In Drosophila, the autosomal recessive pr and cn mutations cause brown and bright-red eyes, respectively (wild-type flies have brick-red eyes). Flies who are homozygous recessive at both pr and cn have orange eyes. A female who has wild-type eyes is crossed to an orange-eyed male. Their progeny have the following distribution of eye colors:

10 wild-type
240 brown
240 bright red
10 orange

The mother of these progeny resulted from a cross between flies from two true-breeding lines.
What are the genotypes of these two lines?

Option B

Explanation:

Complete question is

The original mother resulted from a cross between two flies from true breeding lines.

What are the genotypes of these two lines? (Note that the chromosomes are NOT

indicated this time).

a) pr+ pr+ cn cn and pr pr cn+ cn+  

b) pr+ pr+ cn+ cn+ and pr pr cn cn  

c) pr+ pr+ cn cn and pr pr cn cn  

d) pr pr cn+ cn+ and pr+ pr+ cn cn  

e) either a) or d) could be true.

Solution

True breeding species will have the same  allele pairs for their characteristic traits.

It is given that mother of the progeny has arisen from two  true breeding lines

Thus,  both the parents must have same allele in the allele pair of their genotype

Also the female has wild type eyes, thus her genotype would be pr+cn/pr

Therefore the genotype of parents is

pr+ pr+ cn+ cn+ and pr pr cn cn

There by producing all offspring with genotype  pr pr + / cn cn+

Hence, option B is correct

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