Interaction of genes
Genes interact, so traits do not always segregate 3 : 1. Interaction of allelic genes: in complete dominance the dominant allele masks the recessive; in incomplete dominance the heterozygote has an intermediate trait (four-o’clock, Mirabilis: red × white → pink; F2 1 red : 2 pink : 1 white); in codominance both alleles show fully (human blood group AB: IᴬIᴮ); in multiple allelism a gene has more than two alleles (ABO system: Iᴬ, Iᴮ, i). Interaction of non-allelic genes: in complementarity non-allelic genes complete each other and produce a new trait (F2 9 : 7, 9 : 6 : 1, 9 : 3 : 4 or 9 : 3 : 3 : 1); in epistasis one gene (an inhibitor) suppresses the expression of another non-allelic gene (13 : 3 or 12 : 3 : 1 in dominant epistasis); in polygeny several non-allelic genes act on one trait in the same direction: in cumulative polygeny the trait grows with the number of dominant alleles (with two gene pairs F2 is 1 : 4 : 6 : 4 : 1; quantitative traits such as height, skin colour, grain mass), while in non-cumulative polygeny one dominant allele is enough (F2 15 : 1). In pleiotropy one gene affects several traits. In all cases Mendel’s laws remain the basis: only the phenotypic ratio changes.
“Blood-group cards”: parents’ genotypes are given on cards and each group lists the blood groups possible in the children.