The sex of birds,some insects,and other organisms is determined by a ZW chromosomal arrangement in which the males have like sex chromosomes (ZZ)and females are ZW (similar to XY in humans).Assume that a recessive lethal allele on the Z chromosome causes death of an embryo in birds.What sex ratio would result in the offspring if a cross were made between a male heterozygous for the lethal allele and a normal female?

Question

Quizplus · Accepted Answer

Since the male is heterozygous for the lethal allele, he has one normal Z chromosome and one Z chromosome with the lethal allele. When he produces sperm, half will carry the normal Z chromosome and half will carry the lethal Z chromosome. The female is normal and has two Z chromosomes. When the two gametes combine, there are four possible offspring: ZZ (normal male), ZL (male with lethal allele), ZW (normal female), and ZWL (female with lethal allele). However, the ZWL genotype is lethal, so those embryos will not survive. Therefore, the viable offspring are ZZ (normal males), ZL (males with lethal allele), and ZW (normal females). Since the male-to-female ratio is 2:1, the answer is B.

The Sex of Birds,some Insects,and Other Organisms Is Determined by a ZW