Q: A wild-type fruit fly (grey body, red eyes) is crossed with a black fruit fly with white eyes. The F1 offspring are mated with black fruit flies with white eyes. The F2 progeny are: wild type (684), black-white (701), grey-white (31), black-red (48). What is the recombination frequency between the body color and eye color loci?

Calculate: $$\frac{31+48}{684+701+31+48} \times 100 = \frac{79}{1464} \times 100 \approx 5.4\%$$

Q: In a maize cross, the following progeny were obtained: Coloured, green (88); Coloured, yellow (12); Colourless, green (8); Colourless, yellow (92). What is the recombination frequency between the kernel color and plant color genes?

Calculate: $$\frac{12+8}{88+12+8+92} \times 100 = \frac{20}{200} \times 100 = 10\%$$

Question 1

Which of the following best describes the relationship between linked genes on a chromosome?

Accepted Answer

Linked genes are located close together on the same chromosome and tend to be inherited together.

Question 2

In Morgan’s experiments with Drosophila, what was the observed effect of gene linkage on the expected phenotypic ratio in a testcross?

Accepted Answer

The ratio deviated from 1:1:1:1, with most offspring showing parental phenotypes and some showing recombinant phenotypes.

Question 3

If the recombination frequency between two genes is 9%, what is the distance between them on a chromosome map?

Accepted Answer

9 map units (mu) or 9 centiMorgans (cM)

Question 4

Given the following recombination frequencies: $$b to$$ $$cn is 9$$%, $$cn to$$ $$vg is 9.5$$%, and $$b to$$ $$vg is 17$$%, what is the correct order of these genes on the chromosome?

Accepted Answer

b - cn - vg

Question 5

A wild-type fruit fly (grey body, red eyes) is crossed with a black fruit fly with white eyes. The F1 offspring are mated with black fruit flies with white eyes. The F2 progeny are: wild type (684), black-white (701), grey-white (31), black-red (48). What is the recombination frequency between the body color and eye color loci?

Accepted Answer

Calculate: $$\frac{31+48}{684+701+31+48} 	imes 100 = \frac{79}{1464} 	imes 100 \approx 5.4\%$$

Question 6

What mechanism did Morgan propose to explain the occurrence of recombinant phenotypes in his Drosophila crosses?

Accepted Answer

Crossing over between homologous chromosomes during meiosis.

Question 7

Why do linkage maps provide only a relative picture of gene locations on chromosomes?

Accepted Answer

Because map units indicate relative distance and order, not precise locations, and crossing over frequency is not uniform.

Question 8

In a maize cross, the following progeny were obtained: Coloured, green (88); Coloured, yellow (12); Colourless, green (8); Colourless, yellow (92). What is the recombination frequency between the kernel color and plant color genes?

Accepted Answer

Calculate: $$\frac{12+8}{88+12+8+92} 	imes 100 = \frac{20}{200} 	imes 100 = 10\%$$

Question 9

Which of the following statements about crossing over is correct?

Accepted Answer

Crossing over occurs during meiosis and increases genetic diversity.

Question 10

If two genes are completely linked, what phenotypic ratio would you expect in a testcross?

Accepted Answer

1:1:0:0

Question 11

Which of the following best explains why recombination frequencies can be used to construct chromosome maps?

Accepted Answer

Recombination frequencies are directly proportional to the physical distance between genes.

Question 12

A geneticist finds that the recombination frequency between genes $$rb$$ and $$vg is 16$$%. What is the distance between these genes?

Accepted Answer

16 map units (mu) or 16 centiMorgans (cM)

Question 13

Which of the following is a limitation of using recombination frequencies to map genes?

Accepted Answer

Recombination frequencies are not uniform across the chromosome, so map units do not represent exact physical distances.

Question 14

In a testcross involving Drosophila, the observed offspring numbers are: wild type (965), black-vestigial (944), gray-vestigial (206), black-normal (185). What is the recombination frequency between the body color and wing shape genes?

Accepted Answer

Calculate: $$\frac{206+185}{965+944+206+185} 	imes 100 = \frac{391}{2300} 	imes 100 \approx 17\%$$

Question 15

Which of the following best describes a map unit (centiMorgan)?

Accepted Answer

A map unit is equivalent to 1% recombination frequency between two genes.

Question 16

Which scenario would most likely result in a higher recombination frequency between two genes?

Accepted Answer

The genes are located far apart on the same chromosome.

Question 17

What is the significance of cytological maps in genetics?

Accepted Answer

They indicate the positions of genes with respect to chromosomal features, such as banding patterns.

Question 18

If a geneticist observes a 50% recombination frequency between two genes, what can be concluded about their location?

Accepted Answer

The genes are located far apart on the same chromosome or on different chromosomes.

Question 19

Which of the following is NOT a reason why observed results of crosses with linked genes deviate from Mendel’s law of independent assortment?

Accepted Answer

Linked genes are always located on different chromosomes.

Linkage, Recombination, and Chromosome Mapping in Genetics

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