Question 1

A person is genetically XX and develops as a male.How can this be explained?

Accepted Answer

This condition is called XX male syndrome or de la Chapelle syndrome. It occurs when a piece of the Y chromosome, including the SRY gene, is transferred to one of the X chromosomes during meiosis. This causes the individual to develop male reproductive organs and often results in infertility.

Question 2

The X chromosome in humans is

Accepted Answer

The X chromosome is present in both males and females, but females have two copies of the X chromosome while males have one X and one Y chromosome.

Question 3

A female carrier of the X-linked recessive gene for red-green color blindness will display a phenotype of

Accepted Answer

A female carrier of the X-linked recessive gene for red-green color blindness will display a normal color vision phenotype, as she has one normal X chromosome that can compensate for the defective X chromosome. The XC allele is dominant over the Xc allele.

Question 4

Autosomal dominant diseases are exhibited by anyone who carries at least one dominant allele for that gene.How is it that dominant lethal genes,such as the one that causes Huntington disease,can persist in a population?

Accepted Answer

Dominant lethal genes that cause diseases such as Huntington's disease usually take effect later in life after people have had children. This means that affected individuals may have already passed on the disease-carrying allele to their offspring before they even know that they have the disease. Additionally, some individuals may have a new mutation that causes the disease and may not have a family history of the condition. Therefore, the disease-causing allele can persist in the population.

Question 5

Which of the following statements would indicate a pedigree that shows the inheritance pattern of a recessive X-linked disease?

Accepted Answer

X-linked recessive diseases are more commonly seen in males, as they only have one X chromosome, whereas females have two X chromosomes and could potentially have a normal copy to offset the effects of the disease-causing gene.

Question 6

Why are X-linked recessive genetic disorders more commonly seen in males?

Accepted Answer

Females have two X chromosomes, so they need to inherit two copies of the recessive allele to exhibit the disorder. Males, on the other hand, have only one X chromosome, so if they inherit the recessive allele on that X chromosome, they will exhibit the disorder.

Question 7

An X-linked carrier is a female individual with a ________ genotype,on her X chromosomes,who does not express the recessive trait but can pass it along to offspring.

Accepted Answer

A female carrier has one normal X chromosome and one X chromosome with the recessive allele. As females have two X chromosomes, they exhibit X-linked disorders in a recessive pattern only if both X chromosomes carry the recessive allele. Therefore, the female carrier is heterozygous for the recessive trait, meaning she has one copy of the normal allele and one copy of the recessive allele on her X chromosomes.

Question 8

Most inherited human genetic disorders are inherited as ________ alleles.

Accepted Answer

Most inherited human genetic disorders, such as cystic fibrosis and sickle cell anemia, are caused by recessive alleles. This means that an affected individual must inherit two copies of the mutated allele, one from each parent. Dominant alleles only require one copy to cause the disorder, and sex-linked alleles are located on the X or Y chromosome and thus affect males and females differently. Homologous refers to pairs of chromosomes that carry the same genes but may have different versions of those genes.

Question 9

Which of the following genotypes represents a human male?

Accepted Answer

Human males have an XY genotype, with the presence of a Y chromosome determining male sex.

Question 10

If a father is affected by an X-linked recessive condition and the mother is a carrier,what is the probability of their children being affected?

Accepted Answer

In the case of an X-linked recessive condition, if the father is affected (having only one X chromosome, which carries the condition) and the mother is a carrier (having one affected X chromosome and one unaffected X chromosome), their sons have a 50% chance of being affected. This is because sons inherit their X chromosome from their mother and their Y chromosome from their father. Daughters inherit one X chromosome from each parent, but since they receive the father's affected X chromosome, they will be carriers if they inherit the mother's unaffected X chromosome.

Question 11

In humans,the "master sex switch" that determines whether an embryo will become a male is the

Accepted Answer

The SRY gene on the Y chromosome is responsible for initiating male development in embryos. In the absence of the SRY gene, the embryo will develop as female. Therefore, the SRY gene is often referred to as the "master sex switch." The X chromosome may influence certain aspects of sex determination, but it is not responsible for determining whether an embryo will become male or female. Locus and androgen switch are not commonly used terms in relation to sex determination in humans.

Question 12

A female who is a carrier of the sex-linked gene A has the genotype

Accepted Answer

Since the gene is sex-linked, it must be on the X chromosome. Females have two X chromosomes, so the possible genotypes for a carrier are XAXa or XaXa. Therefore, option D is the best choice as it represents a carrier female with one normal allele and one allele for the sex-linked disorder. The other options do not account for the fact that the gene is sex-linked or do not represent a carrier genotype.

Question 13

The genotype of a female carrier of the X-linked recessive gene for Duchene muscular dystrophy is represented as

Accepted Answer

The X-linked recessive gene for Duchenne muscular dystrophy is located on the X chromosome. In females, there are two X chromosomes, one from each parent. A female carrier has one normal X chromosome and one X chromosome with the recessive gene for Duchenne muscular dystrophy.

The genotype of a female carrier can be represented as XDXd, where X represents the normal allele and Xd represents the allele for Duchenne muscular dystrophy. Choice A (XDY) is incorrect because it represents the genotype of a male with one X and one Y chromosome. Choice C (XdY) is incorrect because it represents the genotype of a male with one X and one Y chromosome, where the X chromosome carries the recessive gene for Duchenne muscular dystrophy. Choice D (XDXD) is incorrect because it represents the genotype of a female with two normal X chromosomes.

Question 14

The pedigree shown below diagrams an X-linked gene.The individual indicated is ________ for the gene.

Accepted Answer

The answer of The pedigree shown below diagrams an X-linked...

Question 15

An allele that appears more frequently in males is MOST likely

Accepted Answer

An allele that appears more frequently in males is likely sex-linked, meaning it is located on the X or Y chromosome. Since males only inherit one X chromosome from their mother, any allele carried on the X chromosome will be expressed in males even if it is recessive. Therefore, sex-linked alleles are more likely to be expressed in males, leading to a higher frequency.

Question 16

Both members of a couple are carriers for a recessive disease allele.If the couple has four children,which of the following statements must be true?

Accepted Answer

When both parents are carriers of a recessive disease allele, each child has a 25% chance of having the disease, a 50% chance of being a carrier, and a 25% chance of not inheriting the allele at all. Therefore, it's possible that 50% of the children could be carriers, but none of the other outcomes are guaranteed for any specific number of children.

Question 17

Because an individual with an XX genotype is a female,is an individual with an XO (no second sex chromosome)a male?

Accepted Answer

The Y chromosome contains the SRY gene which triggers the development of male characteristics. Without this gene, the embryo would develop as a female. Therefore, an individual with an XO genotype would be female as they lack the gene for male development.

Question 18

Which of the following is true of the genetic cross shown in the figure below?

Accepted Answer

Based on the Punnett square shown in the figure, we can see that the parents are both carriers (heterozygous) for the recessive disorder. Therefore, each of their offspring has a 25 percent chance of inheriting two copies of the mutant allele and having the disorder, a 50 percent chance of inheriting one copy of the mutant allele and being a carrier, and a 25 percent chance of inheriting two normal alleles and not having the disorder. Since the disorder is X-linked, it primarily affects males. A carrier female has one normal X chromosome and one X chromosome with the mutant allele. When she has children, each child has a 50 percent chance of inheriting the normal X chromosome and a 50 percent chance of inheriting the X chromosome with the mutant allele. Therefore, their daughters have a 50 percent chance of being carriers, and their sons have a 50 percent chance of inheriting the X chromosome with the mutant allele and having the disorder.

Question 19

In which of the following situations would looking at a pedigree be MOST useful?

Accepted Answer

Pedigrees are most useful when trying to determine patterns of inheritance within families. The fact that the three young boys are first cousins suggests that there may be a genetic component to their neurological disorder. Looking at a pedigree could help determine if other family members are affected and if the disorder is inherited in a dominant or recessive manner.

Question 20

The gene for a certain sex-linked trait is found only on the Y chromosome.If the male parent has this gene,the trait will be expressed in

Accepted Answer

Since the gene for the sex-linked trait is found only on the Y chromosome, it means that only males can inherit and express the trait. Therefore, if the male parent has the gene, all of his male offspring will inherit and express the trait. None of his female offspring will express the trait because they do not have a Y chromosome.

Quiz 8: Chromosomes and Human Genetics