Q: A geneticist observes the following offspring numbers in a testcross: 800 parental type 1, 750 parental type 2, 50 recombinant type 1, and 40 recombinant type 2. Calculate the map distance between the two genes using the formula $$\text{Map distance} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100.$$

$$\frac{90}{1640} \times 100 = 5.5 m.u.$$

Question 1

Which of the following best explains why genes that are close together on the same chromosome tend to be inherited together?

Accepted Answer

They are physically linked and less likely to be separated by crossing over.

Question 2

In Bateson and Punnett's sweet pea experiment, the observed F2 phenotypic ratios deviated from the expected 9:3:3:1 ratio. What was the main reason for this deviation?

Accepted Answer

The two genes were linked on the same chromosome, resulting in coupling.

Question 3

Which of the following statements about crossing over is CORRECT?

Accepted Answer

Crossing over occurs between non-sister chromatids of homologous chromosomes during prophase I of meiosis.

Question 4

Thomas Hunt Morgan's experiments with Drosophila demonstrated that the genes for body color and wing shape are linked. Which result from his testcross best supports this conclusion?

Accepted Answer

The parental (nonrecombinant) phenotypes were much more frequent than the recombinant phenotypes.

Question 5

If two genes assort independently, what is the expected phenotypic ratio in the offspring of a testcross involving a dihybrid individual?

Accepted Answer

1:1:1:1

Question 6

A geneticist observes the following offspring numbers in a testcross: 800 parental type 1, 750 parental type 2, 50 recombinant type 1, and 40 recombinant type 2. Calculate the map distance between the two genes using the formula $$	ext{Map distance} = \frac{	ext{Number of recombinant offspring}}{	ext{Total number of offspring}} 	imes 100.$$

Accepted Answer

$$\frac{90}{1640} 	imes 100 = 5.5 m.u.$$

Question 7

Which of the following is the best definition of a 'linkage group'?

Accepted Answer

A group of genes that are physically linked together on the same chromosome.

Question 8

In a three-gene cross involving genes y, w, and m on the X chromosome of Drosophila, which pair of genes is closest together if the recombinant offspring for y-w is much less frequent than for w-m?

Accepted Answer

y and w

Question 9

A chi-square test is performed to determine if two genes assort independently. The observed chi-square value is 2,109.8 with 1 degree of freedom. What is the correct interpretation?

Accepted Answer

The deviation is too large to be due to chance, so the genes are likely linked.

Question 10

Which experiment provided direct cytological evidence that genetic recombination is due to physical exchange of chromosome segments?

Accepted Answer

Stern's experiment with Bar and carnation eye genes in Drosophila

Question 11

Which of the following is NOT a use of genetic maps?

Accepted Answer

Determining the exact DNA sequence of a gene

Question 12

If the recombinant frequency (RF) between two genes is 0.01, what is the map distance between them?

Accepted Answer

1 centimorgan (cM)

Question 13

Which of the following best describes the relationship between genetic linkage and Mendel's law of independent assortment?

Accepted Answer

Genetic linkage is an exception to the law of independent assortment for genes located close together on the same chromosome.

Question 14

A double crossover between three linked genes is described as 'very uncommon.' Why are double crossovers rare?

Accepted Answer

They require two independent crossover events to occur simultaneously in a small chromosomal region.

Question 15

Which scientist first proposed that the percentage of recombinants could be used to measure the distance between genes, leading to the construction of the first genetic map?

Accepted Answer

Alfred Sturtevant

Question 16

A dihybrid testcross yields the following offspring: 400 parental type 1, 400 parental type 2, 100 recombinant type 1, and 100 recombinant type 2. What is the recombination frequency (RF) and map distance between the two genes?

Accepted Answer

$$	ext{RF} = \frac{200}{1000} = 0.2$$, map distance = 20 cM

Question 17

Which of the following best describes 'synteny'?

Accepted Answer

Two or more genes located on the same chromosome

Question 18

Why might two genes on the same chromosome still assort independently?

Accepted Answer

They are far apart, so crossing over occurs frequently between them.

Question 19

Which of the following is the correct formula for calculating the chi-square value in a genetic linkage test?

Accepted Answer

$$\$$chi^2$$ = \sum \frac{(O - E)^2$}{E}$$

Question 20

In a genetic mapping experiment, why is it important to use a large number of offspring when calculating recombination frequencies?

Accepted Answer

To reduce the impact of random chance and obtain more accurate estimates of map distances.

Genetic Linkage and Mapping in Eukaryotes: Study Notes

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