Q: In a testcross between AaBb and aabb, the following progeny numbers are observed: 40 AaBb, 10 Aabb, 10 aaBb, and 40 aabb. What is the recombination frequency between the A and B genes?

$$\frac{10+10}{40+10+10+40} \times 100\% = 20\%$$

Question 1

Which of the following best describes the principle of genetic linkage?

Accepted Answer

Genes located close together on the same chromosome tend to be inherited together.

Question 2

In a testcross between AaBb and aabb, the following progeny numbers are observed: 40 AaBb, 10 Aabb, 10 aaBb, and 40 aabb. What is the recombination frequency between the A and B genes?

Accepted Answer

$$\frac{10+10}{40+10+10+40} 	imes 100\% = 20\%$$

Question 3

Which of the following statements about crossing over and linkage is correct?

Accepted Answer

Linkage keeps genes together, while crossing over can separate them.

Question 4

A geneticist observes that two genes have a recombination frequency of 8%. How many map units apart are these genes?

Accepted Answer

8 map units

Question 5

If the recombination frequency between gene A and gene B is 5%, and between gene B and gene C is 10%, what is the expected recombination frequency between gene A and gene C, assuming no interference?

Accepted Answer

15%

Question 6

In a three-point testcross, the nonrecombinant progeny are st+ e+ ss+ and st e ss. The double-crossover progeny are st e+ ss+ and st+ e ss. Which gene is in the middle?

Accepted Answer

e

Question 7

A testcross between an individual with genotype AB/ab and an individual with genotype ab/ab produces 90% nonrecombinant and 10% recombinant offspring. What is the distance between the A and B genes?

Accepted Answer

10 centiMorgans

Question 8

Which of the following is true regarding coupling and repulsion configurations?

Accepted Answer

In coupling, dominant alleles are on the same chromosome; in repulsion, they are on different chromosomes.

Question 9

A three-point testcross yields the following recombination frequencies: A-B = 8%, B-C = 12%, and A-C = 20%. What is the most likely gene order?

Accepted Answer

A-B-C

Question 10

If the observed number of double crossovers in a three-point testcross is 2, but the expected number is 5, what is the coefficient of coincidence?

Accepted Answer

$$\frac{2}{5} = 0.4$$

Question 11

Given the coefficient of coincidence is 0.4, what is the interference?

Accepted Answer

$$1 - 0.4 = 0.6$$

Question 12

A genetic map shows the following distances: A—5 m.u.—B—10 m.u.—C. What is the expected recombination frequency between A and C?

Accepted Answer

15%

Question 13

Which of the following best explains why the maximum observable recombination frequency between two genes is 50%?

Accepted Answer

At 50% recombination, genes assort independently and appear unlinked.

Question 14

In a real-world scenario, a plant breeder wants to keep two desirable traits together in the next generation. Which genetic principle should they consider?

Accepted Answer

Select for traits controlled by genes that are closely linked on the same chromosome.

Question 15

A testcross between a heterozygote for two genes (in repulsion) and a homozygous recessive individual produces mostly parental-type offspring. What does this indicate about the genes?

Accepted Answer

The genes are linked and close together on the same chromosome.

Question 16

If a three-point testcross in mice yields the following progeny: 88 a+mg+Rg+, 76 a mg Rg, 36 a+mg Rg, 16 a mg+Rg+, 15 a+mg+Rg, and 9 a mg Rg+, what is the most likely gene order?

Accepted Answer

a - Rg - mg

Question 17

A geneticist calculates the recombination frequency between two genes as $$\frac{53+8+7}{123} 	imes 100\%. $$What does this value represent?

Accepted Answer

The percentage of recombinant progeny, which equals the map distance in centiMorgans.

Question 18

Which of the following is a correct statement about map units (centiMorgans)?

Accepted Answer

One map unit corresponds to a 1% recombination frequency between two genes.

Question 19

A researcher observes that the recombination frequency between genes D and E is 23%. What does this suggest about their physical relationship?

Accepted Answer

Genes D and E are linked but not very close together.

Question 20

In a three-point cross, the expected number of double crossovers is 6, but only 3 are observed. What is the interference?

Accepted Answer

$$1 - \frac{3}{6} = 0.5$$

Linkage and Mapping in Genetics: Principles, Notation, and Genetic Mapping

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