mitochondrial DNA, maternally inherited to both sons and daughters, is used in an analogous way to trace the matrilineal line.
Brain function
Research is currently investigating whether male-pattern neural development is a direct consequence of Y-chromosome-related gene expression or an indirect result of Y-chromosome-related
androgenic hormone
An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This inc ...
production.
Microchimerism
The presence of male chromosomes in fetal cells in the blood circulation of women was discovered in 1974.
In 1996, it was found that male fetal progenitor cells could persist postpartum in the maternal blood stream for as long as 27 years.
A 2004 study at the
Fred Hutchinson Cancer Research Center
The Fred Hutchinson Cancer Center, formerly known as the Fred Hutchinson Cancer Research Center and also known as Fred Hutch or The Hutch, is a cancer research institute established in 1975 in Seattle, Washington.
History
The center grew out o ...
, Seattle, investigated the origin of male chromosomes found in the peripheral blood of women who had not had male progeny. A total of 120 subjects (women who had never had sons) were investigated, and it was found that 21% of them had male DNA. The subjects were categorised into four groups based on their case histories:
* Group A (8%) had had only female progeny.
* Patients in Group B (22%) had a history of one or more miscarriages.
* Patients Group C (57%) had their pregnancies medically terminated.
* Group D (10%) had never been pregnant before.
The study noted that 10% of the women had never been pregnant before, raising the question of where the Y chromosomes in their blood could have come from. The study suggests that possible reasons for occurrence of male chromosome microchimerism could be one of the following:
* miscarriages,
* pregnancies,
* vanished male twin,
* possibly from sexual intercourse.
A 2012 study at the same institute has detected cells with the Y chromosome in multiple areas of the brains of deceased women.
See also
*
Genealogical DNA test
A genealogical DNA test is a DNA-based test used in genetic genealogy that looks at specific locations of a person's genome in order to find or verify ancestral genealogical relationships, or (with lower reliability) to estimate the ethnic mixt ...
*
Genetic genealogy
*
Haplodiploid sex-determination system
Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid. Haplodiploidy is sometimes called arrhenotoky.
Haplodiploidy determines the sex ...
*
Human Y chromosome DNA haplogroups
In human genetics, a human Y-chromosome DNA haplogroup is a haplogroup defined by mutations in the non- recombining portions of DNA from the male-specific Y chromosome (called Y-DNA). Many people within a haplogroup share similar numbers of s ...
*
List of Y-STR markers
The following list of Y-STR markers are commonly used in forensic and genealogical DNA testing.
DYS454 is the least diverse, and multi-copy marker DYS464 is the most diverse Y-STR marker.
The location on the Y-chromosome of numbered Y-STR ...
*
Muller's ratchet
In evolutionary genetics, Muller's ratchet (named after Hermann Joseph Muller, by analogy with a ratchet effect) is a process through which, in the absence of recombination (especially in an asexual population), an accumulation of irreversible d ...
*
Single nucleotide polymorphism
*
Y chromosome Short Tandem Repeat (STR)
*
Y linkage
Y linkage, also known as holandric inheritance (from Ancient Greek ὅλος ''hólos'', "whole" + ἀνδρός ''andrós'', "male"), describes traits that are produced by genes located on the Y chromosome. It is a form of sex linkage.
Y  ...
*
Y-chromosomal Aaron
Y-chromosomal Aaron is the name given to the hypothesized most recent common ancestor of the patrilineal Jewish priestly caste known as ''Kohanim'' (singular "Kohen", also spelled "Cohen"). According to the traditional understanding of the Hebrew ...
*
Y-chromosomal Adam
In human genetics, the Y-chromosomal most recent common ancestor (Y-MRCA, informally known as Y-chromosomal Adam) is the patrilineal most recent common ancestor (MRCA) from whom all currently living humans are descended. He is the most recent mal ...
*
Y-chromosome haplogroups in populations of the world
References
External links
Genetic Genealogy: About the use of mtDNA and Y chromosome analysis in ancestry testingEnsembl genome browser* http://www.ncbi.nlm.nih.gov/mapview/maps.cgi?taxid=9606&chr=Y
Human Genome Project InformationHuman Chromosome Y Launchpad
From the Whitehead Institute for Biomedical Research
focus on the Y chromosome
National Human Genome Research Institute (NHGRI)Use of Novel Mechanism Preserves Y chromosome Genes
Ysearch.org – Public Y-DNA database
Y chromosome Consortium (YCC)
{{DEFAULTSORT:Y Chromosome
Andrology
Chromosomes
Chromosome Y
The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abs ...
Male
Sex-determination systems
Sexual dimorphism