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?

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In this cross, the male bird (ZZ, with one Z carrying the lethal allele) can pass either the normal Z or the Z with the lethal allele to the offspring. The normal female (ZW) will pass either the Z or the W. Offspring receiving the Z with the lethal allele from the father and a Z from the mother will not survive, leading to a reduction in the number of male offspring (ZZ). Thus, for every ZW (female) offspring, there will be one ZZ (male) offspring that survives and one potential ZZ (male) that does not survive due to the lethal allele. This results in a 2:1 male to female sex ratio among the surviving offspring.

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