935
CHAPTER TWENTY-FOUR
Genetics and Genomics
3. Different dominance relationships
a. In incomplete dominance, a heterozygote has a
phenotype intermediate between those of both
homozygotes.
b. In codominance, each of the alleles in the
heterozygote is expressed.
24.3
FACTORS THAT AFFECT EXPRESSION OF
SINGLE GENES (PAGE 924)
1. Penetrance and expressivity
a. A genotype is incompletely penetrant if not all
individuals inheriting it express the phenotype.
b. A genotype is variably expressive if it is expressed
to different degrees in different individuals.
2. Pleiotropy
a. A pleiotropic disorder has several symptoms,
different subsets of which are expressed among
individuals.
b. Pleiotropy refl ects a gene product that is part of
more than one biochemical reaction or is in several
organs or structures.
3. Genetic heterogeneity
a. Genetic heterogeneity refers to a phenotype
resulting from mutation in more than one gene.
b. The same symptoms may result from mutations
in genes whose products are enzymes in the same
biochemical pathway.
24.4
MULTIFACTORIAL TRAITS (PAGE 924)
1. A trait caused by the action of a single gene is
monogenic, and by the action of more than one gene,
polygenic.
2. A trait caused by the action of one or more genes and
the environment is multifactorial.
3. Height, skin color, eye color, and many common
illnesses are multifactorial traits.
4. A frequency distribution for a polygenic trait forms a
bell curve.
24.5
MATTERS OF SEX (PAGE 927)
A female has two X chromosomes; a male has one X and
one Y chromosome. The X chromosome has many more
genes than the Y.
1. Sex determination
a. A male zygote forms when a Y-bearing sperm
fertilizes an egg. A female zygote forms when an
X-bearing sperm fertilizes an egg.
b. A gene on the Y chromosome, called
SRY,
switches
on genes in the embryo that promote development
of male characteristics.
c. Lack of
SRY
and activation of
Wnt4
directs
development as a female.
2. Genes on the sex chromosomes
a. Genes on the sex chromosomes follow different
inheritance patterns than those on autosomes.
CHAPTER SUMMARY
24.1
INTRODUCTION (PAGE 917)
1. Genes are DNA sequences that encode proteins.
2. Genetics is the study of inheritance of characteristics
and human variation.
3. Genetic information is passed from generation to
generation through meiosis and fertilization, when
the haploid genomes of the parents join.
4. The human genome assembles many more proteins
than there are genes by combining gene parts.
5. Genetic information functions at the biochemical,
cell, tissue, individual, family, and population levels.
6. Genes infl
uence each other and their expression
responds to environmental infl
uences.
24.2
MODES OF INHERITANCE (PAGE 918)
1. Chromosomes and genes come in pairs
a. Chromosome charts are called karyotypes.
b. Chromosomes 1 through 22, numbered in
decreasing size order, are autosomes. They do not
have genes that determine sex.
c. The X and Y chromosomes are sex chromosomes.
They have genes that determine sex.
d. Chromosomes and the genes they carry are paired.
e. An allele is an alternate form of a gene. An
individual can have two different alleles for a
particular gene. The gene itself can have many
alleles, because a gene consists of many building
blocks, any of which may be altered.
f. An individual with a pair of identical alleles for a
particular gene is homozygous; if the alleles are
different, the individual is heterozygous.
g. The combination of genes present in an
individual’s cells constitutes a genotype; the
appearance of the individual is its phenotype.
h. A wild type allele provides normal or the most
common function. A mutant allele causes disease
or an unusual trait; it is a change from the wild
type condition.
2. Dominant and recessive inheritance
a. In the heterozygous condition, an allele expressed
when the other is not is dominant. The masked
allele is recessive.
b. Recessive and dominant genes may be autosomal
or X-linked or Y-linked.
c. An autosomal recessive condition affects both
sexes and may skip generations. The homozygous
dominant and heterozygous individuals have
normal phenotypes. The homozygous recessive
individual has the condition. The heterozygote is a
carrier. An affected individual inherits one mutant
allele from each parent.
d. An autosomal dominant condition affects both
sexes and does not skip generations. A person
inherits it from one affected parent.
e. Pedigrees and Punnett squares are used to depict
modes of inheritance.
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