Q: Which of the following equations correctly represents the calculation of recombination frequency (RF)?

$$\text{RF} = \frac{\text{number of recombinant offspring}}{\text{total number of offspring}}$$

Question 1

Which of the following best describes the phenomenon of genetic linkage as discovered by Morgan in Drosophila?

Accepted Answer

Genes located close together on the same chromosome tend to be inherited together because crossing over between them is less likely.

Question 2

During homologous recombination, which enzyme is responsible for creating the initial double-strand break in DNA?

Accepted Answer

Endonuclease

Question 3

If two genes are 10 centimorgans (cM) apart, what is the expected recombination frequency between them?

Accepted Answer

$$0.1 or$$ $$10\%$$

Question 4

Which of the following statements about the resolution of Holliday junctions is correct?

Accepted Answer

Resolution can result in either crossover or non-crossover recombinants depending on the orientation of cleavage.

Question 5

A testcross is performed between a double heterozygote (AaBb) and a double homozygous recessive (aabb). If the genes are unlinked, what phenotypic ratio is expected among the offspring?

Accepted Answer

1:1:1:1

Question 6

Which of the following best explains why the sum of individual map distances between three genes may not equal the observed distance between the outermost genes?

Accepted Answer

Double crossovers can occur, which are not detected if only the outermost markers are considered.

Question 7

Which step in the molecular mechanism of meiotic recombination involves DNA polymerase activity?

Accepted Answer

DNA synthesis to extend the invading strand

Question 8

Why is homologous recombination typically restricted to meiosis in eukaryotic cells?

Accepted Answer

It requires the pairing of homologous chromosomes, which occurs during meiosis.

Question 9

If the recombination frequency (RF) between two genes is $$0.25$$, what is the genetic distance between them in centimorgans (cM)?

Accepted Answer

$$25$$\,cM

Question 10

Which of the following is a direct result of crossing over during meiosis?

Accepted Answer

Generation of new combinations of alleles different from either parent (recombinants)

Question 11

In Morgan’s Drosophila experiment, why did the observed phenotypic ratios in the testcross differ from those expected for genes that assort independently?

Accepted Answer

The genes were linked and inherited together more often than not.

Question 12

Which of the following best describes a non-crossover recombinant?

Accepted Answer

A chromatid that has undergone a double-strand break but no exchange of flanking genetic material.

Question 13

Which of the following equations correctly represents the calculation of recombination frequency (RF)?

Accepted Answer

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

Question 14

Which of the following is NOT a step in the molecular mechanism of homologous recombination?

Accepted Answer

DNA ligase joining Okazaki fragments

Question 15

Why is there not a direct relationship between genetic distance (in cM) and physical distance (in base pairs) along a chromosome?

Accepted Answer

Some regions of chromosomes are more susceptible to recombination than others.

Question 16

Which of the following best explains why alleles that are far apart on a chromosome have a greater chance of being recombined?

Accepted Answer

There is a higher probability of crossing over occurring between them.

Question 17

In a three-point testcross, why might the sum of the two inner map distances be less than the outer map distance?

Accepted Answer

Double crossovers between the inner genes are not detected unless all three loci are scored.

Question 18

Which of the following is a key feature of the Holliday model for homologous recombination?

Accepted Answer

Formation of a cross-shaped structure between homologous chromosomes

Question 19

Which of the following best describes the outcome if crossing over occurs between two genes that are very close together on the same chromosome?

Accepted Answer

The recombination frequency will be low, and most offspring will have parental phenotypes.

DNA Recombination and Genetic Mapping: Mechanisms and Applications

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