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Mendel's Second Law


Independent segregation of two or more gene pairs

In addition to studying several phenotypic traits in isolation, Mendel also studied the combined transmission of two or more traits. In one of his experiments, for example, both the color of the seed, which may be yellow or green, and the texture of the seed shell, which may be smooth or rough, were considered simultaneously.

Plants originated from yellow and plain seeds, both dominant traits, were crossed with plants originated from green and rough seeds, recessive traits. All seeds produced in generation F1 they were yellow and smooth.

The generation F2, obtained by self-fertilization of plants originating from seeds of F1, was composed of four types of seeds:

9/16 plain yellow

3/16 rough yellow

3/16 plain green

1/16 rough green

In proportions these fractions represent 9 plain yellow: 3 rough yellow: 3 plain green: 1 rough green.

Based on this and other experiments, Mendel hypothesized that in the formation of gametes, alleles for seed color (Vv) segregate independently of the alleles that condition the shape of the seed (Rr). Accordingly, an allele carrier gamete V can contain both the allele R like the allele r, with equal chance, and the same is true for allele carriers v.

A double heterozygous plant VvRr according to the hypothesis of independent segregation, four gamete types in equal proportion: 1 VR: 1Vr: 1 vR: 1 vr.

Mendel's Second Law

Mendel concluded that independent segregation of factors for two or more characteristics was a general principle, constituting a second law of inheritance. Thus he called this principle the second law of inheritance or the law of independent segregation, later called the second law of Mendel: Factors for two or more traits segregate in the hybrid, distributing themselves independently to gametes, where they combine at random.

The 9: 3: 3: 1 aspect ratio

By studying the simultaneous inheritance of several feature pairs. Mendel always observed, in F2, the phenotypic ratio 9: 3: 3: 1, a consequence of the independent segregation occurring in the double heterozygote, which gives rise to four gamete types.

Independent segregation of 3 allele pairs

Studying 3 characteristic pairs simultaneously, Mendel found that the distribution of the types of individuals in F2 followed the proportion of 27: 9: 9: 9: 3: 3: 3: 1. This indicates that the genes for the 3 traits considered segregate independently in individuals F1, originating 8 types of gametes.

In one of his experiments, Mendel simultaneously considered the color (yellow or green), the bark texture (smooth or rough) and the seed bark color (gray or white).

Crossing between a plant originated from dominant homozygous seed for all three traits (yellow-smooth-gray) and a plant originated from seed with recessive traits (rough-white-green) produces only dominant yellow, flat and gray peas. These individuals are heterozygous for the three gene pairs (VvRrBb). The independent segregation of these three allele pairs in generation F plants1, leads to the formation of 8 types of gametes.

The gametes produced by plants F1 combine in 64 possible ways (8 maternal types X 8 paternal types), giving rise to 8 types of phenotypes.