Two mothers give birth to sons at the same time in a busy hospital. The son of couple 1 is afflicted with hemophilia A, which is a recessive X-linked disease. Neither parent has the disease. Couple 2 has a normal son even though the father has hemophilia A. The two couples sue the hospital in court, claiming that a careless staff member swapped their babies at birth. You appear in court as an expert witness. What do you tell the jury? Make a diagram that you can submit to the jury.

Verified step by step guidance

1. Step 1: First, let's understand the basics of hemophilia A. Hemophilia A is a genetic disorder that is X-linked recessive. This means that the disease is carried on the X chromosome and is recessive, meaning that two copies of the defective gene are needed for the disease to manifest. In males, who have only one X chromosome, one copy of the defective gene is enough to cause the disease.
2. Step 2: Now, let's consider the first couple. The son has hemophilia A, but neither parent has the disease. This is possible if the mother is a carrier of the disease, meaning she has one copy of the defective gene and one normal gene. She can pass the defective gene to her son, who will then have the disease. The father, having a normal X chromosome, cannot be the source of the defective gene.
3. Step 3: For the second couple, the father has hemophilia A, but the son is normal. This is also possible. The father can only pass his Y chromosome to his son, so the son cannot inherit the disease from him. The mother, who is assumed to be normal, provides a normal X chromosome to the son.
4. Step 4: Based on this information, there is no evidence to suggest that the babies were swapped at birth. The genetic outcomes are consistent with the parents' genotypes and the nature of X-linked recessive inheritance.
5. Step 5: To illustrate this, we can draw a Punnett square for each couple. For couple 1, the square would show that half of the sons would have hemophilia A if the mother is a carrier. For couple 2, the square would show that none of the sons would have hemophilia A, as the father can only pass on his Y chromosome.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of genetic transmission of traits located on the X chromosome. In this case, hemophilia A is a recessive disorder linked to the X chromosome, meaning that males (XY) are more likely to express the disease if they inherit the affected X chromosome from their mother, while females (XX) can be carriers without showing symptoms if they have one normal X chromosome.

Genotype and Phenotype

Genotype refers to the genetic makeup of an individual, while phenotype is the observable expression of that genotype. In the scenario, the genotype of the father in couple 2 is XhY (where Xh represents the hemophilia allele), but his normal son has a genotype of XY, indicating he inherited a normal X chromosome from his mother, demonstrating that the mother must be a carrier of the hemophilia allele.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two individuals. It helps visualize the possible combinations of alleles from the parents. In this case, constructing a Punnett square for the parents in both couples can clarify the inheritance patterns and demonstrate that the son of couple 2 could not have inherited hemophilia from his father, supporting the argument against the baby swap claim.