Q: Given the following data from a three-point cross: $$RF_{v-pr} = 22.3\%$$, $$RF_{pr-bm} = 43.4\%. $$What is the expected double crossover frequency?

$$0.223 \times 0.434 = 0.097 = 9.7\%$$

Q: How is the coefficient of coincidence ($$C) $$calculated in a three-point cross?

$$C = \frac{\text{Observed DCO}}{\text{Expected DCO}}$$

Question 1

Which of the following best describes the outcome of independent assortment for two genes located on different chromosomes during meiosis?

Accepted Answer

Both parental and recombinant gametes are produced in equal proportions.

Question 2

If two genes are completely linked (no crossing over occurs), what types of gametes will be produced by a heterozygote with genotype $$AB/ab$$?

Accepted Answer

Only $$AB$$ and $$ab$$ gametes

Question 3

A single crossover occurs between two nonsister chromatids between genes $$A$$ and $$B in a $$heterozygote $$AB/ab. $$What proportion of the resulting gametes will be recombinant?

Accepted Answer

50%

Question 4

Which of the following statements about linkage groups is correct?

Accepted Answer

The number of linkage groups corresponds to the haploid number of chromosomes.

Question 5

If the recombination frequency between genes $$y$$ and $$w is 0.5$$%, and between $$w$$ and $$m is 34.5$$%, what is the expected recombination frequency between $$y$$ and $$m$$?

Accepted Answer

$$35.0\%$$

Question 6

What is the definition of a map unit (centiMorgan, cM) in genetic mapping?

Accepted Answer

The distance corresponding to 1% recombination frequency between two genes.

Question 7

In a three-point test cross, which class of offspring is expected to occur in the smallest proportion?

Accepted Answer

Double-crossover (DCO) phenotypes

Question 8

Given the following data from a three-point cross: $$RF_{v-pr} = 22.3\%$$, $$RF_{pr-bm} = 43.4\%. $$What is the expected double crossover frequency?

Accepted Answer

$$0.223 	imes 0.434 = 0.097 = 9.7\%$$

Question 9

How is the coefficient of coincidence ($$C) $$calculated in a three-point cross?

Accepted Answer

$$C = \frac{	ext{Observed DCO}}{	ext{Expected DCO}}$$

Question 10

If the observed double crossover frequency is 7.8% and the expected is 9.7%, what is the interference ($$I$$)?

Accepted Answer

$$I = 1 - 0.804 = 0.196 = 19.6\%$$

Question 11

Which of the following is true regarding the detection of crossovers between two genes of interest?

Accepted Answer

Only crossovers that occur between the two genes of interest are detected.

Question 12

In a mapping experiment, you observe 150 single crossovers between $$y$$ and $$w$$, 400 single crossovers between $$w$$ and $$ec$$, and 6 double crossovers, out of 10,000 total offspring. What is the recombination frequency between $$y$$ and $$w$$?

Accepted Answer

$$RF_{y-w} = \frac{150 + 6}{10000} = 0.0156 = 1.56\%$$

Question 13

Which of the following is a criterion for a successful three-point mapping cross?

Accepted Answer

The parent must be heterozygous for all three genes.

Question 14

Why does the observed double crossover frequency often differ from the expected frequency?

Accepted Answer

Because of interference, which reduces the number of observed double crossovers.

Question 15

In a three-point cross, how can you determine which gene is in the middle?

Accepted Answer

By identifying the gene that switches association between the parental and double-crossover classes.

Question 16

If the recombination frequency between two genes is 50%, what does this indicate about their genetic relationship?

Accepted Answer

They assort independently and are likely on different chromosomes or far apart on the same chromosome.

Question 17

Which of the following best explains why genes that are close together on a chromosome tend to be inherited together?

Accepted Answer

There is a lower probability of a chiasma forming between them.

Question 18

A Drosophila female heterozygous for three X-linked genes ($$a$$, $$b$$, $$c) is $$crossed with a male mutant for all three genes. The offspring numbers are: $$+bc (460$$), $$a++ (450$$), $$abc (32$$), $$+++ (38$$), $$a+c (11$$), $$+b+ (9). $$Which are the parental (noncrossover) classes?

Accepted Answer

$$+bc$$ and $$a++$$

Question 19

Which formula correctly calculates the total recombination frequency ($$RF) $$between two genes, accounting for both single and double crossovers?

Accepted Answer

$$RF = \frac{SCO + DCO}{Total}$$

Linkage and Chromosome Mapping in Eukaryotes

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