A modified ratio 15:1 (derived from standard ratio 9:3:3:1) was observed by G.H. Shull
from his studies on the plant known as Shepherd's purse
belonging to the genus Capsella.
Two kinds of phenotypes with respect to fruit shape were known—(i) triangular capsules
and (ii) top shaped capsules.
When races with these phenotypes were crossed, in F1
generation only triangular capsules were observed. When such F1
individuals with triangular capsules were intercrossed among themselves, in F2
progeny, plants with triangular capsules and top shaped capsules were obtained in 15 : 1 ratio (Table 3.5). Obviously the top shaped capsules result from
double recessive genotype. If ‘A’
are two genes, top shaped capsules will be obtained on plants with the genotype aabb.
Plants with the triangular capsules can be AABB, AAbb
and other genotypes with heterozygosity. It shows that even a single dominant gene is enough to give rise to triangular capsules. Presence of both dominants has the same effect.
The above examples, which are only few from a large number of them demonstrating interaction of genes, indicate that the basic dihybrid ratio, 9:3:3:1 can be modified giving rise to 9 : 3 : 4, 12 : 3 : 1, 13 : 3, 9 : 7 and 15 : 1 ratios depending upon the kind of interaction involved. Such an interaction may also be possible between three gene pairs and ratios, which are modifications of 27:9:9:9:3:3:3:1, will be obtained.