ANSWERS TO REVIEW QUESTIONS
CHAPTER 13
1. Essential parts of a chromosome are telomeres, centromere, and origin of replication sites. A
centromere includes repeats of alpha satellites, centromere-associated proteins, and
centromere protein A.
2. Protein-encoding genes become denser from the telomeres inward toward the centromere.
3. Centromeres and telomeres contribute to chromosome stability and have many repeats.
4. A euploid cell has the normal chromosome number. Aneuploidy refers to the loss or gain of
individual chromosomes. Polyploid cells have extra chromosome sets.
5.
a. Homologs do not separate in meiosis I or II, leading to a gamete with an extra or missing
chromosome.
b. DNA replicates, but is not apportioned into daughter cells, forming a diploid gamete.
c. Increased tendency for nondisjunction in the chromosome 21 pair.
d. Crossing over in the male yields unbalanced gametes, which can fertilize oocytes, but too
much or too little genetic material halts development.
e. A gamete that includes just one translocated chromosome will have too much of part of
the chromosome and too little of other parts. Excess chromosome 21 material causes
Down syndrome.
6. Patches of octaploid cells in liver tissue may arise as a result of abnormal mitosis in a few
liver cells early in development.
7. a. A XXX individual has no symptoms, but she may conceive sons with XXY syndrome by
producing XX oocytes.
b. A female with XO Turner syndrome has wide-set nipples, flaps of skin on the neck, and
no secondary sexual development.
c. A female with trisomy 21 Down syndrome. Phenotype includes short, sparse, straight
hair, wide-set eyes with epicanthal folds, a broad nose, protruding tongue, mental
retardation, and increased risk of a heart defect, suppressed immunity, and leukemia.
8. Basketball players may have an extra Y chromosome that makes them tall.
9. A trisomy is one extra chromosome; a triploid is one extra chromosome set.
10. A trisomy is an additional chromosome. Mosaic trisomy has an additional chromosome in
some cell populations. Translocation moves a chromosome segment.
11. Most triploids have severe abnormalities. Trisomy 21 is the least severe trisomy, and involves
the smallest chromosome. Klinefelter syndrome symptoms are worse if there is more than
one extra X chromosome.
12. A balanced translocation causes duplications or deletions when a gamete contains one
translocated chromosome, plus has extra or is missing genes from one of the chromosomes
involved in the translocation. A paracentric or pericentric inversion can cause duplications or
deletions if a crossover occurs between the inverted chromosome and its homolog.
Isochromosomes result from centromere splitting in the wrong plane, duplicating one
chromosome arm but deleting the other.
13. In a Robertsonian translocation, the two long arms of nonhomologous chromosomes fuse,
creating one large translocation chromosome. The short arms are lost. In a reciprocal
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translocation, two nonhomologous chromosomes exchange parts. Insertional translocations
insert a sequence from one chromosome into a nonhomologous chromosome.
14. Chromosomes would not contort during meiosis because their genes are aligned.
15. a. FISH: Fluorescently-labeled DNA probes bind homologous regions on chromosomes.
b. Amniocentesis: Fetal cells and fluid are removed from around a fetus. Cells are cultured
and their chromosomes stained or exposed to DNA probes, and karyotyped.
c. Chorionic villus sampling: Chromosomes in chorionic villus cells are directly karyotyped.
d. Fetal cell sorting: A fluorescence activated cell sorter separates fetal from maternal cells,
and fetal chromosomes are karyotyped.
16. Trisomies 5 and 16 are lethal before birth.
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18. A female cannot have Klinefelter syndrome because the presence of the Y chromosome
containing the SRY gene determines sex. A male cannot have Turner syndrome because
there is no Y chromosome to stimulate male development.
19. Nondisjunction in oogenesis, ring chromosome or isochromosome X, or large deletions of the
X chromosome.
20. Some genes are missing and some are present in two copies.
ANSWERS TO APPLIED QUESTIONS
1. Telomeres, centromere, and origin of replication sites
2. The person is a girl missing part of the short arm of chromosome 5. This is Cri-du- chat
syndrome, and she will be mentally retarded with a cat-like cry.
3. FISH
4. a. Reciprocal translocation
b. She doesn't have extra or missing genes.
c. She might have a child with translocation Down syndrome.
5. At the second mitotic division, replicated chromosomes failed to separate, yielding one of four
cells with an extra two sets of chromosomes.
6. Down syndrome caused by aneuploidy produces an extra chromosome 21 in each cell. In
mosaic Down syndrome, the extra chromosome is only in some cells. In translocation Down
syndrome, unbalanced gametes lead to an individual with extra chromosome 21 material in
each cell.
7. The XXY son could have gotten two X chromosomes from his mother, or an XY-bearing
sperm from his father.
8. a. The translocation carrier can produce an unbalanced gamete that lacks chromosome 22
material.
b. The microdeletion may be more extensive than the deleted region in the translocation
individuals.
c. Translocation family members might be infertile or have offspring with birth defects.
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9. Paracentric inversion
10. If several individuals with the translocation have children only among themselves, a species
with 22 different chromosomes could arise, rather than with 23 chromosome types.
ANSWERS TO WEB ACTIVITIES
1. The Trisomy 18 Foundation website describes how families deal with difficulty feeding
(perhaps using a feeding tube), heart and kidney defects, fluid on the brain, and occupational
therapy.
2 a. Example: Cri-du-Chat syndrome
b. Deletion at 5p15.2
c. Severe, mental and physical abnormalities. High pitched cry
3. Students create a karyotype.
4. Chromosome 13:
cataract-clouded lens, MIM 601885
deafness, OMIM 220290
leukemia/lymphoma  blood cancer, MIM 602221
ectodermal dysplasia  no hair or nails, dark skin, MIM 129500
ANSWERS TO CASE STUDIES AND RESEARCH RESULTS
1. a. Total number of chromosomes and number of copies of chromosome 2
b. Nondisjunction
c. Genes on chromosome 2 produce more severe mental and physical disabilities
when trisomic
d. When the nondisjunction occurred and which cells are mosaic
2. One of the Watkins probably has a balanced translocation, because there is more than one
Down syndrome case. The two spontaneous abortions were the result of unbalanced
gametes. Their problems are likely to repeat with a predictable and high frequency, because
the translocated chromosome is in half of the carrier parent's gametes. In contrast, the
Phelps' child with Down syndrome is more likely the result of nondisjunction, which is unlikely
to repeat. The Phelps child has trisomy 21 Down syndrome; the Watkins' child may only have
a partial extra copy of chromosome 21.
3. a. The long arm
b. Crossover between the inverted and noninverted homologs
c. Deletion
4. No. The parents are healthy.
5. a. Interstitial duplication
b. He has the normal amount of genetic material.
c. Normal for both chromosomes or 10, 10q- and 17q+, 17 like Andre
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