Essential Cell Biology 5th edition

Essential Concepts
687
and function as “normal” humans in today’s world despite the enormous
variations across the human population.
ESSENTIAL CONCEPTS
• Sexual reproduction involves the cyclic alternation of diploid and
haploid states: diploid germ-line cells divide by meiosis to form haploid
gametes, and the haploid gametes from two individuals fuse at
fertilization to form a new diploid cell—the zygote.
• During meiosis, the maternal and paternal homologs are parceled
out to gametes such that each gamete receives one copy of each
chromosome. Because the segregation of these homologs occurs
randomly, and crossing-over occurs between them, many genetically
different gametes can be produced from a single individual.
• In addition to enhancing genetic mixing, crossing-over helps ensure
the proper segregation of chromosomes during meiosis.
• Although most of the mechanical features of meiosis are similar to
those of mitosis, the behavior of the chromosomes is different: meiosis
produces four genetically distinct haploid cells by two consecutive
cell divisions, whereas mitosis produces two genetically identical
diploid cells by a single cell division.
• Mendel unraveled the laws of heredity by studying the inheritance of
a handful of discrete traits in pea plants.
• Mendel’s law of segregation states that the maternal and paternal
alleles for each trait separate from one another during gamete formation
and then reunite randomly during fertilization.
• Mendel’s law of independent assortment states that, during gamete
formation, different pairs of alleles segregate independently of one
another.
• The behavior of chromosomes during meiosis explains both of
Mendel’s laws.
• If two genes are close to each other on a chromosome, they tend
to be inherited as a unit; if they are far apart, they will typically be
separated by crossing-over. The frequency with which two genes
are separated by crossovers can be used to construct a genetic map
that shows their order on a chromosome.
• Mutant alleles can be either dominant or recessive. If a single copy
of the mutant allele alters the phenotype of an individual that also
possesses a wild-type allele, the mutant allele is dominant; if not, it
is recessive.
• Complementation tests reveal whether two mutations that produce
the same phenotype affect the same gene or different genes.
• Mutant organisms can be generated by treating animals with mutagens,
which damage DNA. Such mutants can then be screened to
identify phenotypes of interest and, ultimately, to isolate the responsible
genes.
• With the possible exception of identical twins, no two human genomes
are alike. Each of us carries a unique set of polymorphisms—variations
in nucleotide sequence that in some cases contribute to our
individual phenotypes.
• Some of the common polymorphisms—including SNPs, indels, and
CNVs—provide useful markers for genetic mapping.
• The human genome consists of large haplotype blocks—segments of
nucleotide sequence that have been passed down intact from our
distant ancestors and, in most individuals, have not yet been broken