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 daughter with a man who does not have OTD, what is the chance the daughter will be a heterozygous carrier of OTD? What is the chance the daughter will have OTD?

Verified step by step guidance

1. Determine 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 are more likely to express the disorder because they have only one X chromosome.
2. Analyze the genetic makeup of the woman. Since her father has OTD (and males pass their X chromosome to their daughters), the woman must have inherited the X chromosome with the OTD allele from her father. Her mother is homozygous for the wild-type allele, so the woman inherited a wild-type X chromosome from her mother. Therefore, the woman is a heterozygous carrier of OTD.
3. Determine the genetic makeup of the man the woman is having a child with. The man does not have OTD, so his single X chromosome (if he were female, he would have two X chromosomes) must carry the wild-type allele.
4. Consider the possible genetic combinations for the daughter. The daughter will inherit one X chromosome from her mother and one X chromosome from her father. From the mother, there is a 50% chance the daughter will inherit the X chromosome with the OTD allele and a 50% chance she will inherit the X chromosome with the wild-type allele. From the father, the daughter will always inherit the wild-type X chromosome since he does not have OTD.
5. Calculate the probabilities for the daughter. If the daughter inherits the X chromosome with the OTD allele from her mother, she will be a heterozygous carrier (since the X chromosome from her father is wild-type). The chance of this happening is 50%. The daughter cannot have OTD because she will always inherit a wild-type X chromosome from her father, which compensates for the OTD allele. Therefore, the chance the daughter will have OTD is 0%.

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 and Phenotype

Genotype refers to the genetic makeup of an individual, specifically the alleles they possess for a particular gene. Phenotype, on the other hand, is the observable expression of the genotype, influenced by both genetic and environmental factors. In the context of OTD, understanding the genotype of the parents helps predict the potential phenotypes of their offspring, including whether they will be carriers or affected by the disorder.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two individuals. It allows for the visualization of how alleles from each parent combine to form the genotypes of the offspring. In this case, constructing a Punnett square with the mother's genotype (X^OTD X) and the father's genotype (X Y) will help determine the probabilities of their daughter's genotype, including the likelihood of her being a carrier or affected by OTD.