Essential Cell Biology 5th edition

Answers A:51
large amounts of alcohol that have to be metabolized. This
need for more liver cells activates the control mechanisms
that normally regulate cell proliferation. Unless badly
damaged and full of scar tissue, the liver will usually shrink
back to a normal size after the patient stops drinking
excessively. In liver cancer, in contrast, mutations abolish
normal cell proliferation control and, as a result, cells divide
and keep on dividing in an uncontrolled manner, which is
usually fatal.
Chapter 19
ANSWER 19–1 After the first meiotic division, each nucleus
has a diploid amount of DNA; however, that DNA effectively
contains only a haploid set of chromosomes (albeit in two
copies), representing only one or other homolog of each
type of chromosome (although some mixing will have
occurred during crossing-over). Because the maternal and
paternal chromosomes of a pair will carry different versions
of many of the genes, these daughter cells will not be
genetically identical; each one will, however, have lost either
the paternal or the maternal version of each chromosome.
In contrast, somatic cells dividing by mitosis inherit a diploid
set of chromosomes, and all daughter cells are genetically
identical and inherit both maternal and paternal gene
copies. The role of gametes produced by meiosis is to mix
and reassort gene pools during sexual reproduction, and
thus it is a definite advantage for each of them to have a
slightly different genetic constitution. The role of somatic
cells on the other hand is to build an organism that contains
the same genes in all its cells and retains in each cell both
maternal and paternal genetic information.
ANSWER 19–2 A typical human female produces fewer
than 1000 mature eggs in her lifetime (12 per year over
about 40 years); this is less than one-tenth of a percent
of the possible gametes, excluding the effects of meiotic
crossing-over. A typical human male produces billions of
sperm during a lifetime, so in principle, every possible
chromosome combination is sampled many times.
ANSWER 19–3 For simplicity, consider the situation
where a father carries genes for two dominant traits, M
and N, on one of his two copies of human Chromosome 1.
If these two genes were located at opposite ends of this
chromosome, and there was one and only one crossover
event per chromosome as postulated in the question, half
of his children would express trait M and the other half
would express trait N—with no child resembling the father
in carrying both traits. This is very different from the actual
situation, where there are multiple crossover events per
chromosome, causing the traits M and N to be inherited as
if they were on separate chromosomes. By constructing a
Punnett square like that in Figure 19−27, one can see that
in this latter, more realistic case, we would actually expect
one-fourth of the children of this father to inherit both traits,
one-fourth to inherit trait M only, one-fourth to inherit trait
N only, and one-fourth to inherit neither trait.
ANSWER 19–4 Inbreeding tends to give rise to individuals
who are homozygous for many genes. To see why, consider
the extreme case where the consanguineous relationship
takes the form of brother–sister inbreeding (as among
the Pharaohs of ancient Egypt): because the parents are
closely related, there is a high probability that the maternal
and paternal alleles inherited by the offspring will be the
same. Inbreeding continued over many generations gives
rise to individuals who are homozygous for almost every
gene. Because of the randomness of the mechanism of
inheritance, some deleterious alleles will become prevalent
in the descendants. If the gene is important, individuals
that inherit two defective copies will be unhealthy—often
severely so. In another, separate inbred population, the
same thing will happen, but chances are a different set of
deleterious alleles will become prevalent. When individuals
from the two separate inbred populations mate, their
offspring will inherit deleterious alleles of genes A, B, and
C, for example, from the mother, but functional alleles of
those genes from the father; conversely, they will inherit
deleterious alleles of genes D, E, and F from the father, but
functional alleles of those genes from the mother. Because
most deleterious mutations are recessive, the hybrid
offspring—who are heterozygous for these genes—will thus
escape the deleterious effects.
ANSWER 19–5 Although any one of the three explanations
could in principle account for the observed result, A and B
can be ruled out as being implausible.
A. There is no precedent for any instability in DNA so great
as to be detectable in such a SNP analysis; in any case,
the hypothesis would predict a steady decrease in the
frequency of the SNP with age, not a drop in frequency
that begins only at age 50.
B. Human genes change only very slowly over time (unless
a massive population migration brings an influx of
individuals who are genetically different). People born
50 years ago will be, on average, virtually the same
genetically as the population being born today.
C. This hypothesis is correct. A SNP with these properties
has been used to discover a gene that appears to cause
a substantial increase in the probability of death from
cardiac abnormalities.
ANSWER 19–6 Natural selection alone is not sufficient
to eliminate recessive lethal genes from the population.
Consider the following line of reasoning. Homozygous
defective individuals can arise only as the offspring of a
mating between two heterozygous individuals. By the rules
of Mendelian genetics, offspring of such a mating will be
in the ratio of 1 homozygous normal: 2 heterozygous: 1
homozygous defective. Thus, statistically, heterozygous
individuals should always be more numerous than the
homozygous, defective individ uals. And although natural
selection effectively eliminates the defective genes in
homozygous individuals through death, it cannot act to
eliminate the defective genes in heterozygous individuals
because they do not affect the phenotype. Natural selection
will keep the frequency of the defective gene low in
the population, but, in the absence of any other effect,
there will always be a reservoir of defective genes in the
heterozygous individuals.
At low frequencies of the defective gene, another
important factor—chance—comes into play. Chance
variation can increase or decrease the frequency of
heterozygous individuals (and thereby the frequency of
the defective gene). By chance, the offspring of a mating
between heterozygotes could all be normal, which would
eliminate the defective gene from that lineage. Increases