Essential Cell Biology 5th edition

A:52 Answers
in the frequency of a deleterious gene are opposed by
natural selection; however, decreases are unopposed and
can, by chance, lead to elimination of the defective gene
from the population. On the other hand, new mutations
are continually occurring, albeit at a low rate, creating
fresh copies of the deleterious recessive allele. In a large
population, a balance will be struck between the creation
of new copies of the allele in this way, and their elimination
through the death of homozygotes.
ANSWER 19–7
A. True.
B. True.
C. False. Mutations that occur during meiosis can be
propagated, unless they give rise to nonviable gametes.
ANSWER 19–8 In mitosis, two copies of the same
chromosome can end up in the same daughter cell if
one of the microtubule connections breaks before sister
chromatids are separated. Alternatively, microtubules from
the same spindle pole could attach to both kinetochores
of the chromosome. As a consequence, one daughter
cell would receive only one copy of all the genes carried
on that chromosome, and the other daughter cell would
receive three copies. The imbalance of the genes on this
chromosome compared with the genes on all the other
chromosomes would produce imbalanced levels of protein
which, in most cases, is detrimental to the cell. If the
mistake happens during meiosis, in the process of gamete
formation, it will be propagated in all cells of the organism.
A form of mental retardation called Down syndrome,
for example, is due to the presence of three copies of
Chromosome 21 in all of the nucleated cells in the body.
ANSWER 19–9 Meiosis begins with DNA replication,
producing a tetraploid cell containing four copies of each
chromosome. These four copies have to be distributed
equally during the two sequential meiotic divisions into four
haploid cells. Sister chromatids remain paired so that (1) the
cells resulting from the first division receive two complete
sets of chromosomes and (2) the chromosomes can be
evenly distributed again in the second meiotic division. If
the sister chromatids did not remain paired, it would not
be possible in the second division to distinguish which
chromatids belong together, and it would therefore be
difficult to ensure that precisely one copy of each chromatid
is pulled into each daughter cell. Keeping two sister
chromatids paired in the first meiotic division is therefore
an easy way to keep track of which chromatids belong
together.
This biological principle suggests that you might
consider clamping your socks together in matching pairs
before putting them into the laundry. In this way, the
cumbersome process of sorting them out afterward—and
the seemingly inevitable mistakes that occur during that
process—could be avoided.
ANSWER 19–10
A. A gene is a stretch of DNA that codes for a protein
or functional RNA. An allele is an alternative form of
a gene. Within the population, there are often several
“normal” alleles, whose functions are indistinguishable.
In addition, there may be many rare alleles that are
defective to varying degrees. An individual, however,
normally carries a maximum of two alleles of each gene.
B. An individual is said to be homozygous if the two alleles
of a gene are the same. An individual is said to be
heterozygous if the two alleles of a gene are different.
An individual can be heterozygous for gene A and
homozygous for gene B.
C. The genotype is the specific set of alleles present in
the genome of an individual. In practice, for organisms
studied in a laboratory, the genotype is usually specified
as a list of the known differences between the individual
and the wild type, which is the standard, naturally
occurring type. The phenotype is a description of the
visible characteristics of the individual. In practice, the
phenotype is usually a list of the differences in visible
characteristics between the individual and the wild type.
D. An allele A is dominant (relative to a second allele a) if
the presence of even a single copy of A is enough to
affect the phenotype; that is, if heterozygotes (with
genotype Aa) appear different from aa homozygotes.
An allele a is recessive (relative to a second allele A)
if the presence of a single copy makes no difference
to the phenotype, so that Aa individuals look just like
AA individuals. If the phenotype of the heterozygous
individual differs from the phenotypes of individuals that
are homozygous for either allele, the alleles are said to
be co-dominant.
ANSWER 19–11
A. Since the pea plant is diploid, any true-breeding plant
must carry two mutant copies of the same gene—both
of which have lost their function.
B. If each plant carries a mutation in a different gene, this
will be revealed by complementation tests (see Panel
19−1, p. 675). When plant A is crossed with plant B, all
of the F 1 plants will produce only round peas. And the
same result will be obtained when plant B is crossed
with plant C, or when plant A is crossed with plant C. In
contrast, a cross between any two true-breeding plants
that carry loss-of-function mutations in the same gene
should produce only plants with wrinkled peas. This is
true if the mutations themselves lie in different parts of
the gene.
ANSWER 19–12
A. The mutation is likely to be dominant, because roughly
half of the progeny born to an affected parent—in
each of three marriages to hearing partners—are deaf,
and it is unlikely that all these hearing partners were
heterozygous carriers of the mutation.
B. The mutation is not present on a sex chromosome. If it
were, either only the female progeny should be affected
(expected if the mutation arose in a gene on the
grandfather’s X chromosome), or only the male progeny
should be affected (expected if the mutation arose in
a gene on the grandfather’s Y chromosome). In fact,
the pedigree reveals that both males and females have
inherited the mutant form of the gene.
C. Suppose that the mutation was present on one of the
two copies of the grandfather’s Chromosome 12. Each
of these copies of Chromosome 12 would be expected
to carry a different pattern of SNPs, since one of them
was inherited from his father and the other was inherited
from his mother. Each of the copies of Chromosome
12 that was passed to his grandchildren will have gone
through two meioses—one meiosis per generation.
Because two or three crossover events occur per