Question

A specific X-linked recessive allele is lethal. Males that inherit the recessive allele and females that are homozygous recessive will abort before birth. What is the probability that a heterozygous mother will give birth to a child with only one copy of the recessive allele?

Answer 1

The probability that a heterozygous mother will give birth to a child with only one copy of the recessive allele is
`(1)/(3)`

Explanation:

Let the recessive allele causing lethal disease be represented by "r"

Let the dominant allele causing no lethal disease be represented by "R"

Genotype of heterozygous mother will be
`X_RX_r`

Genotype of heterozygous mother will be
`X_rY`

A cross between
`X_RX_r` and
`X_rY` will produce following offspring

`X_RX_r`

`X_RY`

`X_rX_r`

`X_rY`

Thus, out of four offspring only child with genotype
`X_rY` will die

Thus, only three offspring remain alive and out of these three only one child with genotype
`X_RX_r` has one copy of recessive allele.

Thus, the probability that a heterozygous mother will give birth to a child with only one copy of the recessive allele is
`(1)/(3)`