Question

when predicting the phenotypic ratio of a crossing between two heterozygous fruit flies, the ratio was shown to be 9:3:3:1. what would the parental fruit flys genotype have been if the resulting offspring had a 1:1:1:1 ratio instead?
multiple choice
llbb × llbb
libb × iibb
libb × libb
llbb × libb
libb × libb

Explanation:

When dealing with dihybrid crosses, a \(1:1:1:1\) phenotypic ratio in the offspring is characteristic of a test - cross. A test - cross involves one parent that is heterozygous for both traits (\(AaBb\)) and another parent that is homozygous recessive for both traits (\(aabb\)). In fruit - fly notation (assuming \(L\) and \(l\) for one trait and \(B\) and \(b\) for another), this would be \(LlBb\times llbb\).

Answer:

\(LlBb\times llbb\) (assuming the first option is \(LLBB\times llbb\) which is incorrect as \(LLBB\) is homozygous dominant; the second option \(Llbb\times llBb\) is not a standard test - cross; \(LlBb\times LlBb\) gives a \(9:3:3:1\) ratio; \(llbb\times Llbb\) is not a dihybrid test - cross; \(LlBB\times Llbb\) is also not a dihybrid test - cross. So the correct cross for a \(1:1:1:1\) ratio is \(LlBb\times llbb\))