Q: A three-point test cross is performed and the following recombinant classes are observed: parental (400), single crossover between A and B (100), single crossover between B and C (80), double crossover (20). What is the recombination frequency between A and B?

$$\frac{100 + 20}{600} \times 100 = 20\%$$

Question 1

Which of the following best describes a recombination map?

Accepted Answer

A diagram that shows the relative positions of genes on a chromosome based on their recombination frequencies.

Question 2

Thomas Hunt Morgan and Alfred Sturtevant's work with Drosophila melanogaster led to which of the following conclusions?

Accepted Answer

Genes are arranged in a linear sequence on chromosomes, and the frequency of recombination can be used to estimate distances between them.

Question 3

During meiosis, what is the relationship between the frequency of crossing over and the distance between two loci on a chromosome?

Accepted Answer

The frequency of crossing over is directly proportional to the distance between two loci.

Question 4

Which of the following is true regarding independent assortment?

Accepted Answer

It results in equal percentages of all possible gamete combinations when two loci are on different chromosomes.

Question 5

If two genes are located very close together on the same chromosome and no crossing over occurs between them, what is the expected outcome?

Accepted Answer

Only parental (noncrossover) gametes will be produced.

Question 6

A single crossover event between two nonsister chromatids during meiosis results in which of the following?

Accepted Answer

50% of the gametes will be recombinant and 50% will be parental.

Question 7

What is a map unit (mu) or centiMorgan (cM) defined as?

Accepted Answer

1 percent recombination frequency between two genes on a chromosome.

Question 8

In a two-point test cross, which of the following is NOT a required criterion for accurate gene mapping?

Accepted Answer

The F2 generation must be homozygous for both genes under consideration.

Question 9

If 12% of the gametes produced in a test cross are recombinant for two genes, what is the map distance between these genes?

Accepted Answer

$$12 \ 	ext{mu}$$

Question 10

Which of the following statements about double crossovers is correct?

Accepted Answer

Double crossovers can be used to determine the order of three linked genes.

Question 11

A three-point test cross is performed and the following recombinant classes are observed: parental (400), single crossover between A and B (100), single crossover between B and C (80), double crossover (20). What is the recombination frequency between A and B?

Accepted Answer

$$\frac{100 + 20}{600} 	imes 100 = 20\%$$

Question 12

Why might map distances calculated from recombination frequencies be inaccurate for genes that are far apart on a chromosome?

Accepted Answer

Because multiple crossovers between the genes can go undetected, leading to an underestimation of the true distance.

Question 13

Which of the following best explains the concept of crossover interference?

Accepted Answer

The occurrence of one crossover decreases the probability of another crossover nearby.

Question 14

The coefficient of coincidence ($$C) is $$calculated as:

Accepted Answer

$$C = \frac{	ext{observed double crossovers}}{	ext{expected double crossovers}}$$

Question 15

If the coefficient of coincidence ($$C) is 0.6$$, what is the value of interference ($$I$$)?

Accepted Answer

$$I = 1 - C = 0.4$$

Question 16

Which of the following is a real-world application of gene mapping in humans?

Accepted Answer

Using DNA markers such as SNPs to create recombination maps for disease gene localization.

Question 17

A LOD score of $$Z = 3.0 in a $$human pedigree study indicates:

Accepted Answer

There is strong evidence for genetic linkage between two loci.

Question 18

Which of the following statements about chiasmata is correct?

Accepted Answer

Chiasmata are X-shaped intersections where genetic material is exchanged between nonsister chromatids.

Question 19

In a three-point mapping experiment, how is the gene order determined?

Accepted Answer

By comparing the parental class (largest number) and the double crossover class (smallest number) to identify the gene in the middle.

Gene Mapping: Principles and Applications in Genetics

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