Q: A test cross between a heterozygous Drosophila female (SsWw) and a homozygous recessive male (ssww) yields the following offspring: 375 star eyes, normal wings; 130 star eyes, dumpy wings; 128 normal eyes, normal wings; 367 normal eyes, dumpy wings. What is the recombination frequency between the two genes?

$$\frac{130+128}{1000} \times 100 = 25.8\%$$

Q: Which formula is used to calculate the map distance between two linked genes using test cross data?

$$\text{Map units (cM)} = \frac{\text{number of nonparental offspring} \times 100}{\text{total offspring}}$$

Question 1

Which of the following best explains why linked genes do not follow the law of independent assortment?

Accepted Answer

Linked genes are located close together on the same chromosome and tend to be inherited together.

Question 2

A test cross between a heterozygous Drosophila female (SsWw) and a homozygous recessive male (ssww) yields the following offspring: 375 star eyes, normal wings; 130 star eyes, dumpy wings; 128 normal eyes, normal wings; 367 normal eyes, dumpy wings. What is the recombination frequency between the two genes?

Accepted Answer

$$\frac{130+128}{1000} 	imes 100 = 25.8\%$$

Question 3

Which of the following statements about crossing over is correct?

Accepted Answer

Crossing over occurs between non-sister chromatids of homologous chromosomes during meiosis I.

Question 4

In a dihybrid test cross, the expected phenotypic ratio for unlinked genes is:

Accepted Answer

1:1:1:1

Question 5

If the observed offspring from a test cross deviate significantly from the expected 1:1:1:1 ratio, what is the most likely explanation?

Accepted Answer

The genes are linked and located close together on the same chromosome.

Question 6

Which formula is used to calculate the map distance between two linked genes using test cross data?

Accepted Answer

$$	ext{Map units (cM)} = \frac{	ext{number of nonparental offspring} 	imes 100}{	ext{total offspring}}$$

Question 7

Why is mapping distance less accurate for genes that are far apart on a chromosome?

Accepted Answer

Because double crossovers can occur, which may not be detected and thus underestimate the true distance.

Question 8

Which of the following is a parental phenotype in the cross SSWW x ssww (star eyes, normal wings x normal eyes, dumpy wings)?

Accepted Answer

Star eyes, normal wings

Question 9

A chi-square test is performed on the results of a test cross and yields a value much greater than the critical value for 3 degrees of freedom at $$p=0.05. $$What does this indicate?

Accepted Answer

The observed data do not fit the expected ratio, suggesting the genes are linked.

Question 10

In Drosophila, why must the female be doubly heterozygous and the male homozygous recessive for a test cross to map linked genes?

Accepted Answer

Because recombination only occurs during oogenesis in females.

Question 11

Which of the following best describes a nonparental (recombinant) phenotype?

Accepted Answer

A phenotype that results from new combinations of alleles due to crossing over.

Question 12

Which event during meiosis is responsible for the random assortment of alleles for unlinked genes?

Accepted Answer

Random alignment of homologous chromosomes at metaphase I.

Question 13

If two genes are very close together on the same chromosome, what would you expect regarding the frequency of recombinant offspring?

Accepted Answer

A low frequency of recombinant offspring.

Question 14

Which of the following is true about sister chromatid exchanges (SCEs)?

Accepted Answer

SCEs occur during mitosis but do not produce new allelic combinations.

Question 15

A plant heterozygous for two genes (RrYy) produces four types of gametes in equal proportions. Which law explains this observation?

Accepted Answer

Law of Independent Assortment

Question 16

Which of the following would be evidence that two genes are linked?

Accepted Answer

The parental phenotypes greatly outnumber the recombinant phenotypes in a test cross.

Question 17

What is the main reason that the chromosomal theory of inheritance was proposed?

Accepted Answer

To explain how chromosomes, not individual genes, are transmitted from parent to offspring.

Question 18

Which of the following is NOT a possible outcome of crossing over between homologous chromosomes?

Accepted Answer

Suppression of all recombination events.

Question 19

In a mapping experiment, you observe 76 and 75 nonparental offspring and 542 and 537 parental offspring. What is the map distance between the two genes?

Accepted Answer

$$\frac{76+75}{542+537+76+75} 	imes 100 = 12.3\ 	ext{mu}$$

Chromosome Mapping and Linkage in Eukaryotes: Study Notes

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