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In a Drosophila experiment, a cross was made between homozygous wild-type females and yellow-bodied males. All of the resulting F₁s were phenotypically wild type. However, adult flies of the F₂ generation (resulting from matings of the F₁s) had the characteristics shown in the figure. How is the mutant allele for yellow body inherited?
A) It is not X-linked.
B) It is X-linked.
C) It is inherited by X inactivation.
D) It is Y-linked.

Question

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In a Drosophila experiment, a cross was made between homozygous wild-type females and yellow-bodied males. All of the resulting F₁s were phenotypically wild type. However, adult flies of the F₂ generation (resulting from matings of the F₁s) had the characteristics shown in the figure. How is the mutant allele for yellow body inherited?
A) It is not X-linked. 
B) It is X-linked. 
C) It is inherited by X inactivation. 
D) It is Y-linked.

Quizplus · Accepted Answer

Since only males display the yellow-bodied phenotype, it indicates that the mutant allele is located on the X chromosome. Inheritance patterns suggest X-linkage instead of autosomal recessive (which would produce a 3:1 phenotypic ratio in the F₂ generation) or autosomal dominant (which would result in an affected parent producing an affected offspring). The fact that all F₁ offspring were phenotypically wild-type indicates either that the wild-type allele is dominant or that the mutant allele is recessive.

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