What is the law of segregation?
It is the principles that rule the heredity were discovered by a monk named G. Mendel in the 1860's. One of these principles now called the law of segregation of Mandel, states that allele pairs separate or segregate during gamete formation, and randomly bond at fertilization. This law is also known as Mendel's second law or law of purity of gametes.
Description of the law of segregation
When pure breeding red and white flowered varieties were crossed, they formed red flowered individuals only in F1 generation. But, after self-fertilization of F1 generation plants (red), the individuals of F2 generation thus formed were red flowered and white flowered in the ratio of 3: 1. This ratio is known as monohybrid ratio. Thus, the two complementary characters have segregated or separated in F2 generation.
When F3 generation was raised by selfing F2 plants, all recessive F2 plants produced only recessives. They were pure or true breeding. In contrast, only one-third of dominant F2 were pure and the rest ware impure which produced both dominant and recessive offspring likes F1. The expressed or phenotypic ratio of 3 : 1 was in reality a hidden 1:2:1 ratio of pure dominant : impure dominant or hybrid : pure recessive forms.
This is known as law of segregation, which states that when a pair of allelomorphs is brought together in the hybrid (F1), they remain together in the hybrid without blending but separate complete and pure during gamete formation. This law is also known as Mendel's second law or law of purity of gametes. It is sometimes said to be the law of the splitting of hybrids.
Explanation of the Law of Segregation
The law of segregation can be explained as follows:
Pure breeding red (RR) and white (rr) varieties when crossed in F1 generation form hybrid red (Rr). During the gamete formation segregation or separation takes place and only one factor of the pair of alleles alone enters one gamete and the other enters to the other gamete. Thus a gamete can be only either R or r. The F1 hybrid (Rr) in the above cross produces two types of gametes R and r. After self-pollination they result in RR (red), Rr (red), Rr (red) and rr (white) plants in F2 generation. Each parent thus produces two types of gametes which have equal chances of combination together.
The law of segregation is Mendel's most important contribution to biology because it introduced concept of hereditary factors as discrete, physical entities that do not become blended or altered when pres together in the same individual. He disproved the blending theory by showing that although traits caused recessive alleles disappear in the F1 generation, they reappear unchanged in the F2 generation.
Limitations of the law of segregation
The law of segregation applies only to diploid organisms that form haploid gametes to replicate sexually. It further applies only to traits controlled completely a single gene pair in which of the two alleles one is overriding over the other. Therefore the law of segregation dose not applicable to the following:
i) Alleles that are incompletely dominant or co-dominant.
ii) Genes that collaborate or vary in their expressivity.
iii) Genes that are pleiotropic (that means, each gene having two or more phenotypic effects), complementary, or influenced by epistasis (suppression of one gene's effects by another),
iv) Traits caused by many gene pairs.