E Section A (40 Marks)
Answer all questions in this section.
1. In a particular species of tropical beetles, the wings have either red or orange marks. A cross between red marked beetles
with orange marked beetles produces off-springs with yellow marks only. When the F, generation off-springs are selfed.
they produce F, generation in the ratio of 1 red: 2 yellow. I orange.
(1 mark)
(a) Explain the absence of red and orange marks in the F, off-springs.

Question:

In a particular species of tropical beetles, the wings have either red or orange marks. A cross between red marked beetles  with orange marked beetles produces off-springs with yellow marks only. When the F1 generation off-springs are selfed, they produce F2 generation in the ratio of 1 red: 2 yellow: 1 orange.

(a) Explain the absence of red and orange marks in the F1 off-springs.

The red and yellow alleles show incomplete dominance

Explanation:

Let's say the red allele is R, and the orange allele is r.

Since all the offspring in the F1 are different from the parents, they must have a different genotype. They also all have the same phenotype. Let's try to model this. Let's imagine the red parent is the genotype RR and the orange parent rr. The resulting F1 generation would be:

                R            R

r               Rr           Rr

r               Rr           Rr

All the F1 generation would be the same and are heterozygous - perhaps this heterozygous phenotype explains why we get an intermediate phenotype. If red and orange are incompletely dominant - yellow could be the result of blending of the two traits. Let's see if this works with the F2 generation: we cross Rr x Rr

              R             r

R           RR          Rr

r           Rr           rr

The genotypes are 1 RR: 2 Rr: 1 rr, and the phenotypes are 1 red: 2 yellow: 1 orange, which works with our theory that all the F1s are heterozygous Rr, which have a phenotype of yellow. The red allele is incompletely dominant to the orange allele.

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