Two mothers give birth to sons at the same time at a busy...

Two mothers give birth to sons at the same time at a busy urban hospital. The son of mother 1 has hemophilia, a disease caused by an X-linked recessive allele. Neither parent has the disease. Mother 2 has a son without hemophilia, 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?

Key Concepts

X-linked Recessive Inheritance

This concept explains how certain traits, like hemophilia, are passed down when the responsible gene is located on the X chromosome. In an X-linked recessive pattern, males tend to be affected when they inherit a single mutated copy of the gene, because they have only one X chromosome, whereas females, having two X chromosomes, usually must inherit two copies of the mutated gene to be affected. This inheritance pattern is essential for understanding the transmission of hemophilia and for interpreting family medical histories accurately.

Carrier Status and Phenotypic Expression

Carrier status refers to individuals who hold one copy of a gene mutation but do not express the trait due to having a second, normal allele. For X-linked recessive conditions, a woman can be a carrier without showing symptoms, but she can pass the mutated gene to her sons, who then may express the disease. Conversely, if a father is affected, his sons receive his Y chromosome and thus cannot inherit the affected X-linked gene from him, which is a key insight when interpreting conflicting phenotypic outcomes in offspring.

Genetic Testing and Parentage Analysis

Genetic testing, including DNA fingerprinting and analysis of genetic markers, is a precise method to determine biological relationships. It is particularly useful in situations where there is a claim of baby-swapping or misidentification at birth. Such analyses can compare the genetic profiles of the children and the supposed parents, confirming consistency with the expected patterns of inheritance and thereby either supporting or refuting claims of misidentification based on genetic evidence.

Transcript

00:01 Okay, you have two sets of parents.

00:02 One parent, one set of parents had a son with hemophilia a.

00:06 The other had a healthy son, but the dad of that healthy son has hemophilia a.

00:10 And so how can you explain that you can have two healthy parents and still have a son with hemophilia a from those parents? well, you have to first understand that this is a sex -linked characteristic or sex -linked disease.

00:25 And so it's going to be found on the x chromosomes.

00:29 Females have two x chromosomes.

00:31 So if you look at mom, she can donate one x or one x chromosome for each of her eggs.

00:37 Dad, however, can donate an x or a y chromosome.

00:41 But the last bit of information that we need to understand is that hemophilia a is recessive.

00:47 And so if mom has a big a and a little a for the disease, you know, little a being this allele is hemophilia.

01:00 And the big a is the dominant one.

01:03 And so if she has a big a and a little a, that means that she could be a carrier for the disease, but not express it because she has two x chromosomes.

01:13 And dad, however, if he had, like if this was actually a little a on his x chromosome, this would mean that dad would actually have the disease.

01:27 And so looking at their offspring, knowing that dad, if he has the trait on his x chromosome, he's going to have the disease because he doesn't have another x chromosome to cover it up.

01:39 We can then explain using a couple punnet squares what the genotypes of these parents must be in order for the situations with these two sons.

01:48 So first i'll just answer the question here at the top of the page that i have.

01:52 So what's the male genotype for hemophilia a? so how would we denote that? well, it's going to be found on x chromosome...

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