Question 1

A man with normal vision who is a dwarf due to achondroplasia has children with a woman who is colorblind and of average height. Dwarfism caused by achondroplasia is autosomal dominant, and red-green color blindness is X-linked recessive. What proportion of their sons are expected to be color blind and of normal height?

Accepted Answer

Since achondroplasia is autosomal dominant, the father must have one allele for dwarfism and one for normal height. The mother, being of average height, can only contribute alleles for normal height. For color blindness, the mother, being colorblind, must have two X-linked recessive alleles for color blindness (XcXc). The father, having normal vision, has a normal X chromosome (X) and a Y chromosome. Sons inherit their X chromosome from their mother and their Y chromosome from their father. Therefore, all sons will inherit the X chromosome carrying the colorblind gene from their mother and a Y chromosome from their father, making them colorblind. However, they will inherit one allele for normal height from their father and one from their mother, making them of normal height. Thus, 1/2 of their sons are expected to be color blind and of normal height, considering the inheritance of the dwarfism trait separately from the color blindness trait.

Question 2

Glucose-6-phosphate dehydrogenase deficiency (G6PD) is inherited as an X-linked recessive allele in humans. A woman whose father had G6PD is planning a family with a man who has no history of the disease. What proportion of their sons are expected to have the disease?

Accepted Answer

Since G6PD deficiency is X-linked recessive, a woman with a father who had G6PD must be a carrier (XGxg, where XG is a normal allele and Xg is the allele for G6PD deficiency). A man with no history of the disease will have a normal X chromosome (XGY). Their sons will inherit an X chromosome from their mother and a Y chromosome from their father. Half of the sons are expected to inherit the Xg chromosome from their mother, leading to G6PD deficiency, since they cannot inherit a compensatory normal allele from their father.

Question 3

Which of the following events results in X-inactivation in female mammals?

Accepted Answer

X-inactivation in female mammals is primarily driven by the activation of the XIST gene on the X chromosome that is destined to become the Barr body. This gene produces an RNA that coats the chromosome, leading to its inactivation.

Question 4

A man with normal vision who is a dwarf due to achondroplasia has children with a woman with colorblindness who is of average height. Dwarfism caused by achondroplasia is autosomal dominant, and red-green color blindness is X-linked recessive. What proportion of their daughters are expected to be color-blind with achondroplasia?

Accepted Answer

Since achondroplasia is autosomal dominant, the man must have one allele for achondroplasia and one normal allele. For color blindness, which is X-linked recessive, the woman must have two alleles for color blindness (since she is colorblind and females have two X chromosomes). Daughters inherit one X chromosome from their mother and one from their father. All daughters will inherit the X chromosome carrying the normal vision allele from their father (since he is not colorblind and males have only one X chromosome), ensuring they are not colorblind. They will also inherit one allele for achondroplasia from their father, making them affected by achondroplasia. However, since the question asks specifically for daughters who are both color-blind and have achondroplasia, and we've established that all daughters will not be color-blind, the correct answer is 0.

Question 5

In humans, clear sex differentiation does not occur at fertilization. It becomes evident after the second month of gestation. Which of the following statements describes the first event of sex differentiation in human embryos?

Accepted Answer

The first event of sex differentiation in human embryos is the activation of the SRY gene in male embryos, which leads to the masculinization of the gonads. This gene is located on the Y chromosome and is responsible for initiating the development of male sexual characteristics.

Question 6

In birds, sex is determined by ZW chromosome inheritance. Males are ZZ and females are ZW. Within pigeon populations there is a recessive, Z-linked allele that is lethal in embryos. What is the expected sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a wildtype female?

Accepted Answer

In this cross, half of the male offspring (ZZ) will inherit the lethal allele and die, while all female offspring (ZW) will survive because they can only inherit the normal allele from their father. This results in a 2:1 male to female ratio among the surviving offspring.

Question 7

In cats, an X-linked locus is responsible for fur color. There are two known alleles at this locus. One results in black fur color; the other results in orange fur color. A heterozygote animal has patches of orange and black fur (tortoiseshell). Which coat color phenotypes are expected from the cross of a black female and an orange male?

Accepted Answer

Since the fur color gene is X-linked, females have two X chromosomes and can be heterozygous, displaying the tortoiseshell phenotype. Males, having only one X chromosome, will express the allele present on their single X chromosome. A black female (X^bX^b) crossed with an orange male (X^oY) will produce tortoiseshell females (X^bX^o) and black males (X^bY), as males inherit their X chromosome from their mother and their Y chromosome from their father.

Question 8

The SRY gene is best described as ________.

Accepted Answer

The SRY gene is located on the Y chromosome and is responsible for initiating male sex determination by directing the development of the undifferentiated gonadal tissue into testes.

Question 9

Cinnabar eye color is an X-linked, recessive characteristic in fruit flies. If a female having cinnabar eyes is crossed with a male having wild-type, red eyes, what percent of the F1 males will have cinnabar eyes?

Accepted Answer

Since cinnabar eye color is X-linked and recessive, and the female parent has cinnabar eyes, she must have two copies of the recessive allele (being XX). The male, having wild-type eyes, has a normal allele on his X chromosome. All male offspring (XY) will inherit their X chromosome from their mother and their Y chromosome from their father. Therefore, all male offspring will inherit the cinnabar allele from their mother and exhibit cinnabar eyes.

Question 10

A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is color blind has children with a color-blind male. What is the probability that this couple's first son will be color blind?

Accepted Answer

Since the woman has a color-blind father, she must carry one allele for color blindness (Xc) because the trait is X-linked and her father would have passed his only X chromosome to her. The woman's genotype is XcX. The color-blind male has the genotype XcY. When they have a son, the son gets a Y chromosome from his father and either an X or Xc chromosome from his mother. There's a 50% chance he will inherit the Xc from his mother, leading to color blindness.

Question 11

When Thomas Hunt Morgan crossed red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies, but all the white-eyed flies were male. Which of these best explains Morgan's result?

Accepted Answer

The gene for eye color in Morgan's experiments is located on the X chromosome. Since males have only one X chromosome, a single recessive allele (like that for white eyes) will express the trait, explaining why only males showed the white-eyed phenotype.

Question 12

Use the following information to answer the question.  In a Drosophila experiment, a homozygous wild-type female was crossed with a yellow-bodied male. All of the resulting F1 flies were phenotypically wild type. Crossing the F1 flies resulted in F2 flies having the characteristics shown in the figure. Which of the following statements best describes the yellow body allele?

Accepted Answer

The answer of Use the following information to answer the...

Question 13

Which of the following individuals will inherit an X-linked allele from a male parent who carries the allele?

Accepted Answer

Male parents pass their single X chromosome to all of their daughters and their Y chromosome to all of their sons. Therefore, daughters will inherit any X-linked alleles their father carries.

Question 14

Duchenne muscular dystrophy is a serious condition caused by a recessive allele of a gene on the human X chromosome. The patients' muscles weaken over time because they have a lack or decreased levels of dystrophin, a muscle protein. Which of the following correctly predicts the probability of muscular dystrophy in female children?

Accepted Answer

If a man with Duchenne muscular dystrophy (affected) has children with a carrier woman, there is a 50% chance (one-half) that their daughters will inherit the affected X chromosome from their father and the carrier X chromosome from their mother, resulting in the condition.

Question 15

Red-green color blindness is an X-linked recessive trait in humans. Two people with normal color vision have a son with colorblindness. Given this information, the genotypes of the parents are ________.

Accepted Answer

Since red-green color blindness is an X-linked recessive trait, the son must have inherited his Y chromosome from his father and the X chromosome carrying the colorblindness allele (Xn) from his mother. For the son to express colorblindness, his mother must be a carrier of the trait (XNXn), and his father must have normal color vision (XNY), as men only have one X chromosome, which in this case does not carry the colorblindness allele.

Question 16

Males are more often affected by X-linked traits than are females because ________.

Accepted Answer

Males have only one X chromosome (and one Y chromosome), making them hemizygous for genes on the X chromosome. This means that if a male inherits a recessive gene on the X chromosome, there is no corresponding gene on a second X chromosome to potentially mask the recessive gene, as would be the case in females who have two X chromosomes. Therefore, males are more often affected by X-linked traits.

Question 17

In cats, an X-linked locus is responsible for fur color. There are two known alleles at this locus. One results in black fur color; the other results in orange color. A heterozygote animal has patches of orange and black fur (tortoiseshell). Which of the following outcomes represents all of the possible offspring from the cross provided. Assume all cats have two sex chromosomes.

Accepted Answer

The answer of In cats, an X-linked locus is responsible...

Question 18

In cats, an X-linked locus is responsible for fur color. There are two known alleles at this locus. One results in black fur color; the other results in orange fur color. A heterozygote animal has patches of orange and black fur (tortoiseshell). Which of the following explains the patches of color in female heterozygote cats?

Accepted Answer

Random X inactivation in female mammals leads to the mosaic expression of X-linked genes, explaining the patches of orange and black fur in heterozygous (tortoiseshell) cats.

Question 19

Which of the following statements correctly describes the chromosome theory of inheritance as it was understood in the early 20th century?

Accepted Answer

The chromosome theory of inheritance, as understood in the early 20th century, posits that Mendelian genes have specific locations (loci) on chromosomes, and these chromosomes segregate during meiosis, leading to the distribution of genetic material to offspring. This theory bridged the concepts of Mendelian genetics with the behavior of chromosomes during cell division.

Question 20

Male sex determination in mammals is in large part due to the SRY gene found on the Y chromosome. Which of the following scenarios will result in a person with an XX karyotype developing a male phenotype?

Accepted Answer

Translocation of the SRY gene to an X chromosome during gamete formation can result in an XX individual developing male physical characteristics, as the SRY gene is responsible for initiating male sex determination.

Quiz 15: Linkage and Chromosomes