1) The condition that exists when an organism gains or loses one or more chromosomes but not a complete haploid set is known as ________.
- A) polyploidy
- B) euploidy
- C) aneuploidy
- D) triploidy
- E) trisomy
Answer: C
Section: 6.1
2) Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. Although fertility is reduced in both sexes, females have higher fertility rates than males. Van Dyke et al. (1995; Down Syndrome Research and Practice 3[2]:65-69) summarize data involving children born of Down syndrome individuals. Assume that children are born to a female with Down syndrome and a normal 46-chromosome male. What proportion of the offspring would be expected to have Down syndrome?
- A) One-third of the offspring would be expected to have Down syndrome.
- B) Two-thirds of the offspring would be expected to have Down syndrome.
- C) All the children would be expected to have Down syndrome.
- D) None of the offspring would be expected to have Down syndrome.
- E) One-half of the offspring would be expected to have Down syndrome.
Answer: E
Section: 6.2
3) Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. Although fertility is reduced in both sexes, females have higher fertility than males. Van Dyke et al. (1995; Down Syndrome Research and Practice 3[2]:65-69) summarize data involving children born of Down syndrome individuals. Given the fact that conceptuses with 48 chromosomes (four chromosome 21s) are not likely to survive early development, what percentage of surviving offspring would be expected to have Down syndrome if both parents have Down syndrome?
- A) One-third of the surviving offspring would be expected to have Down syndrome.
- B) All the children would be expected to have Down syndrome.
- C) None of the surviving offspring would be expected to have Down syndrome.
- D) Two-thirds of the surviving offspring would be expected to have Down syndrome.
- E) One-half of the surviving offspring would be expected to have Down syndrome.
Answer: D
Section: 6.2
4) The condition known as cri-du-chat syndrome in humans has a genetic constitution designated as ________.
- A) 45, X
- B) heteroplasmy
- C) 46, 5p-
- D) triploidy
- E) trisomy
Answer: C
Section: 6.5
5) Although the most frequent forms of Down syndrome are caused by a random error, nondisjunction of chromosome 21, Down syndrome occasionally runs in families. The cause of this form of familial Down syndrome is ________.
- A) an inversion involving chromosome 21
- B) a chromosomal aberration involving chromosome 1
- C) too many X chromosomes
- D) a translocation between chromosome 21 and a member of the D chromosome group
- E) a maternal age effect
Answer: D
Section: 6.8
6) A genomic condition that may be responsible for some forms of fragile-X syndrome, as well as Huntington disease, involves ________.
- A) F plasmids inserted into the FMR-1 gene
- B) various lengths of trinucleotide repeats
- C) multiple breakpoints fairly evenly dispersed along the X chromosome
- D) multiple inversions in the X chromosome
- E) single translocations in the X chromosome
Answer: B
Section: 6.9
7) Recently, a gene located on chromosome 3 in humans, FHIT, has been shown to be associated with the significant human malady known as ________.
- A) cancer
- B) Huntington disease
- C) “mad-cow” disease
- D) Klinefelter syndrome
- E) XYY/XY mosaicism
Answer: A
Section: 6.9
8) What explanation is generally given for lethality of monosomic individuals?
Answer: Monosomy may unmask recessive lethals that are tolerated in heterozygotes carrying the wild-type allele.
Section: 6.2
9) Describe the maternal age effect associated with Down syndrome.
Answer: For unknown reasons, the nondisjunctional event that produces Down syndrome occurs more frequently during oogenesis in women older than age 35.
Section: 6.2
10) In what way might gene duplication play a role in evolution?
Answer: In 1970, Ohno proposed that gene duplication provides a way in which new genes arise. By duplicating a gene, the duplicated copy or the original gene is able to mutate without necessarily having an adverse influence on the phenotype.
Section: 6.2
11) Name two methods used in genetic prenatal diagnostic testing in humans.
Answer: amniocentesis and chorionic villus sampling (CVS)
Section: 6.2
12) Trisomics are observed in humans; monosomics are not. Why?
Answer: Monosomics are inviable. Such haploinsufficiency combines the loss of multiple genes.
Section: 6.2
13) Name the polyploid condition that is formed from the addition of one or more extra sets of chromosomes identical to the normal haploid complement of the same species.
Answer: autotetraploidy, assuming the normal chromosome complement is diploid
Section: 6.3
14) Given that a human normally contains 46 chromosomes, give the chromosome number for each of the following conditions:
Turner syndrome (female, no Barr bodies)
Klinefelter syndrome (male, one Barr body)
triploid
Down syndrome (trisomic)
trisomy 13
Answer:
Turner syndrome (female, no Barr bodies) 45
Klinefelter syndrome (male, one Barr body) 47
triploid 69
Down syndrome (trisomic) 47
trisomy 13 47
Section: 6.3
15) Colchicine is an alkaloid derived from plants. What is its effect on chromosome behavior?
Answer: By interfering with spindle formation, replicated chromosomes fail to migrate to the poles at anaphase; thus, sister chromatids end up in the same nucleus.
Section: 6.3
16) Deletions are chromosomal aberrations in which some portion of a chromosome is missing. Describe a method using Drosophila deletions to determine the actual, physical location of a gene.
Answer: Cross homozygous mutant flies to homozygous wild-type flies that have been irradiated (or those with a series of known deletions). Select mutant flies in the F1. Those of the F1 that display the mutant phenotype may have resulted from the wild-type allele being deleted by the X-ray treatment. Establish a stock of the exceptional mutant fly; then examine polytene chromosomes in larvae. The mutation in question may be contained in the compensation loop in the homolog of the deleted chromosome. If a series of known deletions is available (and these do exist in stock centers throughout the world), one can test (by the mating described above) the mutation against each deletion. Obviously, this would be a time-consuming task unless one had some prior knowledge as to the general location of the mutant gene. This can be accomplished using balancer chromosomes and standard linkage determination. Once the mutant gene is “exposed” by a given deletion, a series of additional deletions can be used to “fine map” the region and determine a fairly accurate location for the gene.
Section: 6.5
17) Provide an example of gene redundancy that occurs in both eukaryotes and prokaryotes.
Answer: rDNA
Section: 6.6
18) Describe Bar mutations in Drosophila melanogaster.
Answer: Bar mutations are duplications in portions of the X chromosome.
Section: 6.6
19) Inversion heterokaryotypes are often characterized as having reduced crossing over and reduced fertility. Assume that you were examining a strain of organisms you knew to be inversion heterokaryotypes and saw a relatively high number of double chromatid bridges extending between anaphase I nuclei. What would be a likely explanation for this observation? Explain with a labeled diagram.
Answer: A four-strand double crossover in the inversion loop of a paracentric inversion (in the heterozygous state) would generate the double bridge and two acentric fragments (which would be lost).
Section: 6.7
20) Clearly illustrate the pairing configuration of an inversion (paracentric) heterokaryotype.
Answer: The pairing of homologous chromosomes of an inversion heterokaryotype is typically one of an “outside” loop filled by an “inside loop.”
Section: 6.7
21) What is meant by the terms acentric and dicentric?
Answer: A chromosome without a centromere is acentric; a chromosome with two centromeres is dicentric.
Section: 6.7
22) Under what circumstance can an individual with Down syndrome have 46 chromosomes?
Answer: if he or she carries a D/G translocation, 14/21, for example
Section: 6.8
23) Fragile-X syndrome (or Martin-Bell syndrome) is the most common form of inherited mental retardation in humans. Is it more common in males or females? What is FMR1?
Answer: males (1/4000 compared to 1/8000 in females); FMR1 is one of a growing number of genes in which a sequence of three nucleotides is repeated many times, expanding the size of the gene.
Section: 6.9
24) The term aneuploidy is synonymous with the term segmental deletion.
Answer: FALSE
Section: 6.1
25) Nondisjunction is viewed as a major cause of aneuploidy.
Answer: TRUE
Section: 6.1
26) Assume that a species has a diploid chromosome number of 24. The term applied to an individual with 25 chromosomes would be triploid.
Answer: FALSE
Section: 6.2
27) An expected meiotic pairing configuration in a triploid would be a trivalent.
Answer: TRUE
Section: 6.2
28) An individual with Patau syndrome would be called a triploid.
Answer: FALSE
Section: 6.2
29) Chorionic villus sampling (CVS) is sometimes preferred to amniocentesis because results can be provided earlier in the pregnancy.
Answer: TRUE
Section: 6.2
30) In Drosophila melanogaster (2n = 8), a fly with seven chromosomes could be called a haplo-IV.
Answer: TRUE
Section: 6.2
31) Doubling the chromosomes of a sterile species hybrid with colchicine or cold shock is a method used to produce a fertile species hybrid (amphidiploid).
Answer: TRUE
Section: 6.3
32) Assume that an organism has a diploid chromosome number of 14. There would be 28 chromosomes in a tetraploid.
Answer: TRUE
Section: 6.3
33) An autotriploid may arise when three sperm cells are involved in fertilization of a single egg.
Answer: FALSE
Section: 6.3
34) Assume that an organism has a haploid chromosome number of 7. There would be 14 chromosomes in a monoploid individual of that species.
Answer: FALSE
Section: 6.3
35) Assume that a species has a diploid chromosome number of 24. The term applied to an individual with 36 chromosomes would be triploid.
Answer: TRUE
Section: 6.3
36) A deletion may set up a genetic circumstance known as overdominance.
Answer: FALSE
Section: 6.5
37) Gene duplications provide an explanation for the origin of gene families.
Answer: TRUE
Section: 6.6
38) rDNA in eukaryotes is typically redundant.
Answer: TRUE
Section: 6.6
39) A paracentric inversion is one whose break points do not flank the centromere.
Answer: TRUE
Section: 6.7
40) Inversions suppress crossing over by providing a chemical imbalance because of breakpoints within certain genes.
Answer: FALSE
Section: 6.7
41) A pericentric inversion includes the centromere.
Answer: TRUE
Section: 6.7
42) Individuals with familial Down syndrome are trisomic and have 47 chromosomes.
Answer: FALSE
Section: 6.8
43) Familial Down syndrome can be caused by a translocation between chromosomes 1 and 14.
Answer: FALSE
Section: 6.8
44) Familial Down syndrome is caused by a translocation involving chromosome 21.
Answer: TRUE
Section: 6.8
45) A position effect occurs when a gene’s expression is altered by virtue of a change in its position. One might expect position effects to occur with inversions and translocations.
Answer: TRUE
Section: 6.8
46) Translocations may be pericentric or paracentric.
Answer: FALSE
Section: 6.8
47) Inversions and translocations are without evolutionary significance.
Answer: FALSE
Section: 6.8
48) In general, inversion and translocation heterozygotes are as fertile as organisms whose chromosomes are in the standard arrangement.
Answer: FALSE
Section: 6.8
Essentials of Genetics, 8e (Klug)
Chapter 7 Linkage and Chromosome Mapping in Eukaryotes
1) When two genes fail to assort independently, the term normally applied is ________.
- A) discontinuous inheritance
- B) Mendelian inheritance
- C) linkage
- D) tetrad analysis
- E) dominance and/or recessiveness
Answer: C
Section: Introduction
2) Assume that a cross is made between AaBb and aabb plants and that the offspring fall into approximately equal numbers of the following groups: AaBb, Aabb, aaBb, aabb. These results are consistent with ________.
- A) independent assortment
- B) alternation of generations
- C) complete linkage
- D) incomplete dominance
- E) hemizygosity
Answer: A
Section: 7.1
3) Assume that a cross is made between AaBb and aabb plants and that all the offspring are either AaBb or aabb. These results are consistent with ________.
- A) complete linkage
- B) alternation of generations
- C) codominance
- D) incomplete dominance
- E) hemizygosity
Answer: A
Section: 7.2
4) Assume that a cross is made between AaBb and aabb plants and that the offspring occur in the following numbers: 106 AaBb, 48 Aabb, 52 aaBb, 94 aabb. These results are consistent with ________.
- A) sex-linked inheritance with 30% crossing over
- B) linkage with 50% crossing over
- C) linkage with approximately 33 map units between the two gene loci
- D) independent assortment
- E) 100% recombination
Answer: C
Section: 7.2
5) The phenomenon in which one crossover decreases the likelihood of crossovers in nearby regions is called ________.
- A) chiasma
- B) negative interference
- C) reciprocal genetic exchange
- D) positive interference
- E) mitotic recombination
Answer: D
Section: 7.4
6) Methods for determining the linkage group and genetic map in humans involve ________.
- A) DNA markers
- B) twin spots and tetrad analysis
- C) tetrad analysis and bromodeoxyuridine
- D) zygotene and pachytene DNA synthesis
- E) chiasmatype and classical analyses
Answer: A
Section: 7.5
7) In the early 1900s, two scientists noted that there were many more genes than chromosome pairs, thus setting the stage for the suggestion that some gene loci might be linked during meiotic processes. Who were these two scientists?
Answer: Walter Sutton and Theodor Boveri
Section: Introduction
8) What is the expected evolutionary significance of genetic recombination?
Answer: production of genetic variation
Section: Introduction
9) Under what circumstance might two loci be on the same chromosome but behave as if independently assorting in crosses?
Answer: If the genes are far apart, they may show independent assortment.
Section: 7.1
10) At what stage of the meiotic cell cycle and during what chromosomal configuration does crossing over occur?
Answer: at the four-strand stage of meiosis, after synapsis of homologous chromosomes, and before the end of prophase I
Section: 7.1
11) Diagram chromosomal events that will ultimately result in the segregation of alleles (A and a) during meiosis II rather than meiosis I.
Answer:
Section: 7.2
12) What is the relationship between the degree of crossing over and the distance between two genes?
Answer: It is direct: as the distance increases, the frequency of recombination increases.
Section: 7.2
13) The genes for mahogany eyes and ebony body are approximately 25 map units apart on chromosome III in Drosophila. Assume that a mahogany-eyed female was mated to an ebony-bodied male and that the resulting F1 phenotypically wild-type females were mated to mahogany, ebony males. Of 1000 offspring, what would be the expected phenotypes, and in what numbers would they be expected?
Answer: mahogany = 375; ebony = 375; wild-type = 125; mahogany-ebony = 125
Section: 7.3
14) Assume that there are 12 map units between two loci in the mouse and that you are able to microscopically observe meiotic chromosomes in this organism. If you examined 200 primary oocytes, in how many would you expect to see a chiasma between the two loci?
Answer: 48
Section: 7.3
15) Phenotypically wild-type F1 female Drosophila, whose mothers had light eyes (lt) and fathers had straw (stw) bristles, produced the following offspring when crossed to homozygous light-straw males:
Phenotype Number
light-straw 22
wild-type 18
light 990
straw 970
Total: 2000
Compute the map distance between the light and straw loci.
Answer: 2 map units
Section: 7.3
16) Assume that the genes for tan body and bare wings are 15 map units apart on chromosome II in Drosophila. Assume also that a tan-bodied, bare-winged female was mated to a wild-type male and that the resulting F1 phenotypically wild-type females were mated to tan-bodied, bare-winged males. Of 1000 offspring, what would be the expected phenotypes, and in what numbers would they be expected?
Answer: wild type = 425; tan-bare = 425; tan = 75; bare = 75
Section: 7.3
17) Assume that investigators crossed a strain of flies carrying the dominant eye mutation Lobe on the second chromosome with a strain homozygous for the second chromosome recessive mutations smooth abdomen and straw body. The F1 Lobe females were then backcrossed with homozygous smooth abdomen, straw-body males, and the following phenotypes were observed:
smooth abdomen, straw body 820
Lobe 780
smooth abdomen, Lobe 42
straw body 58
smooth abdomen 148
Lobe, straw body 152
(a) Give the gene order and map units for these three loci.
(b) What is the coefficient of coincidence?
Answer:
(a) Lobe is in the middle.
smooth abdomen—5—Lobe———–15————-straw body
(b) zero
Section: 7.3
18) In Drosophila, assume that the gene for scute bristles (s) is located at map position 0.0 and that the gene for ruby eyes (r) is at position 15.0. Both genes are located on the X chromosome and are recessive to their wild-type alleles. A cross is made between scute-bristled females and ruby-eyed males. Phenotypically wild-type F1 females were then mated to homozygous double mutant males, and 1000 offspring were produced. Give the phenotypes and frequencies expected.
Answer: scute = 425; ruby = 425; wild-type = 75; scute-ruby = 75
Section: 7.3
19) Assume that a cross is made between AaBb and aabb plants and that the offspring occur in the following numbers: 106 AaBb, 48 Aabb, 52 aaBb, 94 aabb. These results are consistent with which arrangement of genes?
Answer: In the AaBb parent, the dominant alleles are on one homolog and the recessive alleles are on the other.
Section: 7.3
20) In the fruit fly, Drosophila melanogaster, a spineless (no wing bristles) female fly is mated to a male that is claret (dark eyes) and hairless (no thoracic bristles). Phenotypically wild-type F1 female progeny were mated to fully homozygous (mutant) males, and the following progeny (1000 total) were observed:
Phenotypes Number Observed
spineless 321
wild-type 38
claret, spineless 130
claret 18
claret, hairless 309
hairless, claret, spineless 32
hairless 140
hairless, spineless 12
(a) Which gene is in the middle?
(b) With respect to the three genes mentioned in the problem, what are the genotypes of the homozygous parents used in making the phenotypically wild-type F1 heterozygote?
(c) What are the map distances for the three genes? A correct formula with the values “plugged in” for each distance will be sufficient.
(d) What is the coefficient of coincidence? A correct formula with the values “plugged in” will be sufficient.
Answer:
(a) hairless
(b) cl h +/cl h + and + + sp/+ + sp
(c) cl——-30—–h—10—sp
(d) 0.03/0.03 = 1
Section: 7.3
21) Three loci, mitochondrial malate dehydrogenase that forms a and b (MDHa, MDHb), glucouronidase that forms 1 and 2 (GUS1, GUS2), and a histone gene that forms + and – (H+, H-) are located on chromosome 7 in humans. Assume that the MDH locus is at position 35, GUS at position 45, and H at position 75. A female whose mother was homozygous for MDHa, GUS2, and H+ and whose father was homozygous for MDHb, GUS1, and H- produces a sample of 1000 egg cells. Give the genotypes and expected numbers of the various types of cells she would produce. Assume no chromosomal interference.
Answer:
MDHa GUS2 H+ = 315 MDHa GUS2 H– = 135
MDHb GUS1 H– = 315 MDHb GUS1 H+ = 135
MDHa GUS1 H– = 35 MDHa GUS1 H+ = 15
MDHb GUS2 H+ = 35 MDHb GUS2 H– = 15
Section: 7.3
22) (a) In a three-point mapping experiment, which three general classes of offspring are expected (assuming crossovers occur)? (b) How many different genotypic classes are expected?
Answer:
(a) noncrossovers, single crossovers, double crossovers
(b) 8
Section: 7.3
23) Assume that two genes are 80 map units apart on chromosome II of Drosophila and that a cross is made between a doubly heterozygous female and a homozygous recessive male. What percent recombination would be expected in the offspring of this type of cross?
Answer: 50% (maximum)
Section: 7.3
24) Provide a brief definition for positive interference.
Answer: A crossover in one region decreases the likelihood of crossovers in nearby regions.
Section: 7.4
25) Given that loci A and B in Drosophila are sex-linked and 20 map units apart, what phenotypic frequencies would you expect in male and female offspring resulting from the following crosses? (Assume that A and B are dominant to a and b, respectively.)
Answer: AaBb (dominant alleles on same chromosome) female X ab/Y male; AaBb (dominant alleles on homologs) female X ab/Y male; aabb female X AB/Y male (no crossing over in male Drosophila)
Section: 7.4
26) Describe a convenient method for determining gene order from three-point cross results.
Answer: Compare the double-crossover class with the parental class and ask which gene has switched places. The gene that switched places is in the middle.
Section: 7.3
27) The coefficient of coincidence reflects the frequency of observed double crossovers compared to the frequency of expected double crossovers. What is the relationship between the coefficient of coincidence and interference?
Answer: Interference is one minus the coefficient of coincidence.
Section: 7.4
28) If interference is complete, what is the frequency of double crossovers?
Answer: zero
Section: 7.4
29) What are two commonly used DNA landmarks for mapping human genes?
Answer: microsatellites and restriction fragment length polymorphisms (RFLPs)
Section: 7.5
30) Two lines of work indicated that crossing over actually involves breakage and reunion of chromatid material. What organisms were involved, and who did the work?
Answer: Creighton and McClintock (corn) and Stern (Drosophila)
Section: 7.6
31) What advantage does BrdU (bromodeoxyuridine) have in the study of chromosome structure and recombination?
Answer: Chromatids stained with BrdU in both DNA strands are distinguishable from those with BrdU in only one strand of the double helix.
Section: 7.6
32) Sister chromatid exchanges increase in frequency in the presence of X rays, certain viruses, ultraviolet light, and certain chemical mutagens. In what autosomal recessive disorder is there known to be an increase in sister chromatid exchanges?
Answer: Bloom syndrome
Section: 7.6
33) If two gene loci are on nonhomologous chromosomes, genes at these loci are expected to assort independently.
Answer: TRUE
Section: 7.1
34) The cross GE/ge X ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that the G and E loci assort independently.
Answer: FALSE
Section: 7.2
35) The cross GE/ge X ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that the recombinant progeny are gE/ge and Ge/ge.
Answer: TRUE
Section: 7.2
36) The cross GE/ge X ge/ge produces the following progeny: GE/ge 404, ge/ge 396, gE/ge 97, Ge/ge 103. From these data, one can conclude that there are 20 map units between the G and E loci.
Answer: TRUE
Section: 7.2
37) Linkage (viewed from results of typical crosses) always occurs when two loci are on the same chromosome.
Answer: FALSE
Section: 7.2
38) Positive interference occurs when a crossover in one region of a chromosome reduces crossovers in nearby regions.
Answer: TRUE
Section: 7.4
39) Mendel predicted that some genes would be carried in the same chromosome.
Answer: FALSE
Section: 7.7
