Q: In a test cross, the following progeny are observed: 151 parental type 1, 154 parental type 2, 119 recombinant type 1, and 133 recombinant type 2. Calculate the map distance between the two genes in centiMorgans (cM).

$$\text{Map distance} = \frac{119 + 133}{151 + 154 + 119 + 133} \times 100 = \frac{252}{557} \times 100 \approx 45.3\ \text{cM}$$

Question 1

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

Accepted Answer

They are less likely to be separated by crossing over, resulting in genetic linkage.

Question 2

In a dihybrid test cross between two genes, you observe the following progeny numbers: 400 parental type 1, 400 parental type 2, 100 recombinant type 1, and 100 recombinant type 2. What is the recombination frequency ($$r) $$between these two genes?

Accepted Answer

$$r = \frac{100 + 100}{400 + 400 + 100 + 100} = \frac{200}{1000} = 0.2$$

Question 3

If two genes are 10 centiMorgans (cM) apart, what percentage of gametes produced by a heterozygote will be recombinant for these genes?

Accepted Answer

10%

Question 4

Which of the following statements about crossing over is correct?

Accepted Answer

Crossing over occurs between homologous chromosomes during meiosis I.

Question 5

A three-point test cross is performed with genes A, B, and C. The double crossover progeny are the least frequent. What is the primary purpose of identifying these double crossover progeny?

Accepted Answer

To determine the order of the three genes on the chromosome.

Question 6

In a test cross, the following progeny are observed: 151 parental type 1, 154 parental type 2, 119 recombinant type 1, and 133 recombinant type 2. Calculate the map distance between the two genes in centiMorgans (cM).

Accepted Answer

$$	ext{Map distance} = \frac{119 + 133}{151 + 154 + 119 + 133} 	imes 100 = \frac{252}{557} 	imes 100 \approx 45.3\ 	ext{cM}$$

Question 7

Why do linked genes often result in non-Mendelian phenotypic ratios in dihybrid crosses?

Accepted Answer

Because linked genes violate the law of independent assortment.

Question 8

Which of the following best describes a centiMorgan (cM)?

Accepted Answer

A unit of genetic distance corresponding to a 1% recombination frequency.

Question 9

A pedigree shows that a disease allele always co-segregates with a specific genetic marker. What does this suggest about the relationship between the marker and the disease gene?

Accepted Answer

The marker and the disease gene are likely genetically linked.

Question 10

Which of the following is a reason why recombination rates are often underestimated in genetic mapping?

Accepted Answer

Double crossovers can restore the parental combination, making some recombinants undetectable.

Question 11

If two genes are unlinked, what is the expected proportion of recombinant gametes produced by a heterozygote?

Accepted Answer

50%

Question 12

In a three-point test cross, the observed progeny include 2 double crossover types, 20 single crossover type 1, 18 single crossover type 2, and 80 parental types. What is the recombination frequency between the two genes that are farthest apart?

Accepted Answer

$$r = \frac{20 + 18 + 2 + 2}{120} = \frac{42}{120} \approx 0.35$$

Question 13

Which of the following experimental observations provided evidence that recombination is the result of crossing over?

Accepted Answer

Physical exchange of chromosome segments was observed during meiosis.

Question 14

A geneticist observes that two genes are physically close on a chromosome but show a recombination frequency of 0.5. What is the most likely explanation?

Accepted Answer

Multiple crossovers between the genes make them appear unlinked.

Question 15

Which of the following is true regarding crossover interference?

Accepted Answer

The occurrence of one crossover decreases the probability of another crossover nearby.

Question 16

In a test cross, the recombination frequency between two genes is found to be 0.07. How many map units apart are these genes?

Accepted Answer

7 cM

Question 17

Which of the following best describes the relationship between physical and genetic linkage?

Accepted Answer

Genes can be physically linked but genetically unlinked if recombination frequency is 0.5.

Question 18

A researcher is constructing a linkage map using recombination frequencies between three genes: X-Y = 5%, Y-Z = 10%, and X-Z = 15%. What is the most likely gene order?

Accepted Answer

X-Y-Z

Question 19

Which of the following scenarios would most likely result in an overestimation of the genetic distance between two genes?

Accepted Answer

Scoring double crossovers as two single crossovers.

Question 20

In a pedigree analysis, a genetic marker is found to co-segregate with a disease allele in all affected individuals except one. What is the most likely explanation for this exception?

Accepted Answer

A recombination event occurred between the marker and the disease gene.

Ch 5a: Genetic Linkage and Mapping in Eukaryotes: Study Notes

스터디 가이드 - 연습 문제

학습 가이드 - 플래시카드