Q: A testcross between a dihybrid fruit fly (b+ b vg+ vg) and a double mutant (b b vg vg) yields the following offspring: 965 wild type, 944 black-vestigial, 206 gray-vestigial, 185 black-normal. What is the recombination frequency between the b and vg genes?

$$\frac{206 + 185}{965 + 944 + 206 + 185} \times 100 = 17\%$$

Question 1

Which of the following best explains why Drosophila melanogaster (fruit flies) became a model organism for genetic studies in the early 20th century?

Accepted Answer

They have only four pairs of chromosomes, which are easily visible under a microscope.

Question 2

In Morgan’s famous experiment with white-eyed Drosophila, why were all white-eyed flies in the F2 generation male?

Accepted Answer

The white-eye gene is X-linked and recessive, so males with one mutant X show the trait.

Question 3

A woman who is a carrier for an X-linked recessive disorder (XNXn) has children with a man who is unaffected (XNY). What is the probability that a child will be male AND affected?

Accepted Answer

$$\frac{1}{4}$$

Question 4

Which of the following best describes the chromosomal basis for Mendel’s law of independent assortment?

Accepted Answer

Alleles of genes on nonhomologous chromosomes assort independently during meiosis.

Question 5

If two genes are located on the same chromosome and are very close together, which of the following is most likely true?

Accepted Answer

They are likely to be inherited together, with a recombination frequency less than 50%.

Question 6

A testcross between a dihybrid fruit fly (b+ b vg+ vg) and a double mutant (b b vg vg) yields the following offspring: 965 wild type, 944 black-vestigial, 206 gray-vestigial, 185 black-normal. What is the recombination frequency between the b and vg genes?

Accepted Answer

$$\frac{206 + 185}{965 + 944 + 206 + 185} 	imes 100 = 17\%$$

Question 7

Which chromosomal abnormality is most likely to result from nondisjunction during meiosis I?

Accepted Answer

Aneuploidy, such as trisomy (2n + 1) or monosomy (2n - 1)

Question 8

Which of the following statements about X inactivation in female mammals is correct?

Accepted Answer

One X chromosome is randomly inactivated in each cell, forming a Barr body.

Question 9

A geneticist observes a recombination frequency of 9% between two genes and 9.5% between one of those genes and a third gene. What is the approximate map distance between the two outer genes?

Accepted Answer

$$9\% + 9.5\% = 18.5$$ map units (since map units are additive for small distances)

Question 10

Which of the following chromosomal alterations involves a segment of a chromosome being reversed end to end?

Accepted Answer

Inversion

Question 11

What is the maximum possible recombination frequency between two genes, and what does this indicate about their chromosomal location?

Accepted Answer

$$50\%$$, indicating the genes are on different chromosomes or far apart on the same chromosome.

Question 12

A plant species is found to have six complete sets of chromosomes (6n). What is this condition called, and in which group of organisms is it most commonly tolerated?

Accepted Answer

Polyploidy; most common in plants

Question 13

Which of the following best explains why X-linked recessive disorders are more common in males than in females?

Accepted Answer

Males only need one copy of the mutant allele to express the disorder, while females need two.

Question 14

Which of the following is a real-world example of genetic mosaicism due to X inactivation?

Accepted Answer

A female tortoiseshell cat with patches of orange and black fur

Question 15

If a female carrier for color blindness (an X-linked recessive trait) has children with a color-blind male, what is the probability that a daughter will be color-blind?

Accepted Answer

$$\frac{1}{4}$$

Question 16

Which of the following chromosomal changes is most likely to result in a genetic disorder due to the loss of essential genes?

Accepted Answer

Deletion

Question 17

A geneticist is mapping three genes on a chromosome. The recombination frequencies are: gene A to B = 12%, gene B to C = 7%, gene A to C = 19%. What is the most likely order of these genes?

Accepted Answer

A - B - C

Question 18

Which of the following best describes the outcome of a dihybrid cross (YyRr × yyrr) when the two genes are unlinked?

Accepted Answer

The phenotypic ratio in the offspring will be 1:1:1:1.

Question 19

Which of the following is NOT a possible result of nondisjunction during meiosis?

Accepted Answer

Independent assortment of chromosomes

Lecture 18 (Unit 2)--Chromosomal Basis of Inheritance: Sex-Linked Traits, Gene Linkage, and Chromosomal Alterations

Guida di studio - Domande di pratica

Study guide - Flashcard