Q: A test cross between a heterozygote for two linked genes (AaBb) and a homozygous recessive (aabb) produces 45% AaBb, 45% aabb, 5% Aabb, and 5% aaBb offspring. What is the recombination frequency between the two genes?

$$\frac{5 + 5}{100} \times 100 = 10\%$$

Q: A three-point test cross in maize yields the following offspring numbers: 272 parental type 1, 272 parental type 2, 99 single crossover type 1, 33 single crossover type 2, 61 single crossover type 3, 44 single crossover type 4, 4 double crossover type 1, and 2 double crossover type 2. What is the recombination frequency between the first and second gene?

$$\frac{61 + 44 + 4 + 2}{587} \times 100 = 18.7\%$$

Question 1

Which of the following best explains why genes that are close together on the same chromosome tend to be inherited together?

Accepted Answer

They are less likely to be separated by crossing over due to their physical proximity.

Question 2

In a dihybrid test cross involving two genes on the same chromosome, the observed offspring ratio is 1:1:0:0. What does this indicate about the genes?

Accepted Answer

The genes are completely linked with no crossing over.

Question 3

A test cross between a heterozygote for two linked genes (AaBb) and a homozygous recessive (aabb) produces 45% AaBb, 45% aabb, 5% Aabb, and 5% aaBb offspring. What is the recombination frequency between the two genes?

Accepted Answer

$$\frac{5 + 5}{100} 	imes 100 = 10\%$$

Question 4

If two genes are 50 map units apart on the same chromosome, what is the expected recombination frequency?

Accepted Answer

50%

Question 5

Which of the following statements about genetic mapping is correct?

Accepted Answer

One map unit (centiMorgan) corresponds to a 1% recombination frequency.

Question 6

A three-point test cross in maize yields the following offspring numbers: 272 parental type 1, 272 parental type 2, 99 single crossover type 1, 33 single crossover type 2, 61 single crossover type 3, 44 single crossover type 4, 4 double crossover type 1, and 2 double crossover type 2. What is the recombination frequency between the first and second gene?

Accepted Answer

$$\frac{61 + 44 + 4 + 2}{587} 	imes 100 = 18.7\%$$

Question 7

Which of the following is a key criterion for a successful three-point mapping cross?

Accepted Answer

The heterozygote must be heterozygous for all three genes under consideration.

Question 8

In a three-point cross, the double crossover progeny are used to determine:

Accepted Answer

The gene order on the chromosome.

Question 9

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

Accepted Answer

It is the reduction in the expected number of double crossovers due to the occurrence of a crossover in a nearby region.

Question 10

A geneticist observes 7 double crossovers in a sample where 10 were expected. What is the coefficient of coincidence ($$C) $$and the interference ($$I$$)?

Accepted Answer

$$C = \frac{7}{10} = 0.7$$, $$I = 1 - 0.7 = 0.3$$

Question 11

Which of the following DNA markers is most commonly used in forensic DNA profiling due to its high variability among individuals?

Accepted Answer

Microsatellites (short tandem repeats, STRs)

Question 12

A disease gene is mapped using a DNA marker that is always inherited with the disease in a pedigree. What is the most likely explanation?

Accepted Answer

The marker and the disease gene are closely linked on the same chromosome.

Question 13

Which of the following statements about single nucleotide polymorphisms (SNPs) is correct?

Accepted Answer

SNPs can be located anywhere in the genome and may or may not affect gene function.

Question 14

In a test cross involving two genes, the observed offspring are 965 wild type, 944 black-vestigial, 206 gray-vestigial, and 185 black-normal. What is the recombination frequency between the two genes?

Accepted Answer

$$\frac{206 + 185}{2300} 	imes 100 = 17\%$$

Question 15

Which of the following is true regarding sister chromatid exchanges (SCEs)?

Accepted Answer

SCEs occur between sister chromatids during mitosis and do not result in new allelic combinations.

Question 16

Bloom syndrome is characterized by which of the following genetic features?

Accepted Answer

A mutation in the BLM gene encoding a DNA helicase, leading to increased SCEs and chromosomal instability.

Question 17

In a three-point test cross, which step is essential for determining the gene order?

Accepted Answer

Identify the two most numerous (nonrecombinant) and two least numerous (double crossover) progeny, then compare their phenotypes.

Question 18

Why does the accuracy of genetic mapping decrease as the distance between two genes increases?

Accepted Answer

Because the probability of undetected multiple crossovers increases, leading to underestimation of the true distance.

Question 19

Which of the following best describes the use of somatic cell hybridization in human genetic mapping?

Accepted Answer

It involves fusing two different species' cells to assign human genes to specific chromosomes.

Chromosome Mapping and Linkage in Eukaryotes

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