Q: In a cross involving two linked genes, the following offspring are observed: 400 parental type 1, 400 parental type 2, 100 recombinant type 1, and 100 recombinant type 2. What is the recombination frequency?

$$ \frac{100 + 100}{400 + 400 + 100 + 100} \times 100\% = 20\% $$

Question 1

Which of the following best explains why two genes located far apart on the same chromosome can appear to assort independently in genetic crosses?

Accepted Answer

Because crossing over between them occurs so frequently that recombinant gametes are produced at a rate close to 50%.

Question 2

A testcross between an individual heterozygous for two linked genes (AB/ab) and a homozygous recessive individual (ab/ab) produces the following offspring: 45% AB, 45% ab, 5% Ab, and 5% aB. What is the map distance between the two genes?

Accepted Answer

$$ (5\% + 5\%) = 10\% $$; therefore, the genes are 10 map units apart.

Question 3

Which statement about complete linkage is correct?

Accepted Answer

Complete linkage results in only parental (nonrecombinant) gametes being produced.

Question 4

In Drosophila, the genes for yellow body (y), white eyes (w), and miniature wings (m) are located on the X chromosome. If the recombination frequency between y and w is 0.5% and between w and m is 34.5%, what can you infer about the relative positions of these genes?

Accepted Answer

y and w are very close together, while w and m are much farther apart.

Question 5

Which of the following best describes a map unit (m u) in genetic mapping?

Accepted Answer

A map unit is defined as a 1% recombination frequency between two genes.

Question 6

If two genes are 20 map units apart, what proportion of gametes produced by a heterozygote will be recombinant for these genes?

Accepted Answer

$$ 20\% $$

Question 7

A geneticist observes that the recombination frequency between two genes is 50%. What is the most likely explanation?

Accepted Answer

The genes are either on different chromosomes or so far apart on the same chromosome that they assort independently.

Question 8

Which of the following scenarios would most likely result in the detection of a single crossover event between two genes?

Accepted Answer

The two genes are moderately far apart on the same chromosome.

Question 9

In a cross involving two linked genes, the following offspring are observed: 400 parental type 1, 400 parental type 2, 100 recombinant type 1, and 100 recombinant type 2. What is the recombination frequency?

Accepted Answer

$$ \frac{100 + 100}{400 + 400 + 100 + 100} 	imes 100\% = 20\% $$

Question 10

Why did Alfred Sturtevant's work on recombination frequencies lead to the development of chromosome maps?

Accepted Answer

Because recombination frequencies are additive and can be used to estimate the relative distances between genes.

Question 11

A researcher is mapping three genes (A, B, and C) on a chromosome. The recombination frequency between A and B is 12%, between B and C is 7%, and between A and C is 19%. What is the most likely order of these genes?

Accepted Answer

A - B - C

Question 12

Which of the following best describes the relationship between chiasmata and genetic recombination?

Accepted Answer

Chiasmata are the physical sites where crossing over and genetic recombination occur between homologous chromosomes.

Question 13

If a single crossover occurs between two nonsister chromatids of a homologous chromosome pair, how many of the resulting four gametes will be recombinant?

Accepted Answer

2

Question 14

In a real-world scenario, why is complete linkage between two genes considered rare?

Accepted Answer

Because it is unlikely for two genes to be so close together that crossing over never occurs between them.

Question 15

A genetic counselor is analyzing a pedigree for a rare genetic disorder. The disorder is linked to a gene that is 2 map units away from a known marker gene. What is the probability that a crossover will occur between the disorder gene and the marker gene during meiosis?

Accepted Answer

$$ 2\% $$

Question 16

Which of the following statements about linkage groups is correct?

Accepted Answer

The number of linkage groups in an organism corresponds to its haploid number of chromosomes.

Question 17

A plant geneticist crosses two heterozygous plants for two genes (AaBb) and observes a 1:1:1:1 ratio in the offspring. What does this suggest about the genes?

Accepted Answer

The genes are either unlinked or so far apart on the same chromosome that they assort independently.

Question 18

Which of the following is a direct consequence of crossing over during meiosis?

Accepted Answer

The production of recombinant gametes with new combinations of alleles.

Question 19

In a testcross, if the observed recombination frequency between two genes is much lower than expected, which of the following is the most likely explanation?

Accepted Answer

The genes are very close together on the chromosome, reducing the likelihood of crossing over.

Linkage and Chromosome Mapping in Eukaryotes: Study Notes

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