Q: In a fruit fly cross, the following progeny were observed: 126 gray, normal; 24 gray, vestigial; 26 black, normal; 124 black, vestigial. What is the percentage of crossing over between the two genes?

$$\frac{24+26}{300} \times 100 = 16.7\%$$

Q: Given the following data from a testcross: 40 AB, 40 ab, 10 Ab, 10 aB. What is the recombination frequency between genes A and B?

$$\frac{10+10}{100} \times 100 = 20\%$$

Question 1

Which of the following best describes a linkage group?

Accepted Answer

A group of genes located on the same chromosome that tend to be inherited together.

Question 2

If two genes are completely linked, what percentage of recombinant offspring would you expect in a testcross?

Accepted Answer

$$0\%$$

Question 3

A testcross between AaBb and aabb produces offspring in a 1:1:1:1 ratio. What does this indicate about the genes A and B?

Accepted Answer

They assort independently.

Question 4

In a fruit fly cross, the following progeny were observed: 126 gray, normal; 24 gray, vestigial; 26 black, normal; 124 black, vestigial. What is the percentage of crossing over between the two genes?

Accepted Answer

$$\frac{24+26}{300} 	imes 100 = 16.7\%$$

Question 5

Which of the following best explains why the maximum observed recombination frequency between two genes is $$50\%$$?

Accepted Answer

At $$50\%$$ recombination, genes assort independently, making it indistinguishable from genes on different chromosomes.

Question 6

A three-point testcross is performed to determine the order of genes C, S, and E. The two most frequent progeny types are C S E and c s e. What does this indicate?

Accepted Answer

These are the parental (non-crossover) types.

Question 7

If the observed double crossover frequency is lower than expected, what does this suggest about interference?

Accepted Answer

There is positive interference.

Question 8

Given the following data from a testcross: 40 AB, 40 ab, 10 Ab, 10 aB. What is the recombination frequency between genes A and B?

Accepted Answer

$$\frac{10+10}{100} 	imes 100 = 20\%$$

Question 9

Which of the following is a correct definition of a clone in genetics?

Accepted Answer

All individuals derived by nonsexual propagation from a single original parent.

Question 10

In a three-point cross, how do you determine which gene is in the middle?

Accepted Answer

Identify the gene that differs between the double crossover and parental types.

Question 11

A chromosome map shows the following distances: y to w = 0.5 m.u., w to m = 34.5 m.u. What is the expected recombination frequency between y and m?

Accepted Answer

Less than $$35\%$$ due to double crossovers not being detected.

Question 12

Which process is illustrated by the exchange of genetic material between homologous chromosomes during meiosis?

Accepted Answer

Crossing over

Question 13

In a somatic cell hybridization experiment, which chromosome is likely to carry the gene for thymidine kinase if only clones with chromosome 17 show activity?

Accepted Answer

Chromosome 17

Question 14

Which of the following best explains why double crossovers can lead to an underestimation of genetic distance?

Accepted Answer

Double crossovers restore the parental configuration, making some recombinants undetectable.

Question 15

If the coefficient of coincidence (C) is calculated as $$16\%$$, what is the interference (I)?

Accepted Answer

$$I = 100\% - 16\% = 84\%$$

Question 16

Which of the following statements about gene mapping is correct?

Accepted Answer

The farther apart two genes are, the higher the probability of a crossover occurring between them.

Question 17

A testcross between a dihybrid and a double recessive produces only parental types. What does this indicate?

Accepted Answer

Complete linkage between the two genes.

Question 18

Which of the following is NOT a plausible reason for observing fewer recombinants than expected in a genetic cross?

Accepted Answer

Genes are located on different chromosomes.

Question 19

In a three-point testcross, the observed number of double crossovers is 3, and the expected number is 12. What is the coefficient of coincidence?

Accepted Answer

$$C = \frac{3}{12} = 0.25$$

Question 20

Which of the following best describes the effect of interference in genetic mapping?

Accepted Answer

It reduces the probability of a second crossover occurring nearby.

Gene Linkage and Chromosome Mapping in Eukaryotes

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