Question 1

Which of the following best explains why a person with 30 CAG repeats in the Huntington gene is not at risk for Huntington’s disease, but their children may be at elevated risk?

Accepted Answer

Because the repeats may increase from one generation to the next.

Question 2

According to the table, what is the risk analysis for an individual with 27–35 CAG repeats in the Huntington gene?

Accepted Answer

Intermediate alleles. No risk. May pose risk for next generation.

Question 3

Which of the following is required for successful positional cloning of a disease gene?

Accepted Answer

Symptoms

Question 4

What is the significance of chiasmata observed during Prophase I of meiosis?

Accepted Answer

They signify the occurrence of crossing over between homologous chromosomes.

Question 5

If two genes are completely linked, what phenotypic ratio would you expect in the F2 generation of a test cross?

Accepted Answer

$$1:1$$

Question 6

Which of the following statements about linked genes is correct?

Accepted Answer

Linked genes exist on the same chromosome and do not assort independently.

Question 7

A Drosophila cross involves two recessive mutations: heavy wing vein (hv) and brown eye (bw). If these genes are completely linked, what F2 phenotypic ratio is expected?

Accepted Answer

$$1:2:1$$

Question 8

Which equation best represents the relationship between recombination frequency ($$r) $$and map distance ($$d) in $$centimorgans (cM)?

Accepted Answer

$$d = r 	imes 100$$

Question 9

Why do mapping estimates become less accurate as the distance between two genes increases?

Accepted Answer

Because more double crossovers occur, which are not detected and thus underestimated.

Question 10

Which of the following best describes the outcome of a test cross between an F1 individual (heterozygous for two completely linked genes) and a homozygous recessive individual?

Accepted Answer

Two phenotypes in a 1:1 ratio

Question 11

Which of the following is a correct representation of a wild-type allele in Drosophila genetic notation?

Accepted Answer

$$e^+$$

Question 12

If a person has 41 CAG repeats in the Huntington gene, what is their risk according to the table?

Accepted Answer

Will get HD with 50% chance of transmission to next generation.

Question 13

Which of the following best explains why chromosome maps are useful in genetics?

Accepted Answer

They indicate the relative location of genes on a chromosome based on recombination frequencies.

Question 14

In a cross involving two genes on different chromosomes, what phenotypic ratio is expected in the F2 generation?

Accepted Answer

$$9:3:3:1$$

Question 15

Which of the following is true about crossing over?

Accepted Answer

It occurs between non-sister chromatids during Prophase I.

Question 16

A geneticist observes a 1:1:1:1 ratio in the offspring of a test cross. What does this suggest about the genes involved?

Accepted Answer

The genes are on different chromosomes or far apart on the same chromosome.

Question 17

Which of the following best describes the phenomenon of genetic anticipation in Huntington’s disease?

Accepted Answer

The disease appears earlier and/or more severely in successive generations due to expansion of CAG repeats.

Question 18

If two genes are 10 cM apart, what is the expected recombination frequency?

Accepted Answer

$$0.1$$

Question 19

Which of the following is NOT a requirement for independent assortment during meiosis?

Accepted Answer

Genes must be completely linked.

Question 20

Which of the following best explains why a test cross is used in gene mapping experiments?

Accepted Answer

It allows the detection of recombinant and parental gametes by crossing with a homozygous recessive individual.

Chromosome Mapping and Linkage in Eukaryotes: Study Guide

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