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.


If the woman has a son with a man who does not have OTD, what is the chance the son will have OTD?

Verified step by step guidance

1. Step 1: Understand the inheritance pattern of ornithine transcarbamylase deficiency (OTD). OTD is an X-linked recessive disorder, meaning the gene responsible for the condition is located on the X chromosome, and males (XY) are more likely to express the disorder because they have only one X chromosome.
2. Step 2: Analyze the genetic makeup of the woman. Since her father has OTD, he must have the mutant allele on his X chromosome. Her mother is homozygous for the wild-type allele, meaning both of her X chromosomes carry the normal allele. Therefore, the woman inherited one mutant X chromosome from her father and one wild-type X chromosome from her mother, making her a carrier of OTD.
3. Step 3: Determine the genetic makeup of the man. The problem states that the man does not have OTD, meaning his X chromosome carries the wild-type allele.
4. Step 4: Predict the genetic combinations for the son. Sons inherit their X chromosome from their mother and their Y chromosome from their father. Since the woman is a carrier, there is a 50% chance she will pass on her mutant X chromosome and a 50% chance she will pass on her wild-type X chromosome.
5. Step 5: Conclude the probability of the son having OTD. If the son inherits the mutant X chromosome from his mother, he will have OTD because males have only one X chromosome. If he inherits the wild-type X chromosome, he will not have OTD. Therefore, the chance the son will have OTD is 50%.

Key Concepts

X-Linked Recessive Inheritance

X-linked recessive disorders are caused by mutations on the X chromosome. Males, having one X and one Y chromosome, are more likely to express these disorders if they inherit the affected X. Females, with two X chromosomes, can be carriers if they have one affected X and one normal X, but typically only express the disorder if both X chromosomes are affected.

Genotype of the Parents

In this scenario, the woman is a carrier for ornithine transcarbamylase deficiency (OTD) because her father has the disorder and her mother is homozygous for the wild-type allele. This means the woman has one affected X chromosome (from her father) and one normal X chromosome (from her mother). The man, not having OTD, has a normal X chromosome, which is crucial for determining the genetic outcome for their son.

Punnett Square Analysis

A Punnett square is a tool used to predict the genetic outcomes of a cross between two individuals. By placing the possible gametes from each parent in a grid, one can visualize the potential genotypes of the offspring. In this case, it helps to determine the probability that the son will inherit the affected X chromosome from his mother and thus have OTD.