Question

What is the expected F2 phenotypic ratio when two white flowers of Lathyrus odoratus are crossed with each other?

Answer 1

In Lathyrus odoratus​, flower colour is purple if dominant alleles of two genes are present together (C–P–). The colour is white if the double dominant condition is absent (ccP–, –pp, ccpp). When two white flowered strains (CCpp, ccPP) are crossed of sweet pea, F₁ generation contained all the purple flowered plants (CcPp). When the purple flowered plants are allowed to self breed, both purple and white flowered plants appear in F₂ generation in the ratio of 9:7. The appearance of purple colour in 9/16 popluation shows that the colour is determined by two dominant genes (C and P). When either of the two is absent (ccPP or CCpp, ccPp, Ccpp), the pigment for purple colour does not appear. Thus gene for flower colour are complementary which means genes show a similar effect independently but produce a new trait when present together in the dominant form.