Q: Which formula correctly calculates the genetic distance in map units (centiMorgans) between two genes?

$$\text{map\ distance\ (cM)} = \frac{\text{number\ of\ recombinants}}{\text{total\ offspring}} \times 100$$

Q: A cross between two Drosophila with genotypes y+w+/yw produces 4292 y+w/yw and 4605 yw/yw offspring, along with 86 y+w+/yw and 44 yw/yw. What is the recombination frequency between the y and w genes?

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

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 assorted independently due to being on different chromosomes.

Question 2

A dihybrid cross between two true-breeding pea plants (SS YY x ss yy) produces F1 plants that are all Ss Yy. If these F1 plants are self-fertilized, what is the expected phenotypic ratio in the F2 generation according to Mendel’s law of independent assortment?

Accepted Answer

$$9:3:3:1$$

Question 3

Which stage of meiosis is characterized by the formation of chiasmata, where crossing-over occurs?

Accepted Answer

Pachytene

Question 4

If two genes are located very close together on the same chromosome, which of the following is most likely to be true?

Accepted Answer

They will show genetic linkage, with more parental than recombinant gametes.

Question 5

In a testcross involving two linked genes, 80% of the offspring display parental phenotypes and 20% display recombinant phenotypes. What is the recombination frequency between these genes?

Accepted Answer

$$20\%$$

Question 6

Which formula correctly calculates the genetic distance in map units (centiMorgans) between two genes?

Accepted Answer

$$	ext{map\ distance\ (cM)} = \frac{	ext{number\ of\ recombinants}}{	ext{total\ offspring}} 	imes 100$$

Question 7

Why do double crossovers between two genes lead to an underestimate of the genetic distance between them?

Accepted Answer

Because double crossovers restore the parental combination, making them indistinguishable from non-recombinant gametes.

Question 8

In Morgan’s experiment with Drosophila, the genes for white eyes (w) and miniature wings (m) are 36.9 map units apart. What percentage of recombinant offspring would you expect in a testcross?

Accepted Answer

$$36.9\%$$

Question 9

Which of the following statements about genetic maps is correct?

Accepted Answer

Genetic maps are constructed using recombination frequencies between genes.

Question 10

A cross between two Drosophila with genotypes y+w+/yw produces 4292 y+w/yw and 4605 yw/yw offspring, along with 86 y+w+/yw and 44 yw/yw. What is the recombination frequency between the y and w genes?

Accepted Answer

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

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 first provided evidence for genetic linkage?

Accepted Answer

Bateson and Punnett’s dihybrid cross in sweet peas.

Question 13

If two genes are completely linked (no crossing over), what gamete ratio is expected from a dihybrid individual (e.g., FG/fg)?

Accepted Answer

$$1:1$$ for only parental types

Question 14

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

Accepted Answer

Complete penetrance

Question 15

Why did Sturtevant’s calculated genetic distance between y and r genes (57.6 mu) differ from the value obtained in a single experiment (37.5 mu)?

Accepted Answer

Because double crossovers were not detected in the single experiment.

Question 16

Which of the following best describes the physical basis for recombination between linked genes?

Accepted Answer

Crossing-over between homologous chromosomes during meiosis I.

Question 17

A geneticist observes 60% parental and 40% recombinant gametes in a testcross. What is the map distance between the two genes?

Accepted Answer

$$40\ 	ext{cM}$$

Question 18

Which of the following is a direct consequence of the law of independent assortment at the chromosomal level?

Accepted Answer

Homologous chromosomes align randomly at metaphase I, leading to genetic variation in gametes.

Genetic Linkage and Recombination: Principles, Evidence, and Mapping

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