Q: In a three-point test cross involving genes A, B, and C, the recombination frequency between A and B is $$0.10$$ and between B and C is $$0.20. $$Assuming no interference, what is the expected frequency of double crossover gametes?

$$(0.10) \times (0.20) = 0.02 or$$ $$2\%$$

Question 1

Which of the following statements best describes the relationship between recombination frequency and the physical distance between two genes on a chromosome?

Accepted Answer

Recombination frequency underestimates the actual physical distance between two genes, especially as the distance increases.

Question 2

A dihybrid test cross is performed with two genes, A and B, that are 10 map units apart. What is the expected frequency of recombinant gametes produced by the dihybrid parent?

Accepted Answer

$$0.10 or$$ $$10\%$$

Question 3

In a three-point test cross involving genes A, B, and C, the recombination frequency between A and B is $$0.10$$ and between B and C is $$0.20. $$Assuming no interference, what is the expected frequency of double crossover gametes?

Accepted Answer

$$(0.10) 	imes (0.20) = 0.02 or$$ $$2\%$$

Question 4

Which experimental result would most strongly support the hypothesis that two genes are linked?

Accepted Answer

The observed frequency of parental phenotypes is significantly greater than $$50\% in $$the progeny.

Question 5

In Drosophila, males do not undergo crossing over. What is the expected outcome of a test cross involving two linked genes in male Drosophila?

Accepted Answer

Only parental gametes will be produced, and no recombinant gametes will be observed.

Question 6

A geneticist observes the following progeny from a dihybrid test cross: 800 parental types and 200 recombinant types. What is the recombination frequency ($$r) $$between the two genes?

Accepted Answer

$$r = \frac{200}{1000} = 0.20 or$$ $$20\%$$

Question 7

Which of the following best explains why recombination frequencies greater than $$50\%$$ are not observed between two genes?

Accepted Answer

Independent assortment produces a maximum of $$50\%$$ recombinants, so higher frequencies are not possible.

Question 8

In a three-point test cross, the observed number of double crossovers is less than the expected number. What is the term for this phenomenon, and how is it calculated?

Accepted Answer

Interference, $$I = 1 - c$$, where $$c is $$the coefficient of coincidence.

Question 9

Which of the following factors does NOT typically affect recombination frequency in eukaryotes?

Accepted Answer

The color of the organism's eyes.

Question 10

Why do genetic maps based on recombination frequencies sometimes differ from physical maps based on DNA sequence?

Accepted Answer

Recombination hotspots and coldspots cause discrepancies between genetic and physical distances.

Question 11

A researcher performs a chi-square test on the progeny of a dihybrid cross and obtains a p-value less than $$0.005. $$What does this result indicate?

Accepted Answer

There is a significant deviation from independent assortment, suggesting genetic linkage.

Question 12

Which of the following is the correct formula for calculating recombination frequency ($$r) $$between two genes?

Accepted Answer

$$r = \frac{	ext{number of recombinant progeny}}{	ext{total number of progeny}}$$

Question 13

In a three-point test cross, how can the gene order be determined using the double-crossover progeny?

Accepted Answer

The gene that differs between the parental and double-crossover genotypes is in the middle.

Question 14

Which of the following best describes a linkage group?

Accepted Answer

A cluster of syntenic genes that are linked to one another on the same chromosome.

Question 15

If two genes are completely linked (no crossing over), what proportion of gametes produced by a heterozygote will be recombinant?

Accepted Answer

$$0\%$$

Question 16

Which scientist first suggested that the frequency of recombination could be used to create genetic maps?

Accepted Answer

Alfred Sturtevant

Question 17

What is the effect of interference ($$I) $$when its value is positive in a three-point test cross?

Accepted Answer

Fewer double crossovers are observed than expected.

Question 18

Which of the following is a real-world application of genetic linkage mapping?

Accepted Answer

Identifying the chromosomal location of disease-associated genes in humans.

Question 19

Why are recombination frequencies between distant genes less accurate for estimating physical distances?

Accepted Answer

Double crossovers may not be detected, causing underestimation of the true distance.

CHAP 5: Genetic Linkage and Mapping in Eukaryotes

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