In birds,sex is determined by a ZW chromosome scheme that is much like the typical XY scheme seen in humans and many other organisms,except that the system is reversed:
Males are ZZ (similar to XX in humans)and females are ZW (similar to XY in humans).A lethal recessive allele that causes death of the embryo occurs on the Z chromosome in pigeons.What would be the sex ratio in the offspring of a cross between a male heterozygous for the lethal allele and a normal female?

Question

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The male is heterozygous (ZlZ) and the female is normal (ZW). The possible offspring are ZlZ (male, lethal), ZZ (male, normal), ZlW (female, lethal), and ZW (female, normal). The lethal combinations (ZlZ and ZlW) die, leaving a 2:1 ratio of normal males (ZZ) to normal females (ZW).

In Birds,sex Is Determined by a ZW Chromosome Scheme That