Q: In a testcross involving two linked genes, the following offspring were observed: 450 AB, 440 ab, 55 Ab, 55 aB. What is the recombination frequency between these genes?

$$\frac{55 + 55}{1000} \times 100 = 11\%$$

Question 1

Which of the following best explains why males are more likely than females to express X-linked recessive disorders such as color blindness?

Accepted Answer

Males are hemizygous for X-linked genes, so a single recessive allele on the X chromosome will result in the disorder.

Question 2

A woman who is heterozygous for the X-linked gene for color blindness (XᴺXⁿ) and a man with normal vision (XᴺY) have a son. What is the probability that their son will be color-blind?

Accepted Answer

$$\frac{1}{2}$$

Question 3

In Morgan’s classic experiment with Drosophila, why were all the white-eyed flies in the F₂ generation male?

Accepted Answer

The gene for eye color is located on the X chromosome, and males are hemizygous for X-linked genes.

Question 4

Which of the following best describes dosage compensation in mammals?

Accepted Answer

One X chromosome in each female cell is randomly inactivated, forming a Barr body.

Question 5

A female cat is heterozygous for an X-linked gene controlling fur color (one allele for black, one for orange). Why does she have patches of both colors?

Accepted Answer

Random X inactivation leads to mosaic expression of each allele.

Question 6

If two genes are located close together on the same chromosome, which of the following is most likely true?

Accepted Answer

They are likely to be inherited together due to gene linkage.

Question 7

In a testcross involving two linked genes, the following offspring were observed: 450 AB, 440 ab, 55 Ab, 55 aB. What is the recombination frequency between these genes?

Accepted Answer

$$\frac{55 + 55}{1000} 	imes 100 = 11\%$$

Question 8

Which chromosomal alteration is correctly matched with its description?

Accepted Answer

Duplication: a segment of a chromosome is repeated.

Question 9

A child is born with three copies of chromosome 21. Which term best describes this condition?

Accepted Answer

Trisomy

Question 10

A human cell with 69 chromosomes is best described as:

Accepted Answer

Triploid ($$3n)$$

Question 11

If nondisjunction occurs during meiosis II in one of the two cells, what fraction of the resulting gametes will be abnormal?

Accepted Answer

Half

Question 12

A testcross between a dihybrid female (b⁺ vg⁺ / b vg) and a homozygous recessive male (b vg / b vg) in Drosophila produces mostly parental phenotypes and some recombinants. What does this indicate about the genes?

Accepted Answer

The genes are linked but crossing over occurs between them.

Question 13

Which of the following karyotypes represents Klinefelter syndrome?

Accepted Answer

47, XXY

Question 14

A chromosome segment is reversed end to end but no genetic material is lost or gained. What type of chromosomal alteration is this?

Accepted Answer

Inversion

Question 15

If a fruit fly has a diploid number of 8 (2n = 8), how many chromosomes would be present in a triploid cell?

Accepted Answer

12

Question 16

Which of the following best explains why genes that are far apart on the same chromosome can appear to assort independently?

Accepted Answer

Crossing over occurs so frequently between them that recombination frequency approaches 50%.

Question 17

A person with Turner syndrome has which of the following karyotypes?

Accepted Answer

45, X

Question 18

Which chromosomal alteration is most likely to result in a genetic disorder due to the loss of essential genes?

Accepted Answer

Deletion

Question 19

If a plant has 2n = 14 chromosomes, how many chromosomes would be present in a tetraploid cell?

Accepted Answer

28

Question 20

Which of the following is true about Y-linked inheritance?

Accepted Answer

Y-linked genes are passed from father to all sons.

CH 15 Chromosomal Basis of Inheritance: Sex Linkage, Gene Linkage, Genetic Maps, and Chromosome Alterations

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