Lecture notes on supplementary factors

Supplementary genes are a pair of non-allelic genes, one of which produces its effect independently in the dominant state while the dominant allele of the second gene (supplemen­tary gene) needs the presence of other gene for its expression.

eg.Inheritance of Combs in Fowls (9:3:3:1)

The interaction of two dominant genes, to control the same character was discovered by Bateson and Punnet (1908) in fowls. In fowls, there are four types of combs. They are rose comb, pea comb,Walnut comb and single comb.Rose comb is controlled by a dominant gene R and pea is comb is  controlled by another dominant gene P The recessive alleles of the above genes in the homozygous condition (rrpp) produce single comb, But when the two dominant genes R and P are brought together they interact and produce a new comb called walnut Walnut comb is due to the interaction of two non-allelic dominant genes R and Pand the single comb is due to the interaction of two recessive gene r and p. A pure rose combed (RRpp) chicken is cross with pure pea combed(rrPP)The F1Progeny contains walnut combs (RrPp)

Then the F1 walnut individuals are crossed together. In the F2 all the  four types of combs appear. Walnut, rose, pea and single are in the ratio of 9:3:3:1

 

The  inheritance of the genes for ‘rose’ and ‘pea’ does not differ  from the normal Mendelian inheritance. The differences that distinguish this  from simple dihybrid cross are that (a) the F1 does not resemble with both the  parents and (b)  new or unusual characters (walnut) results from an interaction between two independently inherited genes, and the other (single comb) results from the interaction of their two recessive alleles. These peculiarities are  due to the fact or circumstance that both genes involved happen to express themselves in this case .