Question

The maternal-effect mutation bicoid (bcd) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles (bcd+ /bcd—) and a male homozygous for the mutation (bcd—/bcd—).

a. How is it possible for a male homozygous for the mutation to exist?
b. Predict the outcome (normal vs. failed embryogenesis) in the F1 and F2 generations of the cross described.

Answer 1

1. A homozygous male (bcd⁻/bcd⁻) child is possible only when the mother is heterozygous (bcd⁺/bcd⁻) and the father is homozygous (bcd⁻/bcd⁻).
2. In Normal vs. failed embryogenesis, in the F1 generation of the cross, all of them will have normal embryogenesis.
3. In Normal vs. failed embryogenesis, in the F2 generation of the cross, 75% of them will have normal embryogenesis and 25% of them will not undergo embryogenesis.

Step-by-step explanation:

1. bcd⁻/bcd⁻ × bcd⁺/bcd⁻

↓

bcd⁺/bcd⁻, bcd⁺/bcd⁻, bcd⁻/bcd⁻, bcd⁻/bcd⁻

∴ Male homozygous is possible.

2. Normal embryogenesis = bcd⁺/bcd⁺

Failed embryogenesis = bcd⁻/ bcd⁻

bcd⁺/bcd⁺ × bcd⁻/ bcd⁻

↓

All progeny will be bcd⁺/bcd⁻ in the F1 generation.

∴ All of them will undergo normal embryogenesis.

3. In F2 generation,

bcd⁺/bcd⁻ × bcd⁺/bcd⁻

↓

bcd⁺/bcd⁺ , bcd⁺/bcd⁻, bcd⁺/bcd⁻, bcd⁻/ bcd⁻

∴ 25% of them is normal and will undergo normal embryogenesis, 50% of them will undergo normal embryogenesis as bcd protein is present and 25% of them will not undergo embryogenesis as bcd protein is absent.

∴ 75% will undergo normal embryogenesis and 25% will not undergo embryogenesis.