Q: In a genetic cross, 48 offspring have black fur and red eyes, 16 have black fur and black eyes, 20 have brown fur and red eyes, and 16 have brown fur and black eyes. What is the recombination frequency between the two genes?

$$\frac{20 + 16}{100} \times 100 = 36\%$$

Q: If two genes are 16 map units apart, how many parental offspring would you expect in a mapping cross that produces 500 total offspring?

$$500 \times (1 - 0.16) = 420$$ parental offspring

Q: A dihybrid test cross in peas yields the following F2 phenotypes: 183 yellow round, 20 yellow wrinkled, 5 green round, 192 green wrinkled (total 400). What is the recombination frequency between the two genes?

$$\frac{20 + 5}{400} \times 100 = 6.25\%$$

Q: A researcher observes 537 and 542 parental offspring, and 76 and 75 recombinant offspring in a Drosophila cross. What is the map distance between the two genes?

$$\frac{76 + 75}{537 + 542 + 76 + 75} \times 100 = 12.3$$ map units

Question 1

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

Accepted Answer

The frequency of recombination between two genes, where 1 mu equals 1% recombination.

Question 2

In a genetic cross, 48 offspring have black fur and red eyes, 16 have black fur and black eyes, 20 have brown fur and red eyes, and 16 have brown fur and black eyes. What is the recombination frequency between the two genes?

Accepted Answer

$$\frac{20 + 16}{100} 	imes 100 = 36\%$$

Question 3

If two genes are 16 map units apart, how many parental offspring would you expect in a mapping cross that produces 500 total offspring?

Accepted Answer

$$500 	imes (1 - 0.16) = 420$$ parental offspring

Question 4

A dihybrid test cross in peas yields the following F2 phenotypes: 183 yellow round, 20 yellow wrinkled, 5 green round, 192 green wrinkled (total 400). What is the recombination frequency between the two genes?

Accepted Answer

$$\frac{20 + 5}{400} 	imes 100 = 6.25\%$$

Question 5

Which of the following statements about double crossovers is correct?

Accepted Answer

Double crossovers can go undetected if the arrangement of alleles is restored to the parental type.

Question 6

Given three genes A, G, and D on a chromosome with recombination frequencies: A-G = 11 cM, G-D = 4 cM, and A-D = 15 cM, what is the correct gene order?

Accepted Answer

A-G-D

Question 7

Why does the maximum observable recombination frequency between two genes never exceed 50%?

Accepted Answer

Because at 50% recombination, genes assort independently as if on different chromosomes.

Question 8

A plant heterozygous for two genes (AaBb) is testcrossed with aabb. The following offspring are observed: AaBb, aabb, Aabb, aaBb. Which genotypes represent recombinant offspring?

Accepted Answer

Aabb and aaBb

Question 9

In a mapping experiment, why are gene distances more accurately estimated when genes are close together?

Accepted Answer

Because double crossovers are less likely and less likely to be missed.

Question 10

A researcher observes 537 and 542 parental offspring, and 76 and 75 recombinant offspring in a Drosophila cross. What is the map distance between the two genes?

Accepted Answer

$$\frac{76 + 75}{537 + 542 + 76 + 75} 	imes 100 = 12.3$$ map units

Question 11

Which of the following is a direct consequence of undetected double crossovers in genetic mapping?

Accepted Answer

The map distance between genes is underestimated.

Question 12

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

Accepted Answer

Double crossover recombinants

Question 13

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

Accepted Answer

50%

Question 14

A geneticist is mapping two genes and finds a recombination frequency of 1%. What is the distance between the genes in map units?

Accepted Answer

1 map unit

Question 15

Which of the following best explains why mapping estimates become less accurate as the distance between two genes increases?

Accepted Answer

Multiple crossovers between the genes may not be detected, leading to underestimation of the true distance.

Question 16

In a real-world scenario, why is it important to consider double crossovers when constructing genetic maps for disease genes in humans?

Accepted Answer

Double crossovers can lead to underestimation of distances, potentially misplacing disease genes on the map.

Question 17

A dihybrid plant with genotype YyRr is testcrossed with yyrr. If the genes are completely linked, what phenotypic ratio is expected among the offspring?

Accepted Answer

1:1

Question 18

Which of the following is a key difference between parental and recombinant gametes in a single crossover event?

Accepted Answer

Parental gametes retain the original combination of alleles, while recombinant gametes have new combinations due to crossing over.

Question 19

In a mapping experiment, a plant heterozygous for genes G and H (GgHh) is testcrossed with gghh. If the map distance between G and H is 8 cM, what percentage of the offspring are expected to be recombinants?

Accepted Answer

8%

Question 20

Which of the following best describes the relationship between physical distance and recombination frequency?

Accepted Answer

Recombination frequency increases with physical distance, but only up to a maximum of 50%.

Lecture slide Chapter 7 part2

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