Question

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

Answer 1

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**.