Question 1

Which of Mendel’s postulates is violated by genetic linkage between two genes located close together on the same chromosome?

Accepted Answer

That homologs independently assort in meiosis

Question 2

In a cross involving two genes that are completely linked (no crossing over), what proportion of the gametes will be recombinant?

Accepted Answer

$$0\%$$

Question 3

What is the physical basis for crossing over between homologous chromosomes during meiosis?

Accepted Answer

Formation of chiasmata between non-sister chromatids

Question 4

If two genes are 10 map units apart, what percentage of gametes are expected to be recombinant?

Accepted Answer

$$10\%$$

Question 5

Three genes on the Drosophila X chromosome have the following recombination rates: yellow-white $$0.5\%$$, white-miniature $$34.5\%$$, yellow-miniature $$35.4\%. $$Which two genes are physically closest together?

Accepted Answer

Yellow and white

Question 6

If two genes are 60 map units apart, what is the expected percentage of recombinant gametes?

Accepted Answer

$$50\%$$

Question 7

A single crossover event between two non-sister chromatids during meiosis will produce which of the following gamete types?

Accepted Answer

Two parental and two recombinant gametes

Question 8

The genes A, B, and C occur in that order in fruit flies. A is 10 map units from B, and B is 20 map units from C. How far is A from C?

Accepted Answer

$$30$$ map units

Question 9

If the probability of a single crossover between genes A and B is $$0.20$$ and between B and C is $$0.30$$, what is the expected frequency of double crossovers?

Accepted Answer

$$0.06$$

Question 10

Which of the following is a criterion for three-point mapping in genetic analysis?

Accepted Answer

Phenotypic class must reflect genotype of gametes of parents

Question 11

In a three-point test cross, which class of offspring will always be the least frequent?

Accepted Answer

Double crossover class

Question 12

Why are recombination frequencies between linked genes considered additive?

Accepted Answer

Because each crossover event is independent and the total frequency approximates the sum of individual events

Question 13

If the recombination frequency between gene w and ec is $$4\%$$ and the frequency of double crossovers is $$0.06\%$$, what is the corrected map distance between w and ec?

Accepted Answer

$$4.06$$ map units

Question 14

Which of the following best explains why genes that are very far apart on the same chromosome appear to assort independently?

Accepted Answer

The frequency of crossing over approaches $$50\%$$, making recombinant and parental gametes equally likely

Question 15

A geneticist observes that the percentage of recombinant offspring increases as the distance between two genes increases. What is the most likely explanation?

Accepted Answer

There is a higher probability of chiasma formation between genes that are further apart

Question 16

In a three-point cross, why must a large number of offspring be analyzed?

Accepted Answer

To detect rare double crossover events and accurately estimate recombination frequencies

Question 17

Which of the following statements about map units (centiMorgans) is correct?

Accepted Answer

One map unit corresponds to $$1\%$$ recombination frequency

Question 18

If a single crossover occurs between two genes that are 20 map units apart, what is the expected proportion of recombinant gametes?

Accepted Answer

$$20\%$$

Question 19

Which scientist first proposed the concept of genetic mapping based on recombination frequencies?

Accepted Answer

Alfred Sturtevant

Question 20

In a three-point test cross, which class of offspring is used to determine the gene order?

Accepted Answer

Double crossover class

Chromosome Mapping and Linkage in Eukaryotes

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