Question

If an F2 generation from a self-crossed F1 generation always yields offspring in a 3:1 phenotypic...

If an F2 generation from a self-crossed F1 generation always yields offspring in a 3:1 phenotypic ratio, which of the following P crosses could have occurred?

a. Aa x AA

b. AA x AA

c. aa x Aa

d. aa x aa

e. AA x aa

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Answer #1

Ans. (e) AA × aa

Explaination: The parental cross would be AA × aa. The F​​​​​​1 will be Aa. Selfing of F​​​​​​1 (Aa × Aa) will produce F​​​​​​2.

Aa × Aa will produce AA, Aa and aa in the ratio 1:2:1. The phenotypes produced by AA and Aa are same and dominant whereas aa produces recessive phenotype. Hence, AA and Aa will produce same phenotype (3 out of 4 offsprings in F​​​​​​2​​​ i.e. 75%) and aa will produce recessive phenotype constituting 1 out of 4 offsprings in F​​​​​​2​​ i.e. 25 % .Hence, there is 3:1 phenotypic ratio in F​​​​​​2​ generation.

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