Q: In a testcross between a Drosophila fly heterozygous for two linked genes (s and e) and a fly homozygous recessive for both, the following offspring numbers are observed: 537 long bristles, gray body; 542 short bristles, ebony body; 76 long bristles, ebony body; 75 short bristles, gray body. What is the map distance between the s and e genes? Use the appropriate formula.

$$((76 + 75) / (537 + 542 + 76 + 75)) \times 100 = 12.3$$ map units

Q: A cross between two haploid yeast strains produces the following asci: 40 PD, 10 NPD, and 50 T. What is the map distance between the two genes using the simple formula?

$$((10 + (1/2) \times 50) / 100) \times 100 = 35$$ map units

Q: Which formula correctly calculates the map distance between two genes using tetrad analysis, accounting for both single and double crossovers?

$$\text{Map distance} = \frac{T + 6NPD}{0.5 \times \text{total number of asci}} \times 100$$

Question 1

In a testcross between a Drosophila fly heterozygous for two linked genes (s and e) and a fly homozygous recessive for both, the following offspring numbers are observed: 537 long bristles, gray body; 542 short bristles, ebony body; 76 long bristles, ebony body; 75 short bristles, gray body. What is the map distance between the s and e genes? Use the appropriate formula.

Accepted Answer

$$((76 + 75) / (537 + 542 + 76 + 75)) 	imes 100 = 12.3$$ map units

Question 2

Why does a testcross between two genes never yield more than 50% recombinant offspring, even if the genes are on different chromosomes?

Accepted Answer

Because independent assortment produces a maximum of 50% recombinants, which is indistinguishable from unlinked genes

Question 3

In a three-factor cross involving the genes b, pr, and vg in Drosophila, the double crossover classes are the least frequent. What information do these double crossover classes provide?

Accepted Answer

They reveal which gene is located in the middle of the three

Question 4

Given the following three-factor cross data (total offspring = 1,005): double crossover offspring = 3, expected double crossover offspring = 7.5. What is the coefficient of coincidence (C) and the interference (I)?

Accepted Answer

$$C = 3 / 7.5 = 0.4$$, $$I = 1 - 0.4 = 0.6$$

Question 5

Which of the following best explains why the map distance calculated directly between two flanking genes in a three-factor cross is often less than the sum of the distances between each gene and the middle gene?

Accepted Answer

Double crossovers between the flanking genes are not detected in two-gene comparisons, leading to an underestimate

Question 6

In tetrad analysis of ascomycete fungi, which type of ascus can only be produced by a double crossover involving all four chromatids?

Accepted Answer

Nonparental ditype (NPD)

Question 7

A cross between two haploid yeast strains produces the following asci: 40 PD, 10 NPD, and 50 T. What is the map distance between the two genes using the simple formula?

Accepted Answer

$$((10 + (1/2) 	imes 50) / 100) 	imes 100 = 35$$ map units

Question 8

Which of the following statements about mitotic recombination is correct?

Accepted Answer

Mitotic recombination can produce twin spots, which are adjacent patches of tissue with different phenotypes

Question 9

In ascomycete fungi, what is the significance of analyzing the arrangement of spores within an ascus?

Accepted Answer

It allows direct observation of the products of a single meiosis, enabling gene mapping

Question 10

If two genes in yeast are unlinked, what is the expected ratio of parental ditype (PD) to nonparental ditype (NPD) asci?

Accepted Answer

PD = NPD

Question 11

A Drosophila female is heterozygous for two X-linked genes in trans configuration: y+ sn / y sn+. If a mitotic crossover occurs between y and sn early in development, what is the expected outcome?

Accepted Answer

Two adjacent patches: one with gray body, short bristles and one with yellow body, normal bristles (twin spots)

Question 12

Which formula correctly calculates the map distance between two genes using tetrad analysis, accounting for both single and double crossovers?

Accepted Answer

$$	ext{Map distance} = \frac{T + 6NPD}{0.5 	imes 	ext{total number of asci}} 	imes 100$$

Question 13

In a three-factor cross, the observed number of double crossovers is much lower than expected. What genetic phenomenon does this illustrate?

Accepted Answer

Positive interference

Question 14

Which of the following best describes the relationship between map units (centimorgans) and recombination frequency?

Accepted Answer

1 map unit (1 cM) corresponds to a 1% recombination frequency between two genes

Question 15

Why are double crossovers between two genes often undetected in two-point testcrosses?

Accepted Answer

Because double crossovers restore the parental arrangement of alleles, making them indistinguishable from nonrecombinants

Question 16

In a three-factor cross, the gene order is determined to be b - pr - vg. If the map distance between b and pr is 6.1 cM and between pr and vg is 12.3 cM, what is the expected map distance between b and vg?

Accepted Answer

$$6.1 + 12.3 = 18.4 cM$$

Question 17

Which of the following is a key advantage of genetic mapping in haploid eukaryotes such as fungi compared to diploid organisms?

Accepted Answer

Each spore directly reveals the genotype resulting from a single meiosis, simplifying analysis

Question 18

A geneticist observes a Drosophila fly with two adjacent patches: one with short bristles on a gray body and one with long bristles on a yellow body, while the rest of the fly is wild-type. What is the most likely explanation for this observation?

Accepted Answer

A mitotic crossover occurred between the y and sn loci during early development

Genetic Linkage and Mapping in Eukaryotes: Study Notes

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