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13. Mendelian Genetics

Sex-Linked Inheritance

General Biology

13. Mendelian Genetics

Sex-Linked Inheritance

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0:48 minutes

Problem 17

Textbook Question

Recall that hemophilia is an X-linked recessive disease. If a woman with hemophilia had children with a man without hemophilia, what is the chance that their first child will have the disease? What is the chance that their first child will be a carrier?

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Identify the genotypes of the parents. The mother has hemophilia and is therefore homozygous recessive (XhXh). The father does not have hemophilia and has a normal X chromosome, so his genotype is XY.

Set up a Punnett square with the mother's alleles on one side (Xh and Xh) and the father's alleles on the other side (X and Y).

Fill in the Punnett square to determine the possible genotypes of the offspring. The combinations will be XhX from the mother and X, and XhY from the mother and Y.

Analyze the genotypes of the offspring to determine the probability of each. XhX (daughter with hemophilia) and XhY (son with hemophilia) are the possible genotypes for a child with the disease. XhX (daughter carrier) and XY (son normal) are the possible genotypes for a child who is a carrier or normal.

Calculate the probability of each scenario: having the disease (XhX or XhY) and being a carrier (XhX). Since each child has an equal chance of inheriting each allele, the probability of each genotype can be determined from the Punnett square.

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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 inheritance refers to a pattern of genetic transmission where a gene causing a trait or disorder is located on the X chromosome. Males, having one X and one Y chromosome, are more likely to express X-linked recessive traits because they have only one copy of the X chromosome. Females, with two X chromosomes, can be carriers if they have one affected X chromosome and one normal X chromosome, potentially passing the trait to their offspring.

Genotype and Phenotype

Genotype refers to the genetic constitution of an individual, specifically the alleles they possess for a particular gene. Phenotype, on the other hand, is the observable expression of the genotype, influenced by both genetic and environmental factors. In the context of hemophilia, the genotype of the parents will determine the likelihood of their children inheriting the disease or being carriers, affecting their phenotype.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two individuals. It allows for the visualization of how alleles from each parent combine to produce potential genotypes in their offspring. By filling out a Punnett square for the woman with hemophilia (XhX) and the man without hemophilia (XY), one can easily determine the probabilities of their child being affected by hemophilia or being a carrier.

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Related Practice

Textbook Question

A couple are both phenotypically normal, but their son suffers from hemophilia, a sex-linked recessive disorder.

What fraction of their children are likely to suffer from hemophilia?

What fraction are likely to be carriers?

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Textbook Question

Why do more men than women have colorblindness?

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Textbook Question

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.

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Textbook Question

Heather was surprised to discover she suffered from red-green colorblindness. She told her biology professor, who said, 'Your father is colorblind, too, right?' How did her professor know this? Why did her professor not say the same thing to the colorblind males in the class?

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Multiple Choice

What is the primary difference between sex-linked traits and autosomal traits in humans?

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Multiple Choice

In sex-linked inheritance, when does a recessive trait manifest in a female?