A woman's father has ornithine transcarbamylase deficiency (OTD), an X-linked recessive disorder producing mental deterioration if not properly treated. The woman's mother is homozygous for the wild-type allele.


What proportion of daughters produced by the woman and the man are expected to have OTD? What proportion of sons of the woman and the man are expected to have OTD?

Verified step by step guidance

1. Understand the inheritance pattern: Ornithine transcarbamylase deficiency (OTD) is an X-linked recessive disorder. This means the gene responsible for the disorder is located on the X chromosome, and males (XY) are more likely to express the disorder because they have only one X chromosome. Females (XX) would need two copies of the defective allele to express the disorder.
2. Determine the genotypes of the parents: The woman is a carrier for OTD because her father has the disorder (he passed the defective X chromosome to her) and her mother is homozygous for the wild-type allele. Thus, the woman's genotype is X^OTDX^WT. The man is unaffected, so his genotype is X^WTY.
3. Predict the offspring's genotypes: Use a Punnett square to determine the possible combinations of X and Y chromosomes from the parents. The woman can pass on either X^OTD or X^WT, while the man can pass on either X^WT or Y. This results in four possible combinations: X^OTDX^WT (carrier daughter), X^WTX^WT (wild-type daughter), X^OTDY (affected son), and X^WTY (wild-type son).
4. Analyze the daughters' outcomes: Daughters inherit one X chromosome from each parent. Half of the daughters (X^OTDX^WT) will be carriers, and the other half (X^WTX^WT) will be wild-type. Since OTD is recessive, none of the daughters will express the disorder because they will always have at least one wild-type allele.
5. Analyze the sons' outcomes: Sons inherit the X chromosome from their mother and the Y chromosome from their father. Half of the sons (X^OTDY) will inherit the defective allele and will have OTD, while the other half (X^WTY) will inherit the wild-type allele and will not have OTD.

Key Concepts

X-linked Recessive Inheritance

X-linked recessive inheritance refers to genetic conditions that are associated with genes located on the X chromosome. In this mode of inheritance, males (XY) are more likely to express the disorder because they have only one X chromosome. Females (XX) can be carriers if they have one affected X chromosome and one normal X chromosome, but they typically do not express the disorder unless they are homozygous for the recessive allele.

Genotype of Parents

Understanding the genotypes of the parents is crucial for predicting the inheritance of traits. In this case, the woman is a carrier for OTD (X^OX^d) since her father has the disorder and her mother is homozygous for the wild-type allele (X^O X^O). The man’s genotype is assumed to be normal (X^O Y) unless stated otherwise. This information helps determine the potential genotypes of their offspring.

Punnett Square Analysis

A Punnett square is a tool used to predict the genetic outcomes of a cross between two parents. By arranging the possible gametes from each parent, one can visualize the probabilities of different genotypes in the offspring. In this scenario, constructing a Punnett square with the woman’s and man’s genotypes will reveal the expected proportions of daughters and sons with OTD, aiding in understanding the inheritance pattern.