Q: In a test cross involving two linked genes, the following offspring numbers are observed: 1339 parental type 1, 1195 parental type 2, 151 recombinant type 1, and 154 recombinant type 2. What is the recombination frequency between the two genes? (Use the formula $$ RF = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\% )$$

$$ RF = \frac{151 + 154}{1339 + 1195 + 151 + 154} \times 100\% = \frac{305}{2839} \times 100\% \approx 10.75\% $$

Question 1

Which of the following best describes the difference between leading and lagging strand synthesis during DNA replication?

Accepted Answer

The leading strand is synthesized continuously in the 5' to 3' direction, while the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments.

Question 2

A geneticist observes that two genes on the same chromosome do not assort independently and the offspring ratios deviate from the expected 9:3:3:1. What is the most likely explanation?

Accepted Answer

The genes are linked and located close together on the same chromosome.

Question 3

Which of the following statements about recombination frequency (RF) is correct?

Accepted Answer

Recombination frequency is proportional to the genetic distance between two genes, but not always to the physical distance in base pairs.

Question 4

During meiosis, homologous chromosomes pair and exchange genetic material. What is the main biological significance of this process?

Accepted Answer

It increases genetic diversity in gametes by shuffling alleles between homologous chromosomes.

Question 5

A double-strand break (DSB) in DNA can be repaired by several mechanisms. Which mechanism uses a homologous chromosome as a template and typically does not result in loss or addition of genetic material?

Accepted Answer

Homologous recombination

Question 6

In the double-strand break repair (DSBR) model, what is the role of the 3' overhang generated by exonuclease activity?

Accepted Answer

It invades the homologous duplex DNA to initiate strand exchange and DNA synthesis.

Question 7

Which of the following best explains why recombination frequency between two genes can never exceed 50%?

Accepted Answer

At 50% recombination, genes assort independently as if they are on different chromosomes.

Question 8

A student is mapping three genes (A, B, and C) on a chromosome. The recombination frequencies are: RF(A-B) = 6.4%, RF(B-C) = 13.2%, and RF(A-C) = 18.5%. Which gene is located between the other two?

Accepted Answer

Gene B is between A and C.

Question 9

Which of the following is a key difference between homologous recombination and non-homologous end joining (NHEJ) in double-strand break repair?

Accepted Answer

Homologous recombination uses a homologous template and is generally error-free, while NHEJ directly ligates DNA ends and can result in mutations.

Question 10

In a test cross involving two linked genes, the following offspring numbers are observed: 1339 parental type 1, 1195 parental type 2, 151 recombinant type 1, and 154 recombinant type 2. What is the recombination frequency between the two genes? (Use the formula $$ RF = \frac{	ext{Number of recombinant offspring}}{	ext{Total number of offspring}} 	imes 100\% )$$

Accepted Answer

$$ RF = \frac{151 + 154}{1339 + 1195 + 151 + 154} 	imes 100\% = \frac{305}{2839} 	imes 100\% \approx 10.75\% $$

Question 11

Which of the following scenarios is most likely to result in gene conversion?

Accepted Answer

Mismatch repair of heteroduplex DNA following homologous recombination.

Question 12

During the DSBR mechanism, what is the outcome if both Holliday junctions are resolved by cutting the same strands (either both vertical or both horizontal)?

Accepted Answer

A non-crossover (patch) recombinant is produced.

Question 13

Which of the following best describes the single-strand annealing (SSA) mechanism of double-strand break repair?

Accepted Answer

SSA repairs DSBs between direct repeats by aligning and joining homologous sequences, resulting in loss of the intervening DNA.

Question 14

Why is the 3' overhang, rather than the 5' overhang, used for strand invasion during homologous recombination?

Accepted Answer

DNA polymerases can only extend from a free 3' hydroxyl group.

Question 15

A patient has a genetic disease caused by the loss of DNA between two direct repeats. Which DNA repair mechanism is most likely responsible for this mutation?

Accepted Answer

Single-strand annealing (SSA)

Question 16

Which of the following is NOT a feature of homologous recombination during meiosis?

Accepted Answer

It occurs between sister chromatids only.

Question 17

In a linkage map, why might the sum of the recombination frequencies between two pairs of genes exceed the recombination frequency between the outermost genes? (e.g., $$ RF(B-C) + RF(A-B) \u003e RF(A-C) )$$

Accepted Answer

Double crossovers between the genes can restore the parental configuration, causing the observed recombination frequency between the outermost genes to be less than the sum of the intervals.

Question 18

Which of the following best explains why homologous chromosomes pair during meiosis but not during mitosis?

Accepted Answer

Pairing during meiosis allows for recombination and proper segregation of homologous chromosomes.

DNA Recombination and Chromosome Mapping: Key Concepts in General Biology

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