1106
UNIT 5
Continuity
29
or mutations in mitochondrial genes are responsible for some
problems with oxidative phosphorylation and some rare genetic
diseases.
Genetic Screening, Counseling, and Therapy
(pp. 1103–1105)
Carrier Recognition
(p. 1103)
1.
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
probes.
Fetal Testing
(pp. 1103–1104)
2.
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.
3.
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
(pp. 1104–1105)
4.
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
function.
Environmental Factors in Gene Expression
(p. 1102)
1.
Environmental factors may influence the expression of genotype.
2.
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.
Nontraditional Inheritance
(pp. 1102–1103)
Beyond DNA: Regulation of Gene Expression
(pp. 1102–1103)
1.
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
traditional genetics.
2.
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.
3.
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
(p. 1103)
4.
Cytoplasmic (mitochondrial) genes pass to offspring via the
ovum and help to determine certain characteristics. Deletions
Multiple Choice/Matching
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
1.
Match one of the following terms (a–i) with each of the
descriptions below:
Key:
(a)
alleles
(f)
homozygote
(b)
autosomes
(g)
phenotype
(c)
dominant allele
(h)
recessive allele
(d)
genotype
(i)
sex chromosomes
(e)
heterozygote
____ (1)
genetic makeup
____ (2)
how genetic makeup is expressed
____ (3)
chromosomes that dictate most body characteristics
____ (4)
alternate forms of the same gene
____ (5)
an individual bearing two alleles that are the same for a
particular trait
____ (6)
an allele that is expressed whether in single or double
dose
____ (7)
an individual bearing two alleles that differ for a
particular trait
____ (8)
an allele that must be present in double dose to be
expressed
2.
Match the following types of inheritance (a–f) with the
descriptions below:
Key:
(a)
dominant-recessive
(d)
polygene
(b)
incomplete dominance
(e)
sex-linked
(c)
multiple-allele
(f)
extranuclear
____ (1)
only sons show the trait
____ (2)
homozygotes and heterozygotes have the same
phenotype
____ (3)
heterozygotes exhibit a phenotype intermediate
between those of the homozygotes
____ (4)
phenotypes of offspring may be more varied than those
of the parents
____ (5)
inheritance of ABO blood types
____ (6)
inheritance of stature
____ (7)
reflects activity of mitochondrial DNA
Short Answer Essay Questions
3.
Describe the important mechanisms that lead to genetic
variations in gametes.
4.
Te ability to taste P±C (phenylthiocarbamide) depends on the
presence of a dominant gene
T
; nontasters are homozygous for the
recessive gene
t
. 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
Tt
and
tt
parents. What is the anticipated percentage of tasters? Nontasters?
What proportion can be expected to be homozygous recessive?
Heterozygous? Homozygous dominant?
5.
Most albino children are born to normally pigmented parents.
Albinos are homozygous for the recessive gene (
aa
). What can
you conclude about the genotypes of the nonalbino parents?
Review Questions
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