Q: Calculate the recombination frequency if 206 gray-vestigial and 185 black-normal offspring are observed out of a total of 2,300 offspring in a Drosophila testcross.

$$\frac{206 + 185}{2300} \times 100 = 17\%$$

Question 1

Which of the following statements best describes the chromosome theory of inheritance as proposed by Sutton and Boveri in 1902?

Accepted Answer

Genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns.

Question 2

In Thomas Hunt Morgan's experiments with Drosophila melanogaster, what was the significance of discovering a white-eyed mutant fly?

Accepted Answer

It provided the first solid evidence associating a specific gene with a specific chromosome.

Question 3

A male inherits an X-linked recessive disorder from his mother. Which of the following best explains why X-linked recessive disorders are more common in males than in females?

Accepted Answer

Males are hemizygous for X-linked genes, so a single recessive allele will result in the disorder.

Question 4

Which gene on the Y chromosome is responsible for the development of testes in an embryo?

Accepted Answer

SRY (sex-determining region on the Y)

Question 5

If a female carrier for an X-linked recessive trait (X	extsuperscript{N}X	extsuperscript{n}) mates with a normal male (X	extsuperscript{N}Y), what is the probability that their son will express the recessive trait?

Accepted Answer

$$\frac{1}{2}$$

Question 6

Which of the following best explains why genes located on the same chromosome tend to be inherited together?

Accepted Answer

They do not assort independently due to genetic linkage.

Question 7

In a dihybrid testcross involving linked genes, the appearance of nonparental phenotypes among the offspring is best explained by which process?

Accepted Answer

Genetic recombination due to crossing over during meiosis

Question 8

Calculate the recombination frequency if 206 gray-vestigial and 185 black-normal offspring are observed out of a total of 2,300 offspring in a Drosophila testcross.

Accepted Answer

$$\frac{206 + 185}{2300} 	imes 100 = 17\%$$

Question 9

Which of the following is a direct consequence of nondisjunction during meiosis?

Accepted Answer

Production of gametes with abnormal chromosome numbers

Question 10

A zygote with only one copy of a particular chromosome is described as:

Accepted Answer

Monosomic

Question 11

Down syndrome is caused by which chromosomal abnormality?

Accepted Answer

Trisomy 21 (three copies of chromosome 21)

Question 12

Which of the following increases genetic variation in sexually reproducing populations?

Accepted Answer

Random fertilization and genetic recombination

Question 13

Which law states that alleles of genes on nonhomologous chromosomes assort independently during gamete formation?

Accepted Answer

Law of Independent Assortment

Question 14

A father with a Y-linked trait will pass this trait to:

Accepted Answer

All of his sons

Question 15

Which of the following best describes the difference between homozygous and hemizygous genotypes for X-linked genes?

Accepted Answer

Homozygous refers to two identical alleles in females; hemizygous refers to a single allele in males.

Question 16

Which of the following is NOT a consequence of aneuploidy?

Accepted Answer

Increased genetic variation for natural selection

Question 17

If a woman with genotype X	extsuperscript{X}X	extsuperscript{w} (carrier for white eyes) and a man with genotype X	extsuperscript{w}Y (white-eyed) have children, what is the probability that a daughter will have white eyes?

Accepted Answer

$$\frac{1}{2}$$

Question 18

Which of the following best explains why the frequency of Down syndrome increases with maternal age?

Accepted Answer

There is a greater wait time for eggs to complete meiosis, increasing the risk of nondisjunction.

Question 19

Which process during meiosis is responsible for the production of recombinant chromosomes?

Accepted Answer

Crossing over between homologous chromosomes

Question 20

A testcross between a dihybrid fly (b	extsuperscript{+} vg	extsuperscript{+}/b vg) and a double mutant (b vg/b vg) produces mostly parental-type offspring but also some recombinant types. What does this indicate about the genes involved?

Accepted Answer

The genes are linked but crossing over occurs between them.

Study Notes: The Chromosomal Basis of Inheritance (Chapter 15)

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