Q: A three-point testcross yields the following recombinant frequencies: single crossover between gene A and B is 8%, between B and C is 12%, and double crossover is 2%. What is the map distance between A and C?

$$(0.08 + 0.12 + 2 \times 0.02) \times 100 = 24\ \text{cM}$$

Q: In a testcross, the following numbers of offspring are observed: 179 (parental), 173 (parental), 52 (single crossover region I), 46 (single crossover region I), 22 (single crossover region II), 22 (single crossover region II), 4 (double crossover), 2 (double crossover). What is the recombination frequency between the two outermost genes?

$$\frac{52+46+22+22+2(4+2)}{500} \times 100 = 30.8\%$$

Question 1

Which of the following statements best explains why linked genes do not assort independently during meiosis?

Accepted Answer

Linked genes are located close together on the same chromosome, so they tend to be inherited together unless crossing over occurs between them.

Question 2

In a dihybrid testcross involving two genes, you observe the following offspring ratios: 45% AB, 45% ab, 5% Ab, and 5% aB. What does this suggest about the relationship between the two genes?

Accepted Answer

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

Question 3

If the recombination frequency between two genes is 12%, what is the map distance between them?

Accepted Answer

$$12\ 	ext{cM}$$

Question 4

A three-point testcross yields the following recombinant frequencies: single crossover between gene A and B is 8%, between B and C is 12%, and double crossover is 2%. What is the map distance between A and C?

Accepted Answer

$$(0.08 + 0.12 + 2 	imes 0.02) 	imes 100 = 24\ 	ext{cM}$$

Question 5

Which experimental result would most strongly support the hypothesis that crossing over is responsible for genetic recombination?

Accepted Answer

Cytological evidence of physical exchange between homologous chromosomes correlates with recombinant phenotypes.

Question 6

In a testcross, the following numbers of offspring are observed: 179 (parental), 173 (parental), 52 (single crossover region I), 46 (single crossover region I), 22 (single crossover region II), 22 (single crossover region II), 4 (double crossover), 2 (double crossover). What is the recombination frequency between the two outermost genes?

Accepted Answer

$$\frac{52+46+22+22+2(4+2)}{500} 	imes 100 = 30.8\%$$

Question 7

Why are double crossover events less frequent than single crossover events in three-point testcrosses?

Accepted Answer

Double crossovers require two separate crossover events to occur between three loci, which is statistically less likely.

Question 8

If the expected number of double crossovers is 7.5 but only 3 are observed, what is the coefficient of coincidence (C) and the interference (I)?

Accepted Answer

$$C = \frac{3}{7.5} = 0.4$$, $$I = 1 - 0.4 = 0.6$$

Question 9

Which of the following best describes the effect of positive interference in genetic mapping?

Accepted Answer

The occurrence of one crossover reduces the probability of another crossover occurring nearby.

Question 10

A geneticist observes that the recombination frequency between two genes is 50%. What does this indicate about the genes?

Accepted Answer

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

Question 11

In a three-point testcross, the double crossover progeny help determine:

Accepted Answer

The gene order on the chromosome.

Question 12

Which formula correctly calculates the map distance between two genes using recombination data?

Accepted Answer

$$	ext{Map distance (cM)} = \frac{	ext{Number of recombinant offspring}}{	ext{Total number of offspring}} 	imes 100$$

Question 13

Which of the following is a correct interpretation of a cis (coupling) configuration in a dihybrid organism?

Accepted Answer

Both dominant alleles are on one chromosome and both recessive alleles are on the homologous chromosome.

Question 14

Why are genetic map distances generally considered additive?

Accepted Answer

Because the probability of crossovers between adjacent genes can be summed to estimate the total distance.

Question 15

A testcross between an individual heterozygous for two genes (AB/ab) and a homozygous recessive (ab/ab) produces offspring in a 1:1:1:1 ratio. What does this indicate?

Accepted Answer

The two genes assort independently.

Question 16

In a three-point cross, the following offspring numbers are observed: 30 (gray body, purple eyes, vestigial wings), 28 (black body, red eyes, normal wings), 61 (gray body, red eyes, vestigial wings), 60 (black body, purple eyes, normal wings), 2 (gray body, purple eyes, normal wings), 1 (black body, red eyes, vestigial wings). What is the frequency of double crossovers?

Accepted Answer

$$\frac{1+2}{1005} = 0.003$$

Question 17

Which of the following best describes the product rule as applied to genetic mapping?

Accepted Answer

The probability of a double crossover is the product of the probabilities of each single crossover event.

Linkage and Chromosome Mapping in Eukaryotes

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