Chapter 15
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?
The gene involved is on the X chromosome.
Sturtevant provided genetic evidence for the existence of four pairs of chromosomes in Drosophila in which of these ways?
Drosophila genes cluster into four distinct groups of linked genes.
Which of the following is the meaning of the chromosome theory of inheritance as expressed in the early 20th century?
Mendelian genes are at specific loci on the chromosome and in turn segregate during meiosis.
Thomas Hunt Morganʹs 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?
- its four pairs of chromosomes
- a very large number of visible as well as biochemically mutant phenotypes
- easy and inexpensive maintenance
- short generation time and large number of offspring
I, II, III, IV, and V
A woman is found to have 47 chromosomes, including three X chromosomes. Which of the following describes her expected phenotype?
normal female
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
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?
tortoiseshell females; black males
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?
XCXc and XCY
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%
Calico cats are female because
a male inherits only one of the two X-linked genes 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
In humans, clear gender differentiation occurs, not at fertilization, but after the second month of gestation. What is the first event of this differentiation?
activation of SRY in male embryos and masculinization of the gonads
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?
Very rarely would a woman have this condition; the condition would be due to a chromosome error
Women (and all female mammals) have one active X chromosome per cell instead of two. What causes this?
modification of the XIST gene so that it is active only on one X chromosome, which then becomes inactive
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.
How would one explain a testcross involving F1 dihybrid flies in which more parental-type offspring than recombinant-type offspring are produced?
The two genes are closely linked on the same chromosome.
What does a frequency of recombination of 50% indicate?
The two genes are likely to be located on different chromosomes.
What is the reason that linked genes are inherited together?
They are located close together on the same chromosome.
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?
The first gene is assorting independently from the other two that are linked.
The centimorgan (cM) is a unit named in honor of Thomas Hunt Morgan. To what is it equal?
1% frequency of recombination between two genes
Recombination between linked genes comes about for what reason?
Crossovers between these genes result in chromosomal exchange.
Why does recombination between linked genes continue to occur?
New allele combinations are acted upon by natural selection.
Map units on a linkage map cannot be relied upon to calculate physical distances on a chromosome for which of the following reasons?
The frequency of crossing over varies along the length of the chromosome.
Which of the following two genes are closest on a genetic map of Drosophila?
b and rb
If nondisjunction occurs in meiosis II during gametogenesis, what will be the result at the completion of meiosis?
1/4 of the gametes will be n + 1, 1/4 will be n - 1, and 1/2 will be n.
One possible result of chromosomal breakage is for a fragment to join a nonhomologous chromosome. What is this alteration called?
Translocation
A nonreciprocal crossover causes which of the following products?
deletion and duplication
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?
75%
Of the following human aneuploidies, which is the one that generally has the most severe impact on the health of the individual?
47, +21
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?
1/4 will be normal, 1/4 will have the translocation, and 1/2 will have duplications and deletions.
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?
expression of inappropriate gene products
An inversion in a human chromosome often results in no demonstrable phenotypic effect in the individual. What else may occur?
Some abnormal gametes may be formed.
What is the source of the extra chromosome 21 in an individual with Down syndrome?
nondisjunction or translocation in either parent
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?
No groups have such higher frequency.
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 childʹs condition?
One member of the couple underwent nondisjunction in gamete production.
In 1956 Tijo and Levan first successfully counted human chromosomes. What is the reason it took so many years to do so?
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.
At which phase(s) is it preferable to obtain chromosomes to prepare a karyotype?
late prophase or metaphase
What is a syndrome?
a group of traits typically found in conjunction with a particular chromosomal aberration or gene mutation
Which of the following is known as a Philadelphia chromosome?
a human chromosome 22 that has had a specific translocation
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 post fertilization mitosis
- IV. an error in pairing
I, II, III, or IV
Which of the following is true of aneuploidies in general?
45 X is the only known human live-born monosomy.
A gene is considered to be non-Mendelian in its inheritance pattern if it seems to ʺviolateʺ Mendelʹs laws. Which of the following would be considered Mendelian?
a gene transmitted to males from the maternal line and from fathers to daughters
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?
Methylation must be reversible in ovarian and testicular cells.
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?
chloroplast inheritance
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?
the nervous and muscular systems
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)?
1/2 Dd:1/2 dd; all right coiling
Which of the following produces a Mendelian pattern of inheritance?
a trait acted upon by many genes
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?
All the children will bear their motherʹs imprinting pattern but only daughters will then pass it down.
Between which two genes would you expect the highest frequency of recombination?
A and G
In a series of mapping experiments, the recombination frequencies for four different linked genes of Drosophila were determined as shown in Figure 15.2. What is the order of these genes on a chromosome map?
b-rb-cn-vg
The pedigree in Figure 15.3 shows the transmission of a trait in a particular family. Based on this pattern of transmission, the trait is most likely...
mitochondrial
How many of their daughters might be expected to be color-blind dwarfs?
none
What proportion of their sons would be color-blind and of normal height?
half
They have a daughter who is a dwarf with normal color vision. What is the probability that she is heterozygous for both genes?
100%
Which of the following are the phenotypes of the parents in this cross?
4 and 8
In which progeny phenotypes has there been recombination between genes A and B?
1, 2, 5, and 6
If recombination is equal to distance in centimorgans (cM), what is the approximate distance between genes A and B?
3 cM
What is the greatest benefit of having used a testcross for this experiment?
Progeny can be scored by their phenotypes alone.
The greatest distance among the three genes is between a and c. What does this mean?
Gene a is between b and c