Q: A testcross between a double heterozygote (A/a; B/b) and a homozygous recessive (a/a; b/b) yields the following progeny: 400 A B, 400 a b, 100 A b, 100 a B. What is the recombination frequency between genes A and B?

$$\frac{100 + 100}{400 + 400 + 100 + 100} \times 100 = 25\%$$

Q: If the recombination frequency between gene X and gene Y is 12%, and between gene Y and gene Z is 8%, what is the expected frequency of double crossovers between X and Z?

$$0.12 \times 0.08 = 0.0096 = 0.96\%$$

Question 1

Which of the following best defines genetic linkage?

Accepted Answer

Genes that are inherited together more often than expected due to their close proximity on the same chromosome.

Question 2

In a dihybrid testcross involving two linked genes, which of the following ratios of progeny would indicate linkage?

Accepted Answer

A ratio where parental types are more frequent than recombinant types.

Question 3

What physical process is responsible for genetic recombination between linked genes during meiosis?

Accepted Answer

Crossing-over at chiasmata between homologous chromosomes.

Question 4

Why can the recombination frequency between any two genes never exceed 50%?

Accepted Answer

Because 50% recombination frequency indicates independent assortment, as seen with unlinked genes.

Question 5

A testcross between a double heterozygote (A/a; B/b) and a homozygous recessive (a/a; b/b) yields the following progeny: 400 A B, 400 a b, 100 A b, 100 a B. What is the recombination frequency between genes A and B?

Accepted Answer

$$\frac{100 + 100}{400 + 400 + 100 + 100} 	imes 100 = 25\%$$

Question 6

Which statement about parental and recombinant gametes is correct when genes are linked?

Accepted Answer

Parental gametes are more frequent than recombinant gametes.

Question 7

In a three-point testcross, the most frequent progeny types represent which configuration?

Accepted Answer

Parental configuration of alleles.

Question 8

If the recombination frequency between gene X and gene Y is 12%, and between gene Y and gene Z is 8%, what is the expected frequency of double crossovers between X and Z?

Accepted Answer

$$0.12 	imes 0.08 = 0.0096 = 0.96\%$$

Question 9

Which of the following is a limitation of two-point crosses in genetic mapping?

Accepted Answer

They are unable to determine gene order if genes are close together.

Question 10

What is the purpose of the chi-square test in linkage analysis?

Accepted Answer

To determine the probability that observed progeny ratios deviate from expected ratios due to chance.

Question 11

A chi-square test for linkage yields a p-value of 0.03. What does this indicate?

Accepted Answer

There is a greater than 95% confidence that the genes are linked.

Question 12

Which of the following best describes a genetic map unit (centiMorgan)?

Accepted Answer

The percentage of recombinant progeny between two genes.

Question 13

In Drosophila, two X-linked genes (w and y) show a recombination frequency of 1.1%. What does this suggest about their physical relationship?

Accepted Answer

They are located close together on the same chromosome.

Question 14

Which of the following is true about mitotic recombination?

Accepted Answer

It can result in genetic mosaics such as twin spots in Drosophila.

Question 15

A double heterozygote for two autosomal genes is testcrossed. The progeny are: 2934 parental type 1, 2768 parental type 2, 871 recombinant type 1, and 846 recombinant type 2. What is the recombination frequency?

Accepted Answer

$$\frac{871 + 846}{2934 + 2768 + 871 + 846} 	imes 100 = 23\%$$

Question 16

Which of the following statements about linkage groups is correct?

Accepted Answer

A linkage group is a set of genes connected by linkage relationships, often synonymous with a chromosome.

Question 17

What is the coefficient of coincidence if the observed double crossover frequency is 0.52% and the expected is 0.79%?

Accepted Answer

$$\frac{0.52}{0.79} = 0.66$$

Question 18

Which of the following is a real-world application of knowing a gene's locus on a chromosome?

Accepted Answer

It allows for the isolation of the gene's DNA for further study.

Question 19

In a three-point cross, why must double crossovers be counted twice when calculating map distances between the outer genes?

Accepted Answer

Because each double crossover represents two separate recombination events.

Gene Linkage, Recombination, and Genetic Mapping

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