C h a p t e r 5
Genetic Control of Cell Function and Inheritance
99
some human genes one of the alleles is transcriptional
inactive (no RNA produced), depending on the par-
ent from whom the allele is inherited. For example, an
allele from the mother would be active and that from
the father would be inactive. This process of gene silenc-
ing was given the name
genomic imprinting
by Helen
Crouse in 1960. It is now more commonly known as
genetic imprinting
. In humans, it is estimated that
approximately 100 genes exhibit genetic imprinting.
Evidence suggests that a genetic conflict occurs in the
developing embryo: the male genome attempts to estab-
lish larger offspring, whereas the female genome favors
smaller offspring to conserve her energy for the current
and subsequent pregnancies.
It was the pathologic analysis of ovarian teratomas
(tumors made up of various cell types derived from an
undifferentiated germ cell) and hydatidiform moles (ges-
tational tumors) that yielded the first evidence of genetic
imprinting. All ovarian teratomas were found to have a
46,XX karyotype. The results of detailed chromosomal
polymorphism analysis confirmed that these tumors
developed without the paternally derived genome.
Conversely, analysis of hydatidiform moles suggested
that they were tumors of paternal origin.
A related chromosomal disorder is
uniparental
disomy
. This occurs when two chromosomes of the
same number are inherited from one parent. Normally,
this is not a problem except in cases where a chromo-
some has been imprinted by a parent. If an allele is
inactivated by imprinting, the offspring will have only
one working copy of the chromosome, resulting in pos-
sible problems.
1/4
1/4
1/4
1/4
Female Male
Dd
d
D
D
Dd
DD
dd
Dd
Dd
d
DD=1/4
Dd=1/2
dd=1/4
FIGURE 5-11.
The Punnett square showing all possible
combinations for transmission of a single gene trait (dimpled
cheeks).The example shown is when both parents are
heterozygous (dD) for the trait.The alleles carried by the
mother are on the left and those carried by the father are on
the top.The D allele is dominant and the d allele is recessive.
The DD and Dd offspring have dimples and the dd offspring
does not.
SUMMARY CONCEPTS
■■
The transmission of information from one
generation to the next is vested in genetic
material transferred from each parent at the time
of conception.
■■
Inheritance represents the likelihood of the
occurrence or recurrence of a specific genetic
trait.
■■
The genotype is the total sum of the genetic
information that is stored in the genetic code of
a person, whereas the phenotype represents the
recognizable traits, physical and biochemical,
associated with the genotype.
■■
Expressivity refers to the expression of a gene in
the phenotype, and penetrance is the ability of
a gene to express its function.The point on the
DNA molecule that controls the inheritance of a
particular trait is called a gene locus.
■■
Alleles are the alternate forms of a gene (one from
each parent), and the locus is the position that they
occupy on the chromosome.The alleles at a gene
locus may carry recessive or dominant traits.
■■
Mendelian, or single-gene, patterns of inheritance
include autosomal dominant and recessive traits
that are transmitted from parents to their offspring
in a predictable manner. A recessive trait is one
expressed only when two copies (homozygous)
of the recessive allele are present. Dominant
traits are expressed with either homozygous or
heterozygous pairing of the alleles.
■■
Polygenic inheritance, which involves multiple
genes, and multifactorial inheritance, which
involves multiple genes as well as environmental
factors, are less predictable.
■■
A pedigree is a graphic method for portraying a
family history of an inherited trait.
GeneTechnology
The past several decades have seen phenomenal
advances in the field of genetics. These advances have
included the assembly of physical and genetic maps
through the Human Genome Project; the establishment
of the International HapMap Project (
.
ncbi.nlm.nih.gov) to map the haplotypes of the many
adjacent single-nucleotide polymorphisms in the human
genome; the establishment of the 1000 Genomes Project
), and the development of
methods for applying the technology of these projects to
the diagnosis and treatment of disease.
