Let A and a represent the factors corresponding to the dominant and recessive members of a pair of alternative characters.
First parental generation, Pi.
Gametes of Pi. Into each passes one member of each pair. The gametes of either parent are similar because the individuals were homozygous.
First filial generation, Fi. All individuals are heterozygous. Since A is dominant, they will exhibit the dominant character.
Gametes of Fi. Segregation. Into each gamete passes one member of each pair of factors. Equal numbers of two kinds of gametes are produced. Two series, male and female, are required.
Second filial generation, F2.
Individuals exhibiting the dominant char acter: Homozygotes, AA, . .
Heterozygotes, Aa, . . 2 . . . 3 Individuals exhibiting the recessive char acter: Homozygote, as . . • . 4 It is now known that dominance is not an essential feature of Mendelian inheritance ; it is commonly not exhibited, the hybrid being intermediate in its characterization. What is essential is the orderly reappearance of the characters of and of in in definite numerical proportions.
Mendel's second law is that of the independent assortment of factors. This is illustrated when in one and the same experiment two different pairs of allelomorphic characters are present. The F, exhibit the two dominant members of the two pairs and in there are on the average in every sixteen individ uals nine exhibiting both dominants, three exhibiting the dominant of one pair and the recessive of the other, three exhibiting the other dominant and the other recessive, and one the two reces sives. All possible combinations of the four characters are yielded. For the production of these, there must be free assortment and re combination of the four factors concerned. The 9 : 3 : 3 : I ratio follows from the co-existence of two 3 : I ratios in the same mat ing. The F2 of a trihybrid mating, one involving three pairs of allelomorphs, one member of each pair being dominant, is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. (See HEREDITY, GENETICS, SEX.) See W. Bateson, Mendel's Principles of Heredity (1913) ; R. C. Punnett, Mendelism (1919) ; F. A. E. Crew, Heredity (1928).