Q: A testcross between a dihybrid fly (AaBb) and a double recessive fly (aabb) produces the following offspring: 400 AaBb, 400 aabb, 100 Aabb, 100 aaBb. What is the recombination frequency between the A and B genes?

$$\frac{100 + 100}{400 + 400 + 100 + 100} \times 100\% = 20\%$$

Q: Which formula correctly calculates the recombination frequency between two genes?

$$\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100\%$$

Q: A geneticist observes 86 and 44 recombinant offspring out of a total of 9027 in a cross between two X-linked genes. What is the map distance between these genes?

$$\frac{86 + 44}{9027} \times 100 = 1.4 cM$$

Question 1

Which of the following best explains why Mendel observed a 9:3:3:1 ratio in the F2 generation of his dihybrid crosses?

Accepted Answer

Because the genes for the two traits were on different chromosomes and assorted independently during meiosis.

Question 2

In the sweet pea experiment by Bateson and Punnett, the observed F2 progeny showed an excess of parental phenotypes compared to the expected 9:3:3:1 ratio. What is the most likely explanation for this result?

Accepted Answer

The two genes were genetically linked and located close together on the same chromosome.

Question 3

Which stage of meiosis is characterized by the formation of chiasmata and the occurrence of crossing-over?

Accepted Answer

Pachytene

Question 4

If two genes are 10 map units apart on a chromosome, what percentage of gametes produced will be recombinant for these two genes?

Accepted Answer

$$10\%$$

Question 5

A testcross between a dihybrid fly (AaBb) and a double recessive fly (aabb) produces the following offspring: 400 AaBb, 400 aabb, 100 Aabb, 100 aaBb. What is the recombination frequency between the A and B genes?

Accepted Answer

$$\frac{100 + 100}{400 + 400 + 100 + 100} 	imes 100\% = 20\%$$

Question 6

Which of the following statements about genetic linkage is correct?

Accepted Answer

Linked genes are located on the same chromosome and tend to be inherited together unless crossing-over occurs.

Question 7

Why does the observed genetic distance between two genes sometimes underestimate the actual physical distance?

Accepted Answer

Because double crossovers can occur, which restore the parental combination and are not detected as recombinants.

Question 8

If two genes are completely linked (no crossing-over occurs), what proportion of gametes will be recombinant?

Accepted Answer

$$0\%$$

Question 9

Which formula correctly calculates the recombination frequency between two genes?

Accepted Answer

$$	ext{Recombination frequency} = \frac{	ext{Number of recombinant offspring}}{	ext{Total number of offspring}} 	imes 100\%$$

Question 10

A geneticist observes 86 and 44 recombinant offspring out of a total of 9027 in a cross between two X-linked genes. What is the map distance between these genes?

Accepted Answer

$$\frac{86 + 44}{9027} 	imes 100 = 1.4 cM$$

Question 11

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

Accepted Answer

The farther apart two genes are, the higher the recombination frequency.

Question 12

Which historical experiment provided the first evidence for genetic linkage?

Accepted Answer

Bateson and Punnett's dihybrid cross in sweet peas.

Question 13

What is the maximum recombination frequency that can be observed between two genes, and what does it indicate?

Accepted Answer

$$50\%$$, indicating the genes are unlinked or on different chromosomes.

Question 14

Which of the following is NOT a modification to simple Mendelian inheritance?

Accepted Answer

Complete dominance with 100% penetrance

Question 15

Why did Sturtevant's calculated genetic distance between y and r genes differ when using summed small intervals versus a single experiment?

Accepted Answer

Double crossovers between the genes were not detected in the single experiment, leading to an underestimate.

Question 16

Which of the following best describes a chiasma?

Accepted Answer

A visible X-shaped structure where homologous chromosomes exchange genetic material during meiosis.

Question 17

If a double crossover occurs between two genes, what is the effect on the observed recombination frequency?

Accepted Answer

It can cause the observed recombination frequency to be lower than the actual number of crossover events.

Question 18

Which of the following is a correct interpretation of 1 map unit (centiMorgan)?

Accepted Answer

A 1% chance that a crossover will occur between two genes during meiosis.

Question 19

Which of the following statements about the creation of the first genetic map is correct?

Accepted Answer

Sturtevant used recombination frequencies between pairs of genes to determine their linear order on the chromosome.

Genetic Linkage and Recombination: Principles, Evidence, and Mapping

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