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Chapter 4: Problem 33

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?

Short Answer

Expert verified
Answer: No, the likelihood that the sons were swapped at birth is low. The outcomes are consistent with the expected probabilities for each couple.

Step by step solution

01

Understand X-linked inheritance

Hemophilia is an X-linked recessive disease, meaning that the gene responsible for the disease is located on the X chromosome. In this case, the affected allele will be labeled as "h", and the non-affected allele as "H". Since males have one X chromosome and one Y chromosome (XY), they will be affected by hemophilia if they inherit the X chromosome with the recessive allele (XhY). Females have two X chromosomes (XX), and can only have the disease if they inherit the affected allele from both parents (XhXh); otherwise, they will be carriers (XHXh) or non-carriers (XHXH).
02

Determine the genotypes of the parents

To determine the possible outcomes for their offspring, we first need to know the genotype of each individual in both families. Couple 1: - Mother 1: We're told that she is not affected by hemophilia, but her son is affected. This means she must be a carrier (XHXh). - Father 1: He's not affected, meaning he's genotypically XHY. Couple 2: - Mother 2: Her son is healthy, but his father is affected. We are not given information about whether she's a carrier or not, so we'll assume her genotype is XHXH (non-carrier) for this part of the analysis. - Father 2: He has hemophilia, meaning he has the XhY genotype.
03

Determine the possible offspring genotypes

Next, we will determine the possible offspring genotypes for each couple using Punnett squares. Couple 1 (Mother 1: XHXh, Father 1: XHY): - Offspring genotypes: XHXH (25%), XHXh (25%), XHY (25%), and XhY (25%) - Probability of having a son with hemophilia: 25% Couple 2 (Mother 2: XHXH, Father 2: XhY): - Offspring genotypes: XHXh (50%) and XHY (50%) - Probability of having a healthy son: 50%
04

Compare the expected offspring outcomes

Couple 1 has a 25% probability of having a son with hemophilia, which supports the fact that their son has the disease. Couple 2 has a 50% probability of having a healthy son, so their son being unaffected is also consistent with the probabilities.
05

Assess the likelihood of the allegation

Based on the genetic information provided and our analysis of X-linked inheritance, the swap allegation seems unlikely since the outcomes are consistent with the expected probabilities for each couple. However, additional genetic testing may be needed to provide more definitive information. Without evidence, as a genetic counselor, the information provided to the jury would be that based on the inheritance pattern of hemophilia and the genotypes of the parents, the likelihood that the sons were swapped at birth is low.

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