1 History 2 Mendel's laws
2.1 Law of Segregation of genes (the "First Law") 2.2 Law of Independent Assortment (the "Second Law") 2.3 Law of Dominance (the "Third Law")
3 Mendelian trait 4 Non-Mendelian inheritance 5 See also 6 Notes 7 References 8 Notes 9 External links
Main article: History of genetics
The principles of
Mendel discovered that, when he crossed purebred white flower and
purple flower pea plants (the parental or P generation), the result
was not a blend. Rather than being a mix of the two, the offspring
(known as the F1 generation) was purple-flowered. When Mendel
self-fertilized the F1 generation pea plants, he obtained a purple
flower to white flower ratio in the F2 generation of 3 to 1. The
results of this cross are tabulated in the
Mendel's laws of inheritance
Law of segregation During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.
Law of independent assortment Genes for different traits can segregate independently during the formation of gametes.
Law of dominance Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.
In the pea plant example above, the capital "B" represents the dominant allele for purple flowers and lowercase "b" represents the recessive allele for white flowers. Both parental plants were true-breeding, and one parental variety had two alleles for purple flowers (BB) while the other had two alleles for white flowers (bb). As a result of fertilization, the F1 hybrids each inherited one allele for purple flowers and one for white. All the F1 hybrids (Bb) had purple flowers, because the dominant B allele has its full effect in the heterozygote, while the recessive b allele has no effect on flower color. For the F2 plants, the ratio of plants with purple flowers to those with white flowers (3:1) is called the phenotypic ratio. The genotypic ratio, as seen in the Punnett square, is 1 BB : 2 Bb : 1 bb. Law of Segregation of genes (the "First Law")
Figure 1 Dominant and recessive phenotypes. (1) Parental generation. (2) F1 generation. (3) F2 generation. Dominant (red) and recessive (white) phenotype look alike in the F1 (first) generation and show a 3:1 ratio in the F2 (second) generation.
The Law of Segregation states that every individual organism contains
two alleles for each trait, and that these alleles segregate
(separate) during meiosis such that each gamete contains only one of
the alleles. An offspring thus receives a pair of alleles for a
trait by inheriting homologous chromosomes from the parent organisms:
one allele for each trait from each parent.
Molecular proof of this principle was subsequently found through
observation of meiosis by two scientists independently, the German
Figure 2 Dihybrid cross. The phenotypes of two independent traits show a 9:3:3:1 ratio in the F2 generation. In this example, coat color is indicated by B (brown, dominant) or b (white), while tail length is indicated by S (short, dominant) or s (long). When parents are homozygous for each trait (SSbb and ssBB), their children in the F1 generation are heterozygous at both loci and only show the dominant phenotypes (SsbB). If the children mate with each other, in the F2 generation all combinations of coat color and tail length occur: 9 are brown/short (purple boxes), 3 are white/short (pink boxes), 3 are brown/long (blue boxes) and 1 is white/long (green box).
The Law of Independent Assortment states that alleles for separate traits are passed independently of one another i.e from parents to offspring. That is, the biological selection of an allele for one trait has nothing to do with the selection of an allele for any other trait. Mendel found support for this law in his dihybrid cross experiments (Fig. 1). In his monohybrid crosses, an idealized 3:1 ratio between dominant and recessive phenotypes resulted. In dihybrid crosses, however, he found a 9:3:3:1 ratios (Fig. 2). This shows that each of the two alleles is inherited independently from the other, with a 3:1 phenotypic ratio for each. Independent assortment occurs in eukaryotic organisms during meiotic prophase I, and produces a gamete with a mixture of the organism's chromosomes. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes. Along with crossing over, independent assortment increases genetic diversity by producing novel genetic combinations. There are many violations of independent assortment due to genetic linkage. Of the 46 chromosomes in a normal diploid human cell, half are maternally derived (from the mother's egg) and half are paternally derived (from the father's sperm). This occurs as sexual reproduction involves the fusion of two haploid gametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During gametogenesis—the production of new gametes by an adult—the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy. In independent assortment, the chromosomes that result are randomly sorted from all possible maternal and paternal chromosomes. Because zygotes end up with a random mix instead of a pre-defined "set" from either parent, chromosomes are therefore considered assorted independently. As such, the zygote can end up with any combination of paternal or maternal chromosomes. Any of the possible variants of a zygote formed from maternal and paternal chromosomes will occur with equal frequency. For human gametes, with 23 pairs of chromosomes, the number of possibilities is 223 or 8,388,608 possible combinations. The zygote will normally end up with 23 chromosomes pairs, but the origin of any particular chromosome will be randomly selected from paternal or maternal chromosomes. This contributes to the genetic variability of progeny.
Law of Dominance (the "Third Law") Mendel's Law of Dominance states that recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. The Law of Dominance can be explained easily with the help of a mono hybrid cross experiment:- In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called "dominant" and the one which is suppressed (not expressed) is called "recessive." Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F1 generation is called Dominant. However, the law of dominance is not universally applicable. Mendelian trait A Mendelian trait is one that is controlled by a single locus in an inheritance pattern. In such cases, a mutation in a single gene can cause a disease that is inherited according to Mendel's laws. Examples include sickle-cell anemia, Tay-Sachs disease, cystic fibrosis and xeroderma pigmentosa. A disease controlled by a single gene contrasts with a multi-factorial disease, like arthritis, which is affected by several loci (and the environment) as well as those diseases inherited in a non-Mendelian fashion. Non-Mendelian inheritance
In four o'clock plants, the alleles for red and white flowers show incomplete dominance. As seen in the F1 generation, heterozygous (wr) plants have "pink" flowers—a mix of "red" (rr) and "white" (ww) coloring. The F2 generation shows a 1:2:1 ratio of red:pink:white
Main article: Non-Mendelian inheritance
Mendel explained inheritance in terms of discrete
factors—genes—that are passed along from generation to generation
according to the rules of probability. Mendel's laws are valid for all
sexually reproducing organisms, including garden peas and human
beings. However, Mendel's laws stop short of explaining some patterns
of genetic inheritance. For most sexually reproducing organisms, cases
where Mendel's laws can strictly account for the patterns of
inheritance are relatively rare. Often, the inheritance patterns are
The F1 offspring of Mendel's pea crosses always looked like one of the
two parental varieties. In this situation of "complete dominance," the
dominant allele had the same phenotypic effect whether present in one
or two copies. But for some characteristics, the F1 hybrids have an
appearance in between the phenotypes of the two parental varieties. A
cross between two four o'clock (Mirabilis jalapa) plants shows this
common exception to Mendel's principles. Some alleles are neither
dominant nor recessive. The F1 generation produced by a cross between
red-flowered (RR) and white flowered (WW)
History of Science portal MCB portal
List of Mendelian traits in humans Mendelian diseases (monogenic disease) Mendelian error Particulate inheritance Punnett square
^ Grafen, Alan; Ridley, Mark (2006). Richard Dawkins: How A Scientist
Changed the Way We Think. New York, New York: Oxford University Press.
p. 69. ISBN 0-19-929116-0.
^ a b c Henig, Robin Marantz (2009). The
Bowler, Peter J. (1989). The Mendelian Revolution: The Emergence of
Hereditarian Concepts in Modern Science and Society. Johns Hopkins
Atics, Jean. Genetics: The life of DNA. AND
Khan Academy, video lecture Probability of Inheritance Mendel's principles of Inheritance Mendelian genetics
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