Q: If the recombination frequency between gene A and B is 10% and between B and C is 20%, what is the expected frequency of double recombinant gametes in a trihybrid cross (assuming no interference)?

$$(0.10) \times (0.20) = 0.02 or 2$$%

Question 1

Which of the following statements best describes the difference between syntenic genes and linked genes?

Accepted Answer

Syntenic genes are located on the same chromosome, but only those close enough to not assort independently are considered linked genes.

Question 2

In a dihybrid test cross involving two linked genes, the observed frequency of parental gametes is 80%. What is the recombination frequency ($$r) $$between these genes?

Accepted Answer

$$r = 0.20$$

Question 3

Which experimental result would most strongly support the hypothesis that two genes are completely linked?

Accepted Answer

All offspring display only parental phenotypes, with no recombinants observed.

Question 4

Morgan’s experiments with Drosophila demonstrated that recombination between X-linked genes occurs during which stage of meiosis?

Accepted Answer

Prophase I

Question 5

A test cross between a dihybrid plant (AaBb) and aabb produces the following progeny: 400 AaBb, 400 aabb, 100 Aabb, 100 aaBb. What is the recombination frequency ($$r) $$between the A and B genes?

Accepted Answer

$$r = \frac{100 + 100}{400 + 400 + 100 + 100} = \frac{200}{1000} = 0.20$$

Question 6

Why does recombination frequency underestimate the actual physical distance between two genes when they are far apart on a chromosome?

Accepted Answer

Double crossovers between the genes may not be detected, making the observed recombination frequency lower than the actual number of crossover events.

Question 7

In a three-point test cross, which class of progeny is expected to be the least frequent?

Accepted Answer

Double-crossover progeny

Question 8

If the recombination frequency between gene A and B is 10% and between B and C is 20%, what is the expected frequency of double recombinant gametes in a trihybrid cross (assuming no interference)?

Accepted Answer

$$(0.10) 	imes (0.20) = 0.02 or 2$$%

Question 9

Which of the following factors does NOT influence recombination frequency?

Accepted Answer

The number of chromosomes in the organism

Question 10

What is the main reason why mapping human genes is more challenging than mapping genes in Drosophila or maize?

Accepted Answer

Controlled matings and large numbers of offspring are not possible in humans.

Question 11

A geneticist observes that the number of double crossovers in a three-point test cross is much lower than expected. What is the most likely explanation?

Accepted Answer

Interference is reducing the number of double crossovers.

Question 12

Which of the following best describes a linkage group?

Accepted Answer

A cluster of syntenic genes that are genetically linked to one another.

Question 13

In a cross between $$AABB$$ and $$aabb$$ plants, the $$F_1 is $$test-crossed with $$aabb. If $$the genes are unlinked, what proportion of the progeny will be recombinant?

Accepted Answer

50%

Question 14

Which of the following is a real-world application of genetic linkage mapping?

Accepted Answer

Identifying the chromosomal location of disease-associated genes in humans using DNA markers.

Question 15

If a recombination frequency between two genes is 0.01, what is the map distance between them in centiMorgans (cM)?

Accepted Answer

1 cM

Question 16

Which of the following best explains why recombination hotspots can cause discrepancies between genetic and physical maps?

Accepted Answer

Hotspots increase recombination rates in certain regions, making genes appear farther apart on genetic maps than on physical maps.

Question 17

A chi-square test is performed on the progeny of a dihybrid cross and yields a p-value less than 0.005. What does this indicate about the genes under study?

Accepted Answer

The genes are likely linked, as the observed ratios deviate significantly from independent assortment.

Question 18

Which scientist first used recombination frequencies to construct a genetic map?

Accepted Answer

Alfred Sturtevant

Question 19

In a three-point test cross, how can the gene order be determined using double-crossover progeny?

Accepted Answer

The gene that differs between the parental and double-crossover genotypes is in the middle.

Genetic Linkage and Mapping in Eukaryotes – Study Notes

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