![Autosomal recessive inheritance - Carriers](https://upload.wikimedia.org/wikipedia/commons/0/07/Autosomal_recessive_inheritance_-_Carriers.png)
A hereditary carrier (genetic carrier or just carrier), is a person or other organism that has
inherited a recessive allele for a genetic trait or mutation but usually does not display that
trait or show symptoms of the
disease
A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
. Carriers are, however, able to
pass
Pass, PASS, The Pass or Passed may refer to:
Places
* Pass, County Meath, a townland in Ireland
* Pass, Poland, a village in Poland
* Pass, an alternate term for a number of straits: see List of straits
* Mountain pass, a lower place in a moun ...
the
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 ...
onto their offspring, who may then express the genetic trait.
Carriers in autosomal inheritances
Autosomal
An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosome, allosomal (sex chromosome) pairs, which may have different structures. The DNA in au ...
dominant-recessive
inheritance
Inheritance is the practice of receiving private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
is made possible by the fact that the individuals of most species (including all higher animals and plants) have two
alleles
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 ...
of most hereditary predispositions because the
chromosomes
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 ...
in the
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
are usually present in pairs (
diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...
). Carriers can be female or male as the autosomes are homologous independently from the sex.
In carriers the expression of a certain characteristic is recessive. The individual has both a genetic predisposition for the
dominant trait and a genetic predisposition for the recessive trait, and the dominant expression prevails in the
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 proper ...
. In an individual which is
heterozygous
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 ...
regarding a certain allele, it is not externally recognisable that it also has the recessive allele. But if the carrier has a child, the recessive trait appears in the phenotype, in case the descendant receives the recessive
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 ...
from both parents and therefore does not possess the dominant allele that would cover the recessive trait. According to
Mendelian Law of Segregation of genes an average of 25% of the offspring become
homozygous
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 ...
and express the recessive trait. Carriers can either pass on normal autosomal recessive hereditary traits or an autosomal recessive
hereditary disease
A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
.
Carriers in gonosomal inheritances
Gonosomal
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes.
In some species there are hermap ...
recessive genes are also passed on by carriers. The term is used in
human genetics
Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population gene ...
in cases of hereditary traits in which the observed trait lies on the female
sex chromosome
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 ...
, 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-d ...
. The carriers are always
women
A woman is an adult female human. Prior to adulthood, a female human is referred to as a girl (a female child or Adolescence, adolescent). The plural ''women'' is sometimes used in certain phrases such as "women's rights" to denote female hum ...
.
Men
A man is an adult male human. Prior to adulthood, a male human is referred to as a boy (a male child or adolescent). Like most other male mammals, a man's genome usually inherits an X chromosome from the mother and a Y chro ...
cannot be carriers because they only have one X chromosome. The Y chromosome is not a really homologous chromosome. For this reason, the genetic make-up of the observed trait is not twofold. If a man has a certain recessive genetic disposition on his X chromosome, this is called ''hemizygot'' and it gets phenotypically expressed. Also a recessive genetic disposition on his Y chromosome - also ''hemizygot'' - can come to expression, because there is no homologous chromosome with an allele, which could overlay it. If there is no genetic information on the Y chromosome for a certain trait, the effect of the Y is neutral and the allele on his X chromosome, which ''would'' be recessive in a heterozygous woman, can now ''alone'' come to expression.
Women have two homologous sex chromosomes (XX). Therefore, women can be carriers of
X-linked
Sex linked describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, ...
genes. Examples of traits inherited via the X chromosome are
color blindness
Color blindness or color vision deficiency (CVD) is the decreased ability to color vision, see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may ...
and the most common hereditary form of
Haemophilia
Haemophilia, or hemophilia (), is a mostly inherited genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding. This results in people bleeding for a longer time after an injury, easy bruising, ...
. Men are affected much more often than women.
Queen Victoria
Victoria (Alexandrina Victoria; 24 May 1819 – 22 January 1901) was Queen of the United Kingdom of Great Britain and Ireland from 20 June 1837 until Death and state funeral of Queen Victoria, her death in 1901. Her reign of 63 years and 21 ...
, and her daughters Princesses Alice and Beatrix, were carriers of the
hemophilia
Haemophilia, or hemophilia (), is a mostly inherited genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding. This results in people bleeding for a longer time after an injury, easy bruising, ...
gene (an abnormal allele of a gene, necessary to produce one of the blood clotting factors). Both had children who continued to pass on the gene to succeeding generations of the royal houses of
Spain
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = ''Plus ultra'' (Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, i ...
and
Russia
Russia (, , ), or the Russian Federation, is a List of transcontinental countries, transcontinental country spanning Eastern Europe and North Asia, Northern Asia. It is the List of countries and dependencies by area, largest country in the ...
, into which they married. Since males only have one X chromosome, males who carried the altered gene had hemophilia B. Females have two X chromosomes, so one copy of an
X-linked recessive
X-linked recessive inheritance is a mode of inheritance in which a mutation in a gene on the X chromosome causes the phenotype to be always expressed in males (who are necessarily homozygous for the gene mutation because they have one X and one Y ...
gene would cause them to be an asymptomatic carrier. These females simply passed it to half of their children.
Gonosomal dominant inheritances are also known. There are ''no'' carriers since owners of a dominant hereditary disposition phenotypically express the trait in each case.
References
{{reflist
Genetics