Q: A testcross between two Drosophila with genotypes s+e+/se and se/se yields 76 and 75 recombinant offspring out of a total of 1230. What is the map distance between the s and e genes?

$$\frac{76 + 75}{1230} \times 100 = 12.3$$ map units

Q: A geneticist observes the following F2 offspring from a cross: 321 gray, long; 333 ebony, short; 117 ebony, long; 125 gray, short. What is the recombination frequency between the two genes?

$$\frac{117 + 125}{321 + 333 + 117 + 125} \times 100 = 27\%$$

Question 1

Which stage of prophase I in meiosis is characterized by the formation of chiasmata, where crossing over occurs between homologous chromosomes?

Accepted Answer

Diplotene

Question 2

Bateson and Punnett observed a deviation from the expected 9:3:3:1 ratio in a dihybrid cross involving sweet peas. What was the most likely explanation for the excess of parental phenotypes in the F2 generation?

Accepted Answer

Gene linkage

Question 3

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

Accepted Answer

They will be inherited together more often than not

Question 4

Which of the following best describes the concept of 'synteny'?

Accepted Answer

Physical linkage of genes on the same chromosome

Question 5

A testcross between two Drosophila with genotypes s+e+/se and se/se yields 76 and 75 recombinant offspring out of a total of 1230. What is the map distance between the s and e genes?

Accepted Answer

$$\frac{76 + 75}{1230} 	imes 100 = 12.3$$ map units

Question 6

If the recombination frequency between two genes is 16%, how many recombinant offspring would you expect in a mapping cross that produces 500 total offspring?

Accepted Answer

80

Question 7

Which of the following statements about genetic mapping is FALSE?

Accepted Answer

Genetic maps always provide the exact physical distance between genes

Question 8

Morgan’s experiments with Drosophila showed that three genes (body color, eye color, wing length) were X-linked. What observation supported this conclusion?

Accepted Answer

The traits were inherited together more often than expected by independent assortment

Question 9

Why does the observed recombination frequency between two genes never exceed 50%?

Accepted Answer

Because multiple crossovers can occur, masking recombination events

Question 10

A geneticist observes the following F2 offspring from a cross: 321 gray, long; 333 ebony, short; 117 ebony, long; 125 gray, short. What is the recombination frequency between the two genes?

Accepted Answer

$$\frac{117 + 125}{321 + 333 + 117 + 125} 	imes 100 = 27\%$$

Question 11

Which of the following is a consequence of mitotic recombination occurring early in embryonic development?

Accepted Answer

A patch of tissue with a different genotype (twin spot) may form

Question 12

If the recombination frequency between genes A and B is 20%, between B and C is 15%, and between A and C is 5%, what is the correct order and map distances of these genes?

Accepted Answer

A-5mu-C-15mu-B

Question 13

Which of the following best explains why observed recombination frequencies tend to underestimate the actual physical distance between genes as the distance increases?

Accepted Answer

Multiple crossovers between genes are not detected as recombination

Question 14

Which of the following is NOT a use of genetic maps?

Accepted Answer

Determining the exact nucleotide sequence of a gene

Question 15

In a testcross, the number of recombinant offspring is 124 out of 1000 total offspring. How many parental (non-recombinant) offspring would you expect?

Accepted Answer

876

Question 16

Which of the following statements about linkage groups is TRUE?

Accepted Answer

Each chromosome represents a single linkage group

Question 17

Retinoblastoma is caused by mutations in the RB gene. In individuals who inherit one RB− allele, what event during cell division can lead to tumor formation?

Accepted Answer

Loss of the remaining RB+ allele during mitosis

Question 18

Which of the following is a direct result of crossing over during meiosis?

Accepted Answer

Increased genetic diversity among gametes

Linkage, Recombination, and Genetic Mapping in Eukaryotes

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