NOTES: Ch 14, part 2 –
Extending Mendelian
Genetics
The Spectrum of Dominance
● Complete dominance occurs when the
phenotypes of the heterozygote and
dominant homozygote are identical
CODOMINANCE:
● In codominance 2 dominant alleles affect
the phenotype in separate, distinguishable
ways
● Example: the human blood group MN
CODOMINANCE:
● inheritance
characterized by full
expression of both
alleles in the
heterozygote
Example: in chickens,
BB = black feathers,
bb = white, Bb =
“speckled” (both black
and white feathers)
INCOMPLETE DOMINANCE:
● inheritance where one
allele is not completely
dominant over the
other, so the
heterozygote has a
phenotype that is
intermediate between
the phenotype of the 2
homozygotes
Ex: RR = red flowers,
rr = white, Rr = pink
The Relationship Between Dominance
and Phenotype
● Dominant and recessive alleles do not
really “interact”
● they lead to synthesis of different
proteins that produce a phenotype
Frequency of Dominant Alleles
● Dominant alleles are not necessarily more
common in populations than recessive
alleles
Multiple Alleles
● Most genes exist in populations in more
than two allelic forms
● The ABO blood group in humans is
determined by multiple alleles
Table 14.2
Polygenic Inheritance
● Many human characters vary in the
population along a continuum and are called
quantitative characters
POLYGENIC INHERITANCE:
● mode of inheritance in which the
additive effect of 2 or more genes
determines a single phenotypic
character
Examples: height
hair color (4 gene pairs)
eye color (2 gene pairs)
skin color
● Quantitative
variation usually
indicates polygenic
inheritance
(An additive effect of
2 or more genes on
a single phenotype)
AaBbCc
aabbcc
20⁄
15⁄
6⁄
64
64
64
1⁄
64
Aabbcc
AaBbcc
AaBbCc
AaBbCc
AABbCc
AABBCc AABBCC
PLEIOTROPY:
● the ability of a single gene to have
multiple phenotypic effects
Examples: sickle cell anemia
Siamese cats & tigers
EPISTASIS:
● interaction between 2 nonallelic genes in
which one modifies the phenotypic expression
of the other
Example: gene for pigment deposition is
epistatic to gene for melanin production in
mice
● An example of epistasis:
BbCc
BbCc
Sperm
1⁄
Eggs
4 BC
1⁄
1⁄
1⁄
4
bC
1⁄
4
1⁄
Bc
4
bc
BBCC
BbCC
BBCc
BbCc
bC
BbCC
bbCC
BbCc
bbCc
4
Bc
BBCc
BbCc
BBcc
4
bc
BbCc
bbCc
Bbcc
4
1⁄
1⁄
BC
4
9⁄
16
3⁄
16
Bbcc
4⁄
bbcc
16
Nature and Nurture: The Environmental
Impact on Phenotype
● Another departure from simple Mendelian
genetics arises when the phenotype for a
character depends on environment as well
as on genotype
● The norm of reaction is the phenotypic
range of a particular genotype that is
influenced by the environment
Figure 14.13
MULTIFACTORIAL INHERITANCE:
● A trait depends on many factors; a variety of
genotypes as well as environmental
influences (such as certain chemicals,
medicines, or diet)
Examples of disorders that may be a
result of multifactorial inheritance:
diabetes, heart disease, neural tube defects,
autism, Alzheimer disease, ALS, and many
cancer syndromes
~Same genotype
~Phenotype
depends on acidity
of soil
~height controlled by
several genes
~height influenced by
amount of nourishment
Integrating a Mendelian View of Heredity
and Variation
● An organism’s phenotype includes its
physical appearance, internal anatomy,
physiology, and behavior
● the phenotype reflects its overall genotype
and unique environmental history
Inheritance Patterns for
Genetic Diseases in Humans
Pedigree Analysis
● A pedigree is a family tree that describes
the interrelationships of parents and children
across generations
George
Sandra
Tom
Arlene
Sam
Wilma
Ann
Michael
Carla
Daniel
Alan
Tina
Christopher
● Inheritance patterns of particular traits can
be traced and described using pedigrees
Ww
ww
ww
Ww ww ww Ww
WW
or
Ww
Ww
ww
(a) Dominant trait (widow’s peak)
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff
Ff
Ff
Third
generation
(two sisters)
ww
Widow’s peak
Figure 14.14 A, B
Ww
No Widow’s peak
Attached earlobe
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Free earlobe
(b) Recessive trait (attached earlobe)
1) Autosomal Recessive:
● recessive alleles that
cause human disorders
are usually defective
versions of normal alleles
● defective alleles code for
either a malfunctioning
protein or no protein at all
● heterozygotes can be
phenotypically normal,
if 1 copy of the normal
allele is all that is
needed to produce
sufficient quantities of
the “good” protein
Examples: cystic
fibrosis, Tay-Sachs,
sickle cell anemia
● “Carriers” are
heterozygous
individuals who
carry the recessive
allele but are
phenotypically
normal
Cystic Fibrosis
● Symptoms of cystic fibrosis include:
-Mucus buildup in the some internal organs
-Abnormal absorption of nutrients in the
small intestine
Sickle-Cell Disease
● Sickle-cell disease:
-Affects one out of 400 African-Americans
-Is caused by the substitution of a single
amino acid in the hemoglobin protein in red
blood cells
● Symptoms include:
-Physical weakness, pain, organ damage,
and even paralysis
2) Autosomal Dominant:
● only 1 dominant allele is needed in order to
produce the effects of these diseases
(heterozygous)
● Lethal homozygous dominant condition
results in spontaneous abortion of fetus
● homozygous
recessives are of
normal phenotype
Examples:
achondroplasia,
Huntington’s
Disease
● ACHONDROPLASIA:
a form of dwarfism that
is lethal when
homozygous for the
dominant allele
Figure 14.15
● HUNTINGTON’S DISEASE: a degenerative
disease of the nervous system
-it has no obvious phenotypic effects until
about 35 to 40 years of age
Figure 14.16
Genetic Testing and Counseling
● Genetic counselors can provide information
to prospective parents concerned about a
family history for a specific disease
Counseling Based on Mendelian
Genetics and Probability Rules
● Using family histories
genetic counselors help
couples determine the
odds that their children will
have genetic disorders
Tests for Identifying Carriers
● For a growing number of diseases tests are
available that identify carriers and help
define the odds more accurately
Fetal Testing
● AMNIOCENTESIS: the liquid that bathes
the fetus is removed and tested
● CHORIONIC VILLUS SAMPLING (CVS): a
sample of the placenta is removed and
tested