emerged — wrinkled yellow and round green ones. The ratio between the
different phenotypes was as follows: 9 round yellow; 3 round green; 3
wrinkled yellow and 1 wrinkled green (Fig. 2–9).
In order to give an explanation to these results, Mendel has assumed
that upon forming the gametes the genes of the different pairs determining
the given feature are segregated independently of one another so that the
plants of F₁ of the (AaBb) genotypes will yield gametes of the following
genotypes: (AB), (Ab), (aB), (ab). Of these four types of gametes 16
combinations are obtained. This regularity about the independent
segregation of the hereditary features in the progeny became known as the
second law of Mendel.
image
Figure 2–9. Illustrative presentation of the second law of Mendel on the independent segregation of the features in dihybrid crosses at a ratio of 9:3:3:1 in F₂. The inheritance of both the round (B) and wrinkled (b) forms, and the yellow (A) and green (a) colours of the seeds is traced. Alleles (A) and (B) are dominant; (a) and (b) — recessive.