Question

Use the forked-line method to answer the following question.

In cucumbers, there are 3 genetic traits, as follows: dull fruit is dominant to glossy fruit, orange fruit color is dominant to cream fruit color, bitter cotyledons are dominant to nonbitter cotyledons.You cross 2 plants together that are heterozygous for all 3 genes. Among the next generation, what proportion of the offspring will be

a) dull, orange, bitter

b) glossy, cream, nonbitter

c) heterozygous for all 3 genes (HINT: not same answer as (a) above)

Answer 1

Here 3 genes are considered. So it is a trihybrid cross.

When more than two genes are being considered, the Punnet-square method is not easy for the use. In this case we will use forked-line method to keep track of probabilities in a cross. This method gives the proportions of offspring expected to exhibit each phenotype (or genotype) without the added visual assistance.

Let,

Heterozygous dull fruit = Dd

Heterozygous orange fruit = Oo

Heterozygous bitter cotyledons  = Bb

P = DdOoBb X  DdOoBb

The first step is to separate the trihybrid cross in to its 3 constituent single gene (Mono hybrid) crosses.

First mono hybrid cross will deal only with D and d alleles.

Second mono hybrid cross will deal only with O and o alleles.

Third mono hybrid cross will deal only with B and b alleles.

Next is to create all possible genotype combinations.

a). Proportion of the offspring dull, orange, bitter = 1/64 + 1/32 + 1/32 + 1/16 + 1/32 +1/16 + 1/16 + 1/8 = 27/64 =0.421

b). Proportion of the offspring glossy, cream, non-bitter = 1/64 = 0.0156

c)Proportion of the heterozygous for all 3 genes = 1/8 +1/32 = 40 / 256 or 0.156