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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?

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

1 answer

To determine the expected sex ratio of offspring in a cross between a male pigeon that is heterozygous for a lethal Z-linked allele and a wildtype female, we first need to understand the genetic setup:

  • The male is heterozygous for the lethal allele on the Z chromosome, so his genotype is Z(A)Z, where Z(A) represents the Z chromosome carrying the lethal allele and Z represents the Z chromosome without the lethal allele.
  • The female, being wildtype, has the ZW genotype (the W chromosome is not involved in sex-linked lethals).

Let's set up the possible gametes produced by each parent:

  • The male (Z(A)Z) can produce two types of gametes: Z(A) and Z.
  • The female (ZW) can produce two types of gametes: Z and W.

We can construct a Punnett square to visualize the possible combinations:

                     Z      W
                 ----------------
          Z(A) | Z(A)Z  |  Z(A)W |
                ----------------
          Z     |   ZZ    |   ZW   |

Now let's analyze the offspring:

  1. Z(A)Z: This genotype results in a male (ZZ) that carries the lethal allele, and since it is lethal, these embryos do not survive.
  2. Z(A)W: This genotype results in a female (ZW) who carries the lethal allele but does survive.
  3. ZZ: This genotype results in a male (ZZ) that is unaffected and does survive.
  4. ZW: This genotype results in a female (ZW) that is unaffected.

From this, we can observe the following outcomes:

  • Males that result from the cross:

    • ZZ (survives)
    • Z(A)Z (does not survive, lethal)

  • Females that result from the cross:

    • ZW (survives)
    • Z(A)W (survives)

Therefore, out of the potential offspring:

  • There are 2 male possibilities, but only one (ZZ) survives.
  • There are 2 female possibilities, both of which (ZW and Z(A)W) survive.

As a result:

  • The living males: 1 (ZZ)
  • The living females: 2 (ZW and Z(A)W)

Thus, the expected sex ratio of surviving offspring is:

1 male : 2 females, which can also be expressed as a ratio of 2:1 male to female.

So, the correct answer is:

2:1 male to female.