Q: Which formula is used to calculate the recombination rate ($$r) $$between two genes?

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

Question 1

In a three-point cross involving genes a, b, and c, which class of progeny is expected to be the least frequent?

Accepted Answer

Double crossover between a, b, and c

Question 2

Given the following observed progeny numbers from a three-point cross: 60, 48, 4, 270, 235, 7, 40, 62, which numbers correspond to the double recombinant class?

Accepted Answer

4 and 7

Question 3

If the recombination rate between genes V and Gl is calculated as $$r = \frac{133}{726} = 0.183$$, what does this value represent?

Accepted Answer

The genetic distance between V and Gl in centimorgans

Question 4

Which equation correctly calculates the coefficient of coincidence ($$c) $$given observed double crossovers ($$O) $$and expected double crossovers ($$E$$)?

Accepted Answer

$$c = \frac{O}{E}$$

Question 5

If the coefficient of coincidence is $$c = 0.60$$, what is the value of crossover interference ($$I$$)?

Accepted Answer

$$I = 1 - c = 0.40$$

Question 6

Why do double crossovers occur less frequently than single crossovers in three-point crosses?

Accepted Answer

Because double crossovers require two independent crossover events between three loci

Question 7

Which of the following factors can affect recombination rate in eukaryotes?

Accepted Answer

Age, temperature, sex, and diet

Question 8

In a three-point cross, how can you determine the gene order on a chromosome?

Accepted Answer

By identifying the double recombinant class and comparing it to the parental class

Question 9

If the expected number of double crossovers is $$E = (0.183)(0.136) 	imes 726 = 18.0$$ and the observed number is $$O = 11$$, what is the coefficient of coincidence?

Accepted Answer

$$c = \frac{11}{18.0} = 0.61$$

Question 10

Why does the maximum measurable map distance between two genes not exceed 0.5 (50 cM)?

Accepted Answer

Because at 50% recombination, genes assort independently and cannot be mapped further apart

Question 11

Which of the following best explains why observed recombination frequencies often underestimate the actual genetic distance between genes?

Accepted Answer

Multiple crossovers can restore the parental genotype, making some recombinants undetectable

Question 12

In a three-point cross, which progeny class is used to determine the gene in the middle?

Accepted Answer

Double recombinant class

Question 13

If you observe 60 and 62 progeny for two single crossover classes, and 4 and 7 for double crossover classes, what is the total number of recombinant progeny?

Accepted Answer

60 + 62 + 4 + 7 = 133

Question 14

Which formula is used to calculate the recombination rate ($$r) $$between two genes?

Accepted Answer

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

Question 15

Which historical figure is associated with the development of genetic mapping using three-point crosses in maize?

Accepted Answer

Rollins Emerson

Question 16

Intragenic recombination refers to:

Accepted Answer

Crossing over within a single gene

Question 17

If the observed number of double recombinants is less than the expected number, what does this indicate about crossover interference?

Accepted Answer

There is positive interference, reducing the likelihood of double crossovers

Question 18

Which of the following is a reason to use three-point crosses instead of two-point crosses in genetic mapping?

Accepted Answer

Three-point crosses allow detection of double crossovers and more accurate gene order determination

Question 19

If the recombination rate between Gl and Vc is $$r = \frac{99}{726} = 0.136$$, what is the genetic distance in centimorgans?

Accepted Answer

13.6 cM

Question 20

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

Accepted Answer

Recombination frequency is proportional to genetic map distance for small distances, but underestimates distance for larger values due to undetected multiple crossovers

Ch 5b: Genetic Linkage and Mapping in Eukaryotes: Three-Point Crosses

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