High School Biology : Understanding Autosomal/Sex-Linked

Study concepts, example questions & explanations for High School Biology

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Genetics Principles

Hemophilia is a sex-linked recessive mutation found on the X-chromosome. A healthy male mates with a female that is a carrier for the disease.

Which of the following statements is true?

Possible Answers:

All offspring will be healthy

50% of their sons will have hemophilia

50% of their daughters will have hemophilia

100% of their daughters will have hemophilia

Correct answer:

50% of their sons will have hemophilia

Explanation:

The disease is recessive and found on the X-chromosome. Remember that males only have one X-chromosome, while females have two. As a result, males only require one mutant allele to express a recessive phenotype, while females need to have two mutant alleles in order to have hemophilia.

In this scenario, both parents are healthy, but the female is a carrier for the disease. This means that one of her alleles codes for hemophilia.

Parents: XXh x XY

Since daughters will need two copies of the mutant allele, they will all be healthy; by default they will receive the healthy X-chromosome from the father.

Daughters: XX or XXh; all healthy phenotype

Sons, however, have a 50% chance of receiving the mutant allele from the mother, because the father will always contribute the Y-chromosome to sons.

Sons: XY or XhY; half healthy, half hemophilic

As a result, 50% of the sons will have hemophilia.

Example Question #1 : Understanding Autosomal/Sex Linked

Rett syndrome is a sex-linked dominant disease found on the X-chromosome. If the offspring of a certain cross have a 50% chance of receiving the disease, regardless of gender, which of the following statements is true?

Possible Answers:

The mother does not have the disease

The father has the disease

The mother is homozygous for the disease

The mother is heterozygous for the disease

Correct answer:

The mother is heterozygous for the disease

Explanation:

Since the disease is found on the X-chromosome, we need to find the scenario in which both sons and daughters have an equal 50% probability of getting the disease. Regardless of gender, mothers will always donate one X-chromosome to the offspring. If the mother is heterozygous for the disease, she has a 50% chance of giving an offspring the diseased allele. As a result, a heterozygous mother will have children that display the disease in the observed ratio.

Parents: XXR x XY

Offspring: XX, XXRXYXRY

Note that this ratio of expression is only possible when the allele for the disorder is dominant; otherwise the heterozygous female would be a carrier, and not express the disorder.

Example Question #1 : Genetics Principles

Colorblindness is a sex-linked recessive condition. If a woman is a carrier for the colorblind trait has a child with a man that does not have the trait, what percent chance will their first child (regardless if it is a girl or boy) be colorblind?

Possible Answers:

50%

0%

25%

100%

75%

Correct answer:

25%

Explanation:

First, we have to look at the genotype of the mother and father. The mother is XX, because she is a woman, while the father is XY. The mother is a carrier for the gene, which means she has the traits on one chromosome, but since it is recessive she does not express it physically (she is not colorblind). Therefore, we can denote her genotype to be X Xc (with c being the color blind trait). When we cross the mother and the father (XY), we receive the following possibilities for their children: XX , XcX, XY, XcY. The trait shows up twice, they could have a girl that is a carrier, but no female will express colorblindness from this couple. If they have a boy, there is a 50% chance that he will be color blind (remember, males only have 1 chromosome so they will inherit genes directly from their mother). But, the question is asking what percent of their children will be colorblind, regardless of it being a girl or boy, therefore the answer is 25%.

Example Question #4 : Genetics Principles

Phil is diagnosed with X-linked muscular dystrophy (recessive). His parents do not have the disease. What is the chance that his sister also has muscular dystrophy?

Possible Answers:

50%

0%

100%

75%

25%

Correct answer:

0%

Explanation:

Since Phil is male, he has one x-chromosome and one y-chromosome. Muscular dystrophy is x-linked recessive, which means his x-chromosome carries the disease allele. He must have inherited this x-chromosome from his mother because his father passed his y-chromosome. Since we know that Phil's parents are not affected, we know that his mother is a carrier of the disease. Phil's sister has two x-chromosomes: one from their mother and one from their father. In order to express a recessive trait, all copies of that gene must be the disease allele. Phil's father can only pass on a healthy x-chromosome to his daughter, and Phil's mother has a 50% chance of passing on her healthy x-chromosome. Thus, there is no chance that Phil's sister will be affected with the disease; there is a 50% chance that she is a carrier, though.

Example Question #5 : Genetics Principles

Bill recently has been diagnosed with Fabry disease (X-linked recessive). Bill is planned on getting married to Emily. There is no history of Fabry disease in Emily's family. They both seek a genetic counseling for their future children. What is the percent chance of Bill and Emily having a baby girl with Fabry disease?

Possible Answers:

0

0.25

1.0

0.50

Correct answer:

0

Explanation:

Since Bill is has the disease and is a male, his chromosomes will appear as: XY. Since Emily does not have the disease and there is no family history of the disease, her chromosomes will appear as: XX. The chances of Bill and Emily having a baby girl is 0.5. This is because they can either have a boy (0.5) or a girl (0.5). These events are independent of each other.

0.5 (baby girl) * 1 (X-chromosome from Bill because he will not donate a Y-chromosome to a female) * 1 (X-chromosome from Emily-does not matter which X-chromosome is given since both are normal) * 0 (No matter what, the baby girl cannot have Fabry because it is an X-linked recessive disease. To get the disease, you would need XX. This is not genotypically possible with the genotypes of the parents provided).

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: