Two mothers give birth to sons at the same time at a busy urban hospital. The son of mother 1 is afflicted with hemophilia, a disease caused by an X-linked recessive allele. Neither parent has the disease. Mother 2 has a normal son, despite the fact that the father has hemophilia. Several years later, couple 1 sues the hospital, claiming that these two newborns were swapped in the nursery following their birth. As a genetic counselor, you are called to testify. What information can you provide the jury concerning the allegation?

Verified step by step guidance

1. Step 1: Understand the inheritance pattern of hemophilia, which is an X-linked recessive disorder. This means the gene causing hemophilia is located on the X chromosome, and males (XY) who inherit the affected X chromosome will express the disease because they have only one X chromosome.
2. Step 2: Analyze the genetic status of the first family. Since the son of mother 1 has hemophilia but neither parent shows the disease, the mother must be a carrier (X^H X^h) and the father must have a normal X chromosome (X^H Y). The son inherits the affected X chromosome (X^h) from the mother and a Y chromosome from the father, resulting in hemophilia.
3. Step 3: Analyze the second family. The father has hemophilia (X^h Y), so he can only pass the affected X chromosome to his daughters and the Y chromosome to his sons. Since mother 2 has a normal son, this son must have inherited the Y chromosome from the father and a normal X chromosome (X^H) from the mother, meaning the mother is not a carrier.
4. Step 4: Use this information to assess the claim of swapped babies. The son with hemophilia must have inherited the affected X chromosome from his mother, so the mother of the hemophiliac son must be a carrier. The normal son born to the father with hemophilia must have inherited a normal X chromosome from his mother, who is not a carrier. This genetic evidence supports that the babies were not swapped.
5. Step 5: Summarize for the jury that the genetic patterns of X-linked inheritance and the presence or absence of carrier status in the mothers provide strong evidence against the claim that the babies were swapped in the nursery.

Key Concepts

X-linked Recessive Inheritance

X-linked recessive traits are caused by mutations on the X chromosome. Males, having only one X chromosome, express the disease if they inherit the affected allele, while females must inherit two copies to be affected. Carrier females typically do not show symptoms but can pass the allele to their sons.

Genetic Probability and Carrier Status

In X-linked recessive diseases like hemophilia, an unaffected mother can be a carrier if she has one mutated X chromosome. A son of a carrier mother has a 50% chance of being affected, while a son of a non-carrier mother will be unaffected. Fathers with hemophilia cannot pass the disease to sons but pass the mutated X to all daughters.

Paternity and Maternity Testing Using Genetic Markers

Genetic testing can determine biological relationships by comparing DNA markers. Since hemophilia is X-linked, testing the mother’s carrier status and the child’s genotype can confirm maternity. Similarly, paternity can be confirmed by comparing autosomal markers, helping resolve disputes about infant identity in hospital mix-ups.