In birds, sex is determined by a ZW chromosome scheme.?

D

D B

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In birds, sex is determined by a ZW chromosome scheme. Males are ZZ and females are ZW. A recessive lethal allele that causes death of the embryo is sometimes present on the Z chromosome in pigeons. What would be the sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a normal female?

A) 4:3 male to female
B) 2:1 male to female
C) 3:1 male to female
 
B) 2:1 male to female

There are four equally probable fertilizations:
(+) Male, homozygous for the non-lethal Z.
(+) Male, heterozygous for the non-lethal and lethal Z.
(+) Female with the non-lethal Z.
(-) Female with the lethal Z.

Since the last fertilization does not result in off spring, the male:female ratio will be 2:1.
 
Z|Zi x ZW
_________
Z | ZZ | ZW|
| ----------------|
Zi |_ZZi_| ZiW |
Z W

Zi= lethal But Z is dominat

So for every 2 males, their is a chance that one will be female that will die.

B
 
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