Essential Cell Biology 5th edition

688 CHAPTER 19 Sexual Reproduction and Genetics
up by crossovers. The relative sizes of haplotype blocks can give
us clues to our evolutionary history and help to identify common
disease-associated alleles.
• Many rare, inherited human diseases are due to mutations in a single
gene.
• The most common human disorders are due to many genes acting
together; DNA sequencing studies are identifying mutations in these
genes that increase the risk of developing these diseases.
KEY TERMS
allele
asexual reproduction
bivalent
chiasma (plural chiasmata)
classical genetic approach
complementation test
crossing-over
diploid
fertilization
gain-of-function mutation
gamete
genetic map
genetic screen
genetics
genotype
germ line
haploid
haplotype block
heterozygous
homolog
homologous recombination
homozygous
law of independent assortment
law of segregation
loss-of-function mutation
meiosis
pairing
pedigree
phenotype
polymorphism
sexual reproduction
sister chromatid
SNP (single-nucleotide
polymorphism)
somatic cell
zygote
QUESTIONS
QUESTION 19–6
It is easy to see how deleterious mutations in bacteria,
which have a single copy of each gene, are eliminated by
natural selection: the affected bacteria die and the mutation
is thereby lost from the population. Eukaryotes, however,
have two copies of most genes—that is, they are diploid.
Often an individual with two normal copies of the gene
(homozygous normal) is indistinguishable in phenotype from
an individual with one normal copy and one defective copy
of the gene (heterozygous). In such cases, natural selection
can operate only against an individual with two copies of
the defective gene (homozygous defective). Consider the
situation in which a defective form of the gene is lethal
when homozygous, but without effect when heterozygous.
Can such a mutation ever be eliminated from the population
by natural selection? Why or why not?
QUESTION 19–7
Which of the following statements are correct? Explain your
answers.
A. The egg and sperm cells of animals contain haploid
genomes.
B. During meiosis, chromosomes are allocated so that each
germ cell obtains one and only one copy of each of the
different chromosomes.
C. Mutations that arise during meiosis are not transmitted
to the next generation.
QUESTION 19–8
What might cause chromosome nondisjunction, where
two copies of the same chromosome end up in the same
daughter cell? What could be the consequences of this
event occurring (a) in mitosis and (b) in meiosis?
QUESTION 19–9
Why do sister chromatids have to remain paired in division I
of meiosis? Does the answer suggest a strategy for washing
your socks?
QUESTION 19–10
Distinguish between the following genetic terms:
A. Gene and allele.
B. Homozygous and heterozygous.