Q: A testcross between a dihybrid fly (pr\textsuperscript{+} vg\textsuperscript{+} / pr vg) and a double recessive fly (pr vg / pr vg) yields the following progeny: 1287 red eyes, normal wings; 1204 purple eyes, vestigial wings; 170 red eyes, vestigial wings; 154 purple eyes, normal wings. What is the recombination frequency between the pr and vg genes?

$$\frac{170+154}{1287+1204+170+154} \times 100 = 11.5\%$$

Q: Which equation correctly calculates the map distance (in centiMorgans) between two genes from testcross progeny data?

$$\text{Map distance (cM)} = \frac{\text{Number of recombinant progeny}}{\text{Total number of progeny}} \times 100$$

Q: In humans, approximately how many base pairs correspond to 1 centiMorgan (cM) on average?

$$1\,\text{cM} \approx 1,000,000\,\text{bp}$$

Question 1

Which statement best describes the relationship between genetic maps and physical maps for closely spaced genes?

Accepted Answer

Genetic maps and physical maps are proportional for closely spaced genes.

Question 2

What is the maximum detectable recombination frequency between two genes, and what does this imply about their genetic relationship?

Accepted Answer

50%; the genes assort independently.

Question 3

A testcross between a dihybrid fly (pr	extsuperscript{+} vg	extsuperscript{+} / pr vg) and a double recessive fly (pr vg / pr vg) yields the following progeny: 1287 red eyes, normal wings; 1204 purple eyes, vestigial wings; 170 red eyes, vestigial wings; 154 purple eyes, normal wings. What is the recombination frequency between the pr and vg genes?

Accepted Answer

$$\frac{170+154}{1287+1204+170+154} 	imes 100 = 11.5\%$$

Question 4

In a testcross, why are parental gametes more common than recombinant gametes when two genes are linked?

Accepted Answer

Because linked genes are physically close, reducing the chance of crossover between them.

Question 5

Which of the following best defines the 'phase' of genes in genetics?

Accepted Answer

The orientation or arrangement of alleles on homologous chromosomes.

Question 6

If two genes are in 'cis' phase, which of the following is true?

Accepted Answer

Dominant alleles are on the same chromosome.

Question 7

A plant geneticist crosses AAbb and aaBB pea plants. The F1 is testcrossed and the progeny ratios are: 0.11 AB, 0.13 ab, 0.38 Ab, 0.38 aB. How far apart are the A and B genes?

Accepted Answer

24 map units

Question 8

Which equation correctly calculates the map distance (in centiMorgans) between two genes from testcross progeny data?

Accepted Answer

$$	ext{Map distance (cM)} = \frac{	ext{Number of recombinant progeny}}{	ext{Total number of progeny}} 	imes 100$$

Question 9

In humans, approximately how many base pairs correspond to 1 centiMorgan (cM) on average?

Accepted Answer

$$1\,	ext{cM} \approx 1,000,000\,	ext{bp}$$

Question 10

Why does the proportionality between genetic and physical maps break down for distantly spaced genes?

Accepted Answer

Because distantly spaced genes assort independently, making recombination frequency approach 50%.

Question 11

Which of the following is true about recombination in Drosophila males?

Accepted Answer

Recombination does not occur in Drosophila males.

Question 12

A geneticist is mapping three genes: A, B, and C. The recombination frequency between A and B is 10%, between B and C is 5%, and between A and C is 15%. What is the most likely order of the genes on the chromosome?

Accepted Answer

A - B - C

Question 13

In a testcross, the expected progeny ratio for two unlinked genes is:

Accepted Answer

1:1:1:1

Question 14

Which of the following statements about centromeres and recombination is correct?

Accepted Answer

Regions near the centromere exhibit little or no crossing over.

Question 15

If an organism has the genotype Ab/aB and the genes A and B are 20 map units apart, what proportion of gametes produced will be AB?

Accepted Answer

0%

Question 16

Which of the following is a key difference between a genetic map and a physical map?

Accepted Answer

Genetic maps are based on recombination frequencies; physical maps are based on DNA sequence distances.

Question 17

In a pedigree showing two linked disorders, one autosomal dominant and one autosomal recessive, what information is needed to determine if a gamete is parental or non-parental?

Accepted Answer

The phase of the alleles and the parental combinations.

Question 18

Which of the following best explains why the heterogametic sex (e.g., XY in humans) often has lower recombination rates?

Accepted Answer

Because recombination rates are genome-wide and not confined to sex chromosomes.

Question 19

Which scenario would most likely result in independent assortment of two genes?

Accepted Answer

The genes are located far apart on the same chromosome.

Genetic Mapping and Linkage Analysis in Genetics

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