Question 1

Which of Mendel’s postulates is violated by the phenomenon of genetic linkage?

Accepted Answer

Homologs independently assort in meiosis.

Question 2

If two genes are located far apart on the same chromosome, what is the expected recombination frequency between them?

Accepted Answer

50%

Question 3

A crossover event occurs between two nonsister chromatids during meiosis. What is the result?

Accepted Answer

Two parental and two recombinant gametes are produced.

Question 4

What does it mean if two genes are 6 map units apart?

Accepted Answer

Recombination occurs 6 percent of the time between these two genes.

Question 5

Three genes on the Drosophila X chromosome have the following recombination rates: yellow-white 0.5%, white-miniature 34.5%, yellow-miniature 35.4%. Which two genes are physically closest together?

Accepted Answer

Yellow and white

Question 6

If genes A, B, and C occur in that order in fruit flies, and A is 10 map units from B, and B is 20 map units from C, how far is A from C?

Accepted Answer

30 map units

Question 7

Which statement best describes the relationship between the frequency of crossing over and the distance between two loci on a chromosome?

Accepted Answer

The frequency of crossing over is proportional to the distance between loci.

Question 8

In a testcross involving two genes that are completely linked, what phenotypic ratio is expected in the progeny?

Accepted Answer

1:1

Question 9

Alfred Sturtevant’s chromosome mapping relied on which principle?

Accepted Answer

Recombination frequencies between linked genes are additive.

Question 10

A single crossover occurs between two genes that are 50 map units apart. What is the expected proportion of recombinant gametes?

Accepted Answer

$$\frac{1}{2}$$

Question 11

If two genes are so close together that no crossing over occurs, what is the expected outcome in a dihybrid cross?

Accepted Answer

Only parental types will be observed.

Question 12

Which of the following best explains why the number of linkage groups corresponds to the haploid number of chromosomes?

Accepted Answer

Genes located on the same chromosome show linkage to one another.

Question 13

In a cross involving two X-linked genes in Drosophila, why might the observed ratio of recombinant phenotypes differ between two pairs of genes?

Accepted Answer

The distance between the genes affects the likelihood of crossing over.

Question 14

What is the definition of a map unit (m u) in genetic mapping?

Accepted Answer

1 map unit equals 1% recombination between two genes.

Question 15

Which scenario would result in a gamete with a recombinant genotype?

Accepted Answer

A crossover occurs between two nonsister chromatids.

Question 16

If the recombination frequency between two genes is 35%, what can be inferred about their physical distance?

Accepted Answer

They are far apart on the same chromosome.

Question 17

Which of the following equations correctly represents the additive property of recombination frequencies for three genes A, B, and C, where A-B = 10 m u and B-C = 20 m u?

Accepted Answer

$$A-C = A-B + B-C = 10 + 20 = 30\ 	ext{m u}$$

Question 18

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

Accepted Answer

It is improbable for two randomly selected genes to be so close as to demonstrate complete linkage.

Question 19

Which of the following best describes the chromosomal theory of inheritance as established by genetic mapping studies?

Accepted Answer

Chromosomes contain genes in linear order, equivalent to Mendel’s unit factors.

Question 20

A single crossover event between two genes results in which of the following gamete types?

Accepted Answer

Two noncrossover (parental) and two crossover (recombinant) gametes.

Ch 7P1 Linkage and Chromosome Mapping in Eukaryotes: Study Notes

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