Question 1

Given the following testcross data for genes A and B: 143 offspring with genotype A–B, 146 with a–b, 82 with A–b, and 79 with a–B, calculate the recombination frequency between genes A and B. Express your answer as a percentage and in centimorgans (cM).

Accepted Answer

$$\frac{82 + 79}{143 + 146 + 82 + 79} = 0.36 or$$ $$36\% or$$ $$36\ \mathrm{cM}$$

Question 2

What does a recombination frequency of $$36\%$$ between genes A and B indicate about their genetic relationship?

Accepted Answer

The genes are $$36$$ map units apart on the same chromosome.

Question 3

If the recombination frequency between genes B and C is $$7\%$$, what is the genetic distance between these genes in centimorgans?

Accepted Answer

$$7\ \mathrm{cM}$$

Question 4

If the distance between gene A and B is $$36\ \mathrm{cM}$$ and between B and C is $$7\ \mathrm{cM}$$, what is the expected distance between A and C?

Accepted Answer

$$36 + 7 = 43\ \mathrm{cM}$$

Question 5

Why is the maximum measurable recombination frequency between two genes $$50\%$$?

Accepted Answer

Because at $$50\%$$ recombination, genes assort independently and further crossing over does not increase the observed recombination frequency.

Question 6

You perform a testcross and observe the following offspring: 149 B–C, 151 b–c, 10 B–c, and 12 b–C. What is the recombination frequency between genes B and C?

Accepted Answer

$$\frac{10 + 12}{149 + 151 + 10 + 12} = 0.07 or$$ $$7\%$$

Question 7

In a dihybrid testcross, which offspring types are considered recombinants?

Accepted Answer

Those with phenotypes not matching either parental combination.

Question 8

You are studying two genes that influence trunk height and flower color in trees. Tall height is dominant over short, and red color is dominant over white. If you observe 50 tall white and 50 short red offspring out of 500 total, what is the genetic distance between these genes?

Accepted Answer

$$\frac{50 + 50}{500} = 0.2 or$$ $$20\ \mathrm{cM}$$

Question 9

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

Accepted Answer

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

Question 10

You are studying two linked genes that influence hair type and eye color in mice. Straight hair (H) is dominant over curly (h), and brown eyes (E) are dominant over blue (e). You get the following numbers of offspring from a dihybrid testcross: 101 straight hair, brown eyes; 104 curly hair, blue eyes; 8 straight hair, blue eyes; 7 curly hair, brown eyes. Which chromosome arrangement in the dihybrid is correct?

Accepted Answer

One chromosome with H and E, the other with h and e ($$H E / h e)$$

Question 11

If two genes are $$50\ \mathrm{cM}$$ apart, what is the expected ratio of parental to recombinant offspring in a testcross?

Accepted Answer

1:1:1:1 (all combinations equally likely)

Question 12

Why do genes that are far apart on the same chromosome sometimes appear to assort independently?

Accepted Answer

Because multiple crossovers can occur, making the observed recombination frequency approach $$50\%.$$

Question 13

Which of the following is the correct formula for calculating recombination frequency ($$RF) $$from testcross data?

Accepted Answer

$$RF = \frac{	ext{Number of recombinant offspring}}{	ext{Total number of offspring}}$$

Question 14

A researcher observes a recombination frequency of $$0.07$$ between two genes. What is the expected number of recombinant offspring in a testcross with $$1000$$ total offspring?

Accepted Answer

$$0.07 	imes 1000 = 70$$

Question 15

Which step is NOT part of building a linkage map using testcross data?

Accepted Answer

Measuring gene expression levels in the offspring.

Question 16

If you observe equal numbers of all four possible phenotypes in a dihybrid testcross, what can you infer about the genes?

Accepted Answer

The genes are either on different chromosomes or far apart on the same chromosome.

Question 17

A geneticist wants to determine the order of three genes (A, B, and C) on a chromosome. The recombination frequencies are: A–B: $$36\%$$, B–C: $$7\%$$, A–C: $$43\%. $$What is the correct gene order?

Accepted Answer

A – B – C

Question 18

Which of the following best explains why recombination frequencies are additive for closely linked genes?

Accepted Answer

Because the probability of crossing over between two intervals is the sum of the probabilities for each interval.

Gene Mapping with Recombination: Principles and Applications

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