Mastering Biology Chapter 15

A white-eyed female Drosophila is crossed with a red-eyed male Drosophila. Which statement below correctly describes the results?

None of the females will have white eyes. All of the females will have red eyes because they will inherit the dominant red-eye allele from their male parent.

In humans, what determines the sex of offspring and why?

The male gamete determines sex because each male gamete can contribute either an X or a Y chromosome. In humans, the male produces both X- and Y-bearing sperm, whereas females only produce X-bearing eggs.

Which of the following is true of an X-linked gene, but not of a Y-linked gene?

The gene is present in both males and females. All cells have at least one functional X chromosome.

Males are more often affected by sex-linked traits than females because

Males are hemizygous for the X chromosome.

SRY is best described in which of the following ways?

A gene region present on the Y chromosome that triggers male development

Red-green color blindness is a sex-linked recessive trait in humans. Two people with normal color vision have a color-blind son. What are the genotypes of the parents?

XNXn and XNY

Cinnabar eyes is a sex-linked recessive characteristic in fruit flies. If a female having cinnabar eyes is crossed with a wild-type male, what percentage of the F1 males will have cinnabar eyes?

100%

Which of the following statements is true of linkage?

The closer two genes are on a chromosome, the lower the probability that a crossover will occur between them.

What is a nondisjunction?

An error in cell division that causes homologous chromosomes or sister chromatids to move to the same side of the dividing cell Nondisjunction refers to the failure of pairs of chromosomes or sister chromatids to separate during meiosis or mitosis.

When can nondisjunction occur?

In mitosis, when sister chromatids fail to separate In meiosis, when sister chromatids fail to separate In meiosis, when homologous chromosomes fail to separate Nondisjunction errors can occur in meiosis I, when homologous chromosomes fail to separate, or in either mitosis or meiosis II, when sister chromatids fail to separate.

Which syndrome is characterized by the XO chromosome abnormality?

Turner syndrome Turner syndrome is characterized by a chromosome pattern of XO, or one X chromosome and no Y chromosome.

What kind of cell results when a diploid and a haploid gamete fuse during fertilization?

A triploid cell A triploid cell has three sets of chromosomes: the two from the diploid gamete and the one from the haploid gamete.

Of the following chromosomal abnormalities, which type is most likely to be viable in humans?

Trisomy Some trisomies are viable in humans, but they result in developmental abnormalities, such as Down syndrome (trisomy 21). Only humans with trisomies of the small chromosomes (13, 18, or 21) or the sex chromosomes survive past birth.

If a diploid cell undergoes meiosis and produces two gametes that are normal, and one with n − 1 chromosomes, and one with n + 1 chromosomes, what type of error occurred?

A nondisjunction error occurred in meiosis II, in which both sister chromatids of a chromosome migrated to the same pole of the cell. When this error occurs in meiosis II, only half of the gametes are affected.

If a diploid cell undergoes meiosis and produces two gametes with n + 1 chromosomes and two gametes with n− 1 chromosomes, what type of error occurred?

A nondisjunction error occurred in meiosis I, in which both members of a homologous pair migrated to the same pole of the cell. When this type of error occurs, half of the gametes receive one too many chromosomes, and the other half receive one too few. When all of the gametes are affected, the error probably occurred in meiosis I.

In general, the frequency with which crossing over occurs between two linked genes depends on what?

how far apart they are on the chromosome The farther apart two genes are, the greater the probability that a crossover will occur between them, and therefore the greater the recombination frequency.

Normally, only female cats have the tortoiseshell phenotype because

a male inherits only one allele of the X-linked gene controlling hair color.

In birds, sex is determined by a ZW chromosome scheme. Males are ZZ and females are ZW. A recessive lethal allele that causes death of the embryo is sometimes present on the Z chromosome in pigeons. What would be the sex ratio in the offspring of a cross between a male that is heterozygous for the lethal allele and a normal female?

2:1 male to female

Sex determination in mammals is due to the SRY region of the Y chromosome. An abnormality of this region could allow which of the following to have a male phenotype?

translocation of SRY to an autosome of a 46, XX individual

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man’s father was 6 feet tall, and both the woman’s parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive.

How many of their daughters might be expected to be color-blind dwarfs?

none

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man’s father was 6 feet tall, and both the woman’s parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive.

What proportion of their sons would be color-blind and of normal height?

half

A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man’s father was 6 feet tall, and both the woman’s parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive.

They have a daughter who is a dwarf with normal color vision. What is the probability that she is heterozygous for both genes?

100%

Three genes (A, B, and C) at three loci are being mapped in a particular species. Each gene has two alleles, one of which results in a phenotype that is markedly different from the wild type. The unusual allele of gene A is inherited with the unusual allele of gene B or C about 50% of the time. However, the unusual alleles of genes B and C are inherited together 14.4% of the time. Which of the following describes what is happening?

Gene A is assorting independently of genes B and C, which are linked.

What is one map unit equivalent to?

1% frequency of recombination between two genes

Recombination between linked genes comes about for what reason?

Crossovers between these genes result in chromosomal exchange.

Which statement about recombination between linked genes is correct?

New allele combinations may result in differential reproductive success.

Define deletion

A deletion is the loss of part of a chromosomal segment.

Define duplication

A duplication is the repetition of a segment. The repeated segment may be located next to the original or at a different location, and its orientation may be the same as the original or the reverse.

Define inversion

An inversion is the removal of a segment followed by its reinsertion into the same chromosome in the reverse orientation.

Define translocation

A translocation is the transfer of a segment to a nonhomologous chromosome. Translocations may be reciprocal (two nonhomologous chromosomes exchange segments) or nonreciprocal (one chromosome transfers a segment without receiving one).

Meiosis occurs normal (no disjunction)

3 only

Nondisjunction of one chromosome pair in meiosis I

2 or 4

Nondisjunction of all three chromosome pairs in meiosis I

0 or 6

Nondisjunction of one chromosome in one daughter cell in meiosis II

2, 3, or 4

Nondisjunction of all three chromosomes in one daughter cell in meiosis II

0, 3, or 6

Humans are diploid and have 46 chromosomes (or two sets). How many sets of chromosomes are found in each human gamete?

1 Meiosis cuts the number of chromosomal sets in half.

Humans are diploid and have 46 chromosomes. How many chromosomes are found in each human gamete?

23 23 is one-half of 46.

_____ is the process by which haploid gametes form a diploid zygote.

Fertilization The joining of haploid gametes produces a diploid zygote.

A particular diploid plant species has 48 chromosomes, or two sets. A mutation occurs and gametes with 48 chromosomes are produced. If self-fertilization occurs, the zygote will have _____ set(s) of chromosomes.

4 If self-fertilization occurs, and two gametes (each having 48 chromosomes) unite, the zygote will have 96 chromosomes (or 4 sets).

Which of these terms applies to an organism with extra sets of chromosomes?

polyploid Polyploid individuals have more than two complete sets of chromosomes.

Mutant tetraploid plants _____.

are unable to interbreed with a diploid plant This is why polyploidy is a mechanisms of plant speciation.

Most polyploid plants arise as a result of _____.

hybridization Most polyploid plants are the result of the hybridization of two different parental species.

Which of the following results in a situation in which the chromosome number is either 2n+1 or 2n-1 ?

aneuploidy An aneuploid individual has either too many or too few chromosomes, usually as a result of nondisjunction.

What results if a fragment of a chromosome breaks off and then reattaches to the original chromosome at the same place but in the reverse direction?

an inversion The question describes an inversion.

What is the source of the extra chromosome 21 in an individual with Down syndrome?

nondisjunction or translocation in either parent

What is the probability that a daughter of this mating will be a hemophiliac? Express your answer as a fraction using the slash symbol and no spaces (for example, 1/24), or as a whole number.

0

What is the probability that a son will be a hemophiliac? Express your answer as a fraction (for example, 1/24), or as a whole number.

1/2

If the couple has four sons, what is the probability that all four will be born with hemophilia? Express your answer as a fraction (for example, 1/24), or as a whole number.

1/16

Sex determination in animals

The mechanism of sex determination varies with different animal species. There are various chromosomal systems for sex determination found in animals.