In birds, sex is determined by ZW chromosome inheritance. Males are ZZ and females are ZW. Within pigeon populations there is a recessive, Z-linked allele that is lethal in embryos. What is the expected sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a wildtype female?

Question

1:1 male to female
1:2 male to female
3:1 male to female
2:1 male to female

Answer 1

To determine the expected sex ratio in the offspring of a cross between a heterozygous male (ZZZl, where Zl represents the recessive lethal allele on the Z chromosome) and a wildtype female (ZW), we first need to identify the possible gametes produced by each parent.

1. Male parent (heterozygous): The male can produce two types of gametes:

   • Z (wildtype)
   • Zl (lethal)

2. Female parent (wildtype): The female can only produce one type of gamete:

   • Z (wildtype)

Now we can lay out the possible combinations of these gametes for their offspring:

• From the male (Z or Zl) and the female (Z):
  • ZZ (male): will survive
  • ZW (female): will survive
  • ZlZ (male): Lethal allele, will not survive (embryos die)
  • ZlW (female): Lethal allele, will not survive (embryos die)

Analyzing these combinations:

• ZZ (survives, male)
• ZW (survives, female)
• ZlZ (dies, male)
• ZlW (dies, female)

Since ZlZ and ZlW die, they do not contribute to the live offspring ratio. Only the ZZ and ZW combinations are viable.

Thus, the surviving male to female ratio is:

• Surviving males (ZZ): 1
• Surviving females (ZW): 1

This leads to a sex ratio of 1:1 for the viable offspring.

So, the expected sex ratio in the offspring of this cross is 1:1 male to female.