chapter 15, 16

Question 1

When Thomas Hunt Morgan crossed his red-eyed F1 generation flies to each other, the F2 generation included both red- and white-eyed flies. Remarkably, all the white-eyed flies were male. What was the explanation for this result?

Answer 1

The gene involved is on the X chromosome.

Question 2

Sturtevant provided genetic evidence for the existence of four pairs of chromosomes in Drosophila in which of these ways

Answer 2

Drosophila genes cluster into four distinct groups of linked genes.

Question 3

Which of the following is the meaning of the chromosome theory of inheritance as expressed in the early 20th century?

Answer 3

Mendelian genes are at specific loci on the chromosome and in turn segregate during meiosis.

Question 4

Thomas Hunt Morgan’s choice of Drosophila melanogaster has been proven to be useful even today. Which of the following has/have continued to make it a most useful species?

I. its four pairs of chromosomes
II. a very large number of visible as well as biochemically mutant phenotypes
III. easy and inexpensive maintenance
IV. short generation time and large number of offspring

Answer 4

I, II, III, IV, and V

Question 5

A woman is found to have 47 chromosomes, including three X chromosomes. Which of the following describes her expected phenotype?

Answer 5

normal female

Question 6

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

Answer 6

males are hemizygous for the X chromosome.

Question 7

SRY is best described in which of the following ways?

Answer 7

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

Question 8

In cats, black fur color is caused by an X-linked allele; the other allele at this locus causes orange color. The heterozygote is tortoiseshell. What kinds of offspring would you expect from the cross of a black female and an orange male?

Answer 8

tortoiseshell females; black males

Question 9

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?

Answer 9

XCXc and XCY

Question 10

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?

Answer 10

100%

Question 11

Calico cats are female because

Answer 11

a male inherits only one of the two X-linked genes controlling hair color.

Question 12

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?

Answer 12

2:1 male to female

Question 13

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?

Answer 13

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

Question 14

In humans, clear gender differentiation occurs, not at fertilization, but after the second month of gestation. What is the first event of this differentiation?

Answer 14

activation of SRY in male embryos and masculinization of the gonads

Question 15

Duchenne muscular dystrophy (DMD) is caused by a gene on the human X chromosome. The patients have muscles that weaken over time because they have absent or decreased dystrophin, a muscle protein. They rarely live past their 20s. How likely is it for a woman to have this condition?

Answer 15

Very rarely would a woman have this condition; the condition would be due to a chromosome error.

Question 16

Women (and all female mammals) have one active X chromosome per cell instead of two. What causes this?

Answer 16

modification of the XIST gene so that it is active only on one X chromosome, which then becomes inactive

Question 17

Which of the following statements is true of linkage?

Answer 17

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

Question 18

How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?

Answer 18

The two genes are closely linked on the same chromosome.

Question 19

What does a frequency of recombination of 50% indicate?

Answer 19

The two genes are likely to be located on different chromosomes.

Question 20

What is the reason that linked genes are inherited together?

Answer 20

They are located close together on the same chromosome.

Question 21

Three genes at three loci are being mapped in a particular species. Each has two phenotypes, one of which is markedly different from the wild type. The unusual allele of the first gene is inherited with either of the others about 50% of the time. However, the unusual alleles of the other two genes are inherited together 14.4% of the time. Which of the following describes what is happening?

Answer 21

The first gene is assorting independently from the other two that are linked.

Question 22

The centimorgan (cM) is a unit named in honor of Thomas Hunt Morgan. To what is it equal?

Answer 22

1% frequency of recombination between two genes

Question 23

Recombination between linked genes comes about for what reason?

Answer 23

Crossovers between these genes result in chromosomal exchange.

Question 24

Why does recombination between linked genes continue to occur?

Answer 24

New allele combinations are acted upon by natural selection.

Question 25

Map units on a linkage map cannot be relied upon to calculate physical distances on a chromosome for which of the following reasons?

Answer 25

The frequency of crossing over varies along the length of the chromosome.

Question 26

Which of the following two genes are closest on a genetic map of Drosophila?

Answer 26

b and rb

Question 27

If nondisjunction occurs in meiosis II during gametogenesis, what will be the result at the completion of meiosis?

Answer 27

1/4 of the gametes will be n + 1, 1/4 will be n - 1, and 1/2 will be n.

Question 28

) One possible result of chromosomal breakage is for a fragment to join a nonhomologous chromosome. What is this alteration called?

Answer 28

translocation

Question 29

A nonreciprocal crossover causes which of the following products?

Answer 29

deletion and duplication

Question 30

In humans, male-pattern baldness is controlled by an autosomal gene that occurs in two allelic forms. Allele Hn determines nonbaldness, and allele Hb determines pattern baldness. In males, because of the presence of testosterone, allele Hb is dominant over Hn. If a man and woman both with genotype HnHb have a son, what is the chance that he will eventually be bald?

Answer 30

75%

Question 31

Of the following human aneuploidies, which is the one that generally has the most severe impact on the health of the individual?

Answer 31

47, +21

Question 32

A phenotypically normal prospective couple seeks genetic counseling because the man knows that he has a translocation of a portion of his chromosome 4 that has been exchanged with a portion of his chromosome 12. Although he is normal because his translocation is balanced, he and his wife want to know the probability that his sperm will be abnormal. What is your prognosis regarding his sperm?

Answer 32

1/4 will be normal, 1/4 will have the translocation, and 1/2 will have duplications and deletions.

Question 33

Abnormal chromosomes are frequently found in malignant tumors. Errors such as translocations may place a gene in close proximity to different control regions. Which of the following might then occur to make the cancer worse?

Answer 33

expression of inappropriate gene products

Question 34

An inversion in a human chromosome often results in no demonstrable phenotypic effect in the individual. What else may occur?

Answer 34

Some abnormal gametes may be formed.

Question 35

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

Answer 35

nondisjunction or translocation in either parent

Question 36

Down syndrome has a frequency in the U.S. population of ~1/700 live births. In which of the following groups would you expect this frequency to be significantly higher?

Answer 36

No groups have such higher frequency.

Question 37

A couple has a child with Down syndrome. The mother is 39 years old at the time of delivery. Which of the following is the most probable cause of the child’s condition?

Answer 37

One member of the couple underwent nondisjunction in gamete production.

Question 38

In 1956 Tijo and Levan first successfully counted human chromosomes. What is the reason it took so many years to do so?

Answer 38

Chromosomes were piled up on top of one another in the nucleus, chromosomes were not distinguishable during interphase, and a method had not yet been devised to halt mitosis at metaphase.

Question 39

At which phase(s) is it preferable to obtain chromosomes to prepare a karyotype?*

Answer 39

late prophase or metaphase

Question 40

What is a syndrome?

Answer 40

a group of traits typically found in conjunction with a particular chromosomal aberration or gene mutation

Question 41

Which of the following is known as a Philadelphia chromosome?

Answer 41

a human chromosome 22 that has had a specific translocation

Question 42

At what point in cell division is a chromosome lost so that, after fertilization with a normal gamete, the result is an embryo with 45, X?*
I. an error in anaphase I
II. an error in anaphase II
III. an error of the first postfertilization mitosis
IV. an error in pairing

Answer 42

I, II, III, or IV

Question 43

Which of the following is true of aneuploidies in general?

Answer 43

45 X is the only known human live-born monosomy.

Question 44

A gene is considered to be non-Mendelian in its inheritance pattern if it seems to “violate” Mendel’s laws. Which of the following would be considered Mendelian?

Answer 44

gene transmitted to males from the maternal line and from fathers to daughters

Question 45

Genomic imprinting is generally due to the addition of methyl (–CH3) groups to C nucleotides in order to silence a given gene. If this depends on the sex of the parent who transmits the gene, which of the following must be true?

Answer 45

*Methylation must be reversible in ovarian and testicular cells

Question 46

Correns described that the inheritance of variegated color on the leaves of certain plants was determined by the maternal parent only. What phenomenon does this describe?

Answer 46

chloroplast inheritance

Question 47

Mitochondrial DNA is primarily involved in coding for proteins needed for electron transport. Therefore, in which body systems would you expect most mitochondrial gene mutations to be exhibited?

Answer 47

the nervous and muscular systems

Question 48

A certain kind of snail can have a right-handed direction of shell coiling (D) or left-handed coiling (d). If direction of coiling is due to a protein deposited by the mother in the egg cytoplasm, then a Dd egg-producing snail and a dd sperm-producing snail will have offspring of which genotype(s) and phenotype(s)?8

Answer 48

1/2 Dd:1/2 dd; all right coiling

Question 49

Which of the following produces a Mendelian pattern of inheritance

Answer 49

a trait acted upon by many genes

Question 50

Suppose that a gene on human chromosome 18 can be imprinted in a given pattern in a female parent but not in a male parent. A couple in whom each maternal meiosis is followed by imprinting of this gene have children. What can we expect as a likely outcome?

Answer 50

All the children will bear their mother’s imprinting pattern but only daughters will then pass it down.

Question 51

In his transformation experiments, what did Griffith observe?*

Answer 51

Mixing a heat-killed pathogenic strain of bacteria with a living nonpathogenic strain can convert some of the living cells into the pathogenic form.

Question 52

How do we describe transformation in bacteria?

Answer 52

assimilation of external DNA into a cell

Question 53

In trying to determine whether DNA or protein is the genetic material, Hershey and Chase made use

Answer 53

DNA contains phosphorus, whereas protein does not.

Question 54

Which of the following investigators was/were responsible for the following discovery?
In DNA from any species, the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine.

Answer 54

Erwin Chargaff

Question 55

Cytosine makes up 42% of the nucleotides in a sample of DNA from an organism. Approximately what percentage of the nucleotides in this sample will be thymine?

Answer 55

8%

Question 56

Which of the following can be determined directly from X-ray diffraction photographs of crystallized DNA?

Answer 56

the diameter of the helix