or mutations in mitochondrial genes are responsible for some
problems with oxidative phosphorylation and some rare genetic
Genetic Screening, Counseling, and Therapy
Te likelihood of an individual carrying a deleterious recessive
gene may be assessed by constructing a pedigree. Some of
these genes can be deduced by various blood tests and DNA
Amniocentesis is fetal testing based on aspirated samples of
amniotic ﬂuid. Fetal cells in the ﬂuid are cultured for several
weeks, then examined for chromosomal defects (karyotyped) or
for DNA markers of genetic disease. Amniocentesis cannot be
performed until the 14th week of pregnancy.
Chorionic villus sampling is fetal testing based on a sample of the
chorion. Tis tissue is rapidly mitotic, so karyotyping can be done
almost immediately. Samples may be obtained by the 8th week.
Human Gene Therapy
Te Human Genome Project has allowed research on diagnosis
of genetic disease and on its treatment to surge forward. Tus far,
gene therapy has been particularly useful for correcting single-
gene disorders. Te most common approach involves transferring
a corrected gene via a virus to the aﬀected cells to restore normal
Environmental Factors in Gene Expression
Environmental factors may inﬂuence the expression of genotype.
Maternal factors that cross the placenta may alter expression of
fetal genes. Environmentally provoked phenotypes that mimic
genetically determined ones are called phenocopies. Nutritional
deﬁcits or hormonal imbalances may alter anticipated growth and
development during childhood.
Beyond DNA: Regulation of Gene Expression
Control of gene expression occurs at three levels: protein-coding
genes, RNA-only genes, and epigenetic marks. Te latter two
account for many cases of inherited disease that fail to follow
Te products of the RNA-only genes (siRNA and miRNA) may
silence genes or prevent their expression and appear to play a role
in directing apoptosis during development.
Epigenetic marks involve attachment of small chemical groups
(methyl or acetyl) to DNA or histone proteins. In general,
methylation prevents access to the DNA whereas acetylation
provides access to the DNA. Genomic imprinting, which involves
methylation of certain genes during gametogenesis, confers
diﬀerent eﬀects and phenotypes on maternal and paternal genes.
It is reversible, and occurs anew each generation.
Extranuclear (Mitochondrial) Inheritance
Cytoplasmic (mitochondrial) genes pass to oﬀspring via the
ovum and help to determine certain characteristics. Deletions
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
Match one of the following terms (a–i) with each of the
how genetic makeup is expressed
chromosomes that dictate most body characteristics
alternate forms of the same gene
an individual bearing two alleles that are the same for a
an allele that is expressed whether in single or double
an individual bearing two alleles that diﬀer for a
an allele that must be present in double dose to be
Match the following types of inheritance (a–f) with the
only sons show the trait
homozygotes and heterozygotes have the same
heterozygotes exhibit a phenotype intermediate
between those of the homozygotes
phenotypes of oﬀspring may be more varied than those
of the parents
inheritance of ABO blood types
inheritance of stature
reﬂects activity of mitochondrial DNA
Short Answer Essay Questions
Describe the important mechanisms that lead to genetic
variations in gametes.
Te ability to taste P±C (phenylthiocarbamide) depends on the
presence of a dominant gene
; nontasters are homozygous for the
. Tis is a situation of classical dominant-recessive
inheritance. (a) Consider a mating between heterozygous parents
producing three oﬀspring. What proportion of the oﬀspring are
likely to be tasters? What is the chance that all three oﬀspring will
be tasters? Nontasters? What is the chance that two will be tasters
and one will be a nontaster? (b) Consider a mating between
parents. What is the anticipated percentage of tasters? Nontasters?
What proportion can be expected to be homozygous recessive?
Heterozygous? Homozygous dominant?
Most albino children are born to normally pigmented parents.
Albinos are homozygous for the recessive gene (
). What can
you conclude about the genotypes of the nonalbino parents?