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Ch. 4 - Extensions of Mendelian Genetics
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 4 - Extensions of Mendelian GeneticsProblem 33
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?

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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.
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.
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.
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.
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.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.
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X-Inactivation

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.
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Probability

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.
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Maternal Effect
Related Practice
Textbook Question

Discuss the topic of phenotypic expression and the many factors that impinge on it.

Textbook Question

Predict the F₁ and F₂ results of crossing a male fowl that is cock-feathered with a true-breeding hen-feathered female fowl. Recall that these traits are sex limited.

Textbook Question

In goats, the development of the beard is due to a recessive gene. The following cross involving true-breeding goats was made and carried to the F₂ generation:

Offer an explanation for the inheritance and expression of this trait, diagramming the cross. Propose one or more crosses to test your hypothesis.

Textbook Question

Consider the three pedigrees below, all involving a single human trait.

Given your conclusions in part (a), indicate the genotype of the following individuals: II-1, II-6, II-9 If more than one possibility applies, list all possibilities. Use the symbols A and a for the genotypes.

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Textbook Question

Labrador retrievers may be black, brown (chocolate), or golden (yellow) in color (see chapter-opening photo). While each color may breed true, many different outcomes are seen when numerous litters are examined from a variety of matings where the parents are not necessarily true breeding. Following are just some of the many possibilities.

(a) black x brown → all black

(b) black x brown → 1/2 black, 1/2 brown

(c) black x brown → 3/4 black, 1/4 golden

(d) black x golden → all black

(e) black x golden → 4/8 golden 3/8 black 1/8 brown

(f) black x golden → 2/4 golden 1/4 black 1/4 brown

(g) brown x brown → 3/4 brown 1/4 golden

(h) black x black → 9/16 black 4/16 golden, 3/16 brown

Propose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color.

Textbook Question

Contrast penetrance and expressivity as the terms relate to phenotypic expression.