Chromosomal Inheritance II
Outline
• Incomplete Dominance, Codominance, and
Multiple Allelism
• Interaction of genes
• Pedigree Studies
• Genetics and Ethics
Extending Mendel’s Rules
• Incomplete dominance
– heterozygotes have an intermediate phenotype
• Codominance
– Heterozygotes displays the phenotype of both alleles
• multiple allelism
– Multiple distinct genes versions (i.e., alleles) are
present in the population
• polymorphism
– Multiple distinct phenotypes are present in a
population
Multiple Alleles and
Polymorphism
• ABO blood group in
humans are determined
by three alleles : IA, IB,
and i.
Allele
IA
IB
Carbohydrate
A
B
i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
I AI B
AB
ii
O
(b) Blood group genotypes and phenotypes
Pleiotropy
• A gene that influences many traits rather than just one is
pleiotropic.
– Marfan Syndrome (FBN1): defective fibrillinlimbs, spinal chord,
heart
– Cystic fibrosis (CFTR): defective salt transportlungs, pancreas,
sebacious glands, etc.
Lung(s)
pancreas
healthy
CF
Antagonistic pleiotropy
• Some effects are good; some are bad
• Sickle cell anemia (hemoglobin B)
HBB/hbb
– Codominant trait
– HBB/HBB; HBB/hbb; hbb/hbb
Mild sickle
cell disease
Healthy
???
Unhealthy
Malaria
protection
Fig. 14-UN2
Degree of dominance
Description
Complete dominance
of one allele
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
Example
PP
Pp
C RC R
Codominance
Multiple alleles
Pleiotropy
Heterozygotes: Both
phenotypes expressed
In the whole population,
some genes have more
than two alleles
One gene is able to
affect multiple
phenotypic characters
C RC W C W C W
IAIB
ABO blood group alleles
IA , IB , i
Sickle-cell disease
Fig. 14-12
• A gene at one locus
alters the
phenotypic
expression of a
gene at a second
locus
• Coat color in mice
– pigment color (B for
black; b for brown)
– Pigment deposit (C
for color; c for no
color)
Epistasis
BbCc

BbCc
Sperm
1/
4 BC
1/
4 bC
1/
4 Bc
1/
4 bc
Eggs
1/
1/
1/
1/
4 BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
Discrete vs. Quantitative Traits
• Discrete traits.
– seed color in peas—no intermediate phenotypes
• Quantitative traits
– Traits that fall into a continuum
• Frequencies
– form a bell-shaped curve (normal distribution) for a population.
A phenotype distribution that forms a bell-shaped curve.
Normal distribution—bell-shaped curve
Quantitative Traits Result from the Action of Many Genes
Wheat kernel color is a quantitative trait.
Hypothesis to explain inheritance of kernel color
Parental
generation
aa bb cc
(pure-line white)
F1
generation
AA BB CC
(pure-line red)
Aa Bb Cc
(medium red)
Self-fertilization
F2
generation
20
15
6
1
15
6
1

Polygenic Inheritance
AaBbCc
• Traits that vary in the
population along a
continuum
• Additive effect of 2+
genes on a single
phenotype
AaBbCc
Sperm
1/
Eggs
• Skin color in humans is an
example of polygenic
inheritance
1/
8
1/
8
1/
8
1/
8
1/
1/
8
1/
1/
8
8
1/
8
1/
64
15/
8
1/
1/
8
8
8
1/
8
1/
8
1/
8
8
1/
Phenotypes:
64
Number of
dark-skin alleles: 0
6/
64
1
15/
64
2
20/
3
64
4
6/
64
5
1/
64
6
Relationship among
genes
Epistasis
Example
Description
One gene affects
the expression of
another
BbCc
BbCc
BC bC Bc bc
BC
bC
Bc
bc
9
Polygenic
inheritance
A single phenotypic
AaBbCc
character is
affected by
two or more genes
:3
:4
AaBbCc
Applying Mendel’s Rules to Humans
• Humans  terrible genetic models
– Generation time is too long
– Parents produce relatively few offspring
– Breeding experiments are frowned upon
• Human disorders follow 5 patterns
1) Autosomal dominant 2) Autosomal recessive
3) X-linked recessive
4) X-linked dominant
5) Y-linked
• Pedigrees (family trees)
– analyze the human crosses that already exist.
Human Pedigree Reports
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
ww
Ww ww ww Ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
Is a widow’s peak a dominant or recessive trait?
Autosomal Recessive Traits
• If a phenotype is due to an autosomal recessive allele
– trait = homozygous
– parents (w/o trait) = heterozygous carriers.
• Carriers carry the allele and transmit it even though they do
not exhibit the phenotype.
I
Carrier male
Carrier female
Carriers
(heterozygotes)
are indicated
with half-filled
symbols
II
III
Affected
male
IV
Affected
female
Autosomal or Sex-Linked trait?
• Equally often in males and females
– likely to be autosomal.
• Males more likely to have the trait
– usually X-linked.
• Hemophilia is an example of an X-linked trait resulting from a
recessive allele.
I
Queen Victoria
Prince Albert
Female carrier of
hemophilia allele
II
III
IV
Affected
male
Frequency of Dominant Alleles
• Not necessarily more common (NOT always “WT”)
• one baby out of 400 in the United States is born with extra fingers or toes
• Dominant allele; uncommon occurrence
• In this example, the recessive allele is far more prevalent than
the population’s dominant allele
What are the Societal
Implications of this Knowledge?
Fetal Testing
•Tests to determine in utero if a child has a
disorder.
•14th to 16th week of pregnancy
•Blood or amniocentesis
•Fetal tests can reveal a serious disorder
•Trisomy 21, 18, etc.
•Some testing after birth
•eg Type I diabetes
http://www.youtube.com/watch?v=qA25_fiy
h_E&feature=related
Eugenics
•Science of “improving the genetic stock”
of humans
•Old Testament
•Plato’s Republic (description of the
ideal society )
•Francis Galton
•“National Eugenics Laboratory”
•Experimental studies of heredity
•Twins
•Karl Pearson
•The higher birth rate of the poor
•Supplant by "higher" races
US Propaganda and Policy
• The Immigration Act of 1924
– quota for different nationalities
– perceived tendencies towards crime etc.
• Forced Sterilization
Eugenics and the Third Reich
•Nazi Germany
• The Aryan Nation and the Holocaust
•Human races
•Ill-founded concept
•Populations with overlapping gene pools.
•No major difference in the genome sequence