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Modifications to Mendelian Inheritance
I. Allelic, Genic, and Environmental Interactions
Modifications to Mendelian Inheritance
I. Allelic, Genic, and Environmental Interactions
A. Overview:
Environment
The effect of a gene is influenced at three levels:
- Intralocular (effects of other alleles at this locus)
- Interlocular (effects of other genes at other loci)
- Environmental (the effect of the environment on
determining the effect of a gene on the phenotype)
A
a
PHENOTYPE
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
A
a
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
- The presence of one allele is enough
to cause the complete expression of a given
phenotype.
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
- The heterozygote expresses a phenotype
between or intermediate to the phenotypes of the
homozygotes.
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
- Both alleles are expressed completely;
the heterozygote does not have an intermediate
phenotype, it has BOTH phenotypes.
AB Phenotype
ABO Blood Type:
A = ‘A’ surface antigens
B = ‘B’ surface antigens
O = no surface antigen from this
locus
Phenotype
Genotypes
A
AA, AO
B
BB, BO
O
OO
AB
codominance
AB
TT = tall (grows best in warm conditions)
tt = short (grows best in cool conditions)
Tt = Very Tall (has both alleles and so grows
optimally in cool and warm conditions)
Enzyme Activity
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
– the heterozygote expresses a
phenotype MORE EXTREME than either
homozygote
“T”
TEMP
“t”
Enzyme Activity
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
TEMP
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
- Essential genes: many proteins
are required for life. “Loss-of-function” alleles
may not affect heterozygotes, but in
homozygotes may result in the death of the
zygote, embryo, or adult – depending on
when they should be expressed during
development.
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
- Essential genes: many proteins
are required for life. “Loss-of-function” alleles
may not affect heterozygotes, but in
homozygotes may result in the death of the
zygote, embryo, or adult – depending on
when they should be expressed during
development.
Why haven’t they been weeded out of
the population by selection?
Recessive Lethals:
Aa
x
Aa
- 25% reduction in
number of offspring
A
a
A
AA
Aa
a
Aa
aa
Self-crossing the survivors
shows that 1/3 show no
reduction in offspring
number (AA), while 2/3 show
the 25% reduction in number
(Aa)
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Sometimes, the heterozygote has a
different phenotype than the
homozygote. The phenotypic effect
can be ‘dominant’ while the lethal
effect is recessive. AY exerts a
dominant effect on coat color
(expressed in the heterozygote),
but is lethal ONLY in the
homozygous condition (recessive
lethality).
Also an example of pleiotropy –
one gene affecting >1 trait.
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Conditional Lethality:
In this case, the expression of
lethality only occurs under specific
conditions. Favism is caused by a
mutation in the gene that codes for
the enzyme glucose-6-phosphate
dehydrogenase. When afflicted
individuals eat fava beans, their
red blood cells rupture and clog
capillaries, resulting in anemia and
death.
It is the most common enzyme
defect in humans.
Why not selected against?
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Conditional Lethality:
In this case, the expression of
lethality only occurs under specific
conditions. Favism is caused by a
mutation in the gene that codes for
the enzyme glucose-6-phosphate
dehydrogenase. When afflicted
individuals eat fava beans, their
red blood cells rupture and clog
capillaries, resulting in anemia and
death.
It is the most common enzyme
defect in humans.
Why not selected against?
Balanced selection… provides
some protection against
malaria.
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethal Alleles:
Dominant Lethal
Why aren’t they weeded out of
the population?
HC is a neurodegenerative disorder caused by an
autosomal lethal dominant allele.
Dr. Nancy Wexler's work
The fishing villages around Lake Maracaibo in
Venezuela have the highest incidence of
Huntington’s Chorea in the world, approaching
50% in some communities.
The gene was mapped to chromosome 4, and the HC allele was caused by a repeated
sequence of over 35 “CAG’s”. Dr. Nancy Wexler found homozygotes in Maracaibo and
described it as the first truly dominant human disease (most are incompletely dominant and
cause death in the homozygous condition).
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethality:
6. Multiple Alleles:
- not really an interaction, but a departure from
simple Mendelian postulates.
- and VERY important as a source of variation
# Alleles at the Locus
# Genotypes Possible
1 (A)
1 (AA)
2 (A, a)
3 (AA, Aa, aa)
3 (A, a, A’)
6 (AA, Aa, aa, A’A’, A’A, A’a)
4
10
5
15
I. Allelic, Genic, and Environmental Interactions
A. Overview:
B. Intralocular Interactions
1. Complete Dominance:
2. Incomplete Dominance:
3. Codominance:
4. Overdominance :
5. Lethality:
6. Multiple Alleles:
7. Penetrance and Expressivity:
- Penetrance: the percentage of individuals with a
given genotype that actually EXPRESS the associated
phenotype. (Because of environment or other
genes)
- Expressivity: The DEGREE to which an individual
expresses its genetically determined trait. The
degree of “eyeless” expression in Drosophila is
affected by genetic background and environment.
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
- Summary and Implications:
populations can harbor
extraordinary genetic variation at each locus,
and these alleles can interact in myriad ways
to produce complex and variable phenotypes.
-Consider this cross: AaBbCcDd x AABbCcDD
Assume:
The genes assort independently
A and a are codominant
B is incompletely dominant to b
C is incompletely dominant to c
D is completely dominant to d
How many phenotypes are possible in the
offspring?
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
- Summary and Implications:
populations can harbor
extraordinary genetic variation at each locus,
and these alleles can interact in myriad ways
to produce complex and variable phenotypes.
-Consider this cross: AaBbCcDd x AABbCcDD
A
B
2 x
3
C
x
3
D
x
1 = 18
If they had all exhibited complete
dominance, there would have
been only:
1 x
2
x
2
x
1 =4
Assume:
The genes assort independently
A and a are codominant
B is incompletely dominant to b
C is incompletely dominant to c
D is completely dominant to d
How many phenotypes are possible in the
offspring?
So the variety of allelic interactions
that are possible increases
phenotypic variation
multiplicatively. In a population
with many alleles at each locus,
there is an nearly limitless
amount of phenotypic variability.
I. Allelic, Genic, and Environmental
Interactions
A. Overview:
B. Intralocular Interactions
C. Interlocular Interactions
The phenotype can be affected by more than
one gene.
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
There may be several genes that produce
the same protein product; and the
phenotype is the ADDITIVE sum of these
multiple genes.
Creates continuously variable traits.
So here, both genes A and B produce the
same pigment. The double homozygote
AABB produces 4 ‘doses’ of pigment and
is very dark. It also means that there are
more ‘intermediate gradations’ that are
possible.
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
one gene masks/modifies the expression at
another locus; the phenotype in the A,B,O
blood group system can be affected by the
genotype at the fucosyl transferase locus.
This locus makes the ‘H substance’ to which
the sugar groups are added to make the A and
B surface antigens.
A non-function ‘h’ gene makes a nonfunctional foundation and sugar groups can’t
be added – resulting in O blood regardless of
the genotype at the A,B,O locus. This ‘O’ is
called the ‘Bombay Phenotype’ – after a
woman from Bombay (Mumbai) in which it
was first described.
Genotype
at H
Genotype
at A,B,O
Phenotype
H-
A-
A
H-
B-
B
H-
OO
O
H-
AB
AB
hh
A-
O
hh
B-
O
hh
OO
O
hh
AB
O
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
So, what are the phenotypic ratios
from this cross:
HhAO x
HhBO?
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
So, what are the phenotypic ratios
from this cross:
HhAO x
HhBO?
Well, assume they are inherited
independently.
AT H:
¾ H: ¼ h
At A,B,O: ¼ A : ¼ O: ¼ B : ¼ AB
So, the ¼ that is h is O type blood,
regardless.
Then, we have:
¾ H x ¼ A = 3/16 A
¾ H x ¼ O = 3/16 O (+ 4/16 above)
¾ H x ¼ B = 3/16 B
¾ H x ¼ AB = 3/16 AB
Phenotypic Ratios: 3/16 A : 3/16 B : 3/16 AB : 7/16 O = 16/16 (check!)
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all
Process:
enzymes may be needed to produce a
enzyme 1
given phenotype. Absence of either may
produce the same alternative ‘null’.
Precursor 1
enzyme 2
precursor2
product
(pigment)
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
Process:
enzyme 1
Precursor 1
enzyme 2
precursor2
product
(pigment)
-example #2: in a enzymatic process, all
Strain 1:
enzymes may be needed to produce a
enzyme 1
enzyme 2
given phenotype. Absence of either may
produce the same alternative ‘null’.
For example, two strains of white flowers
precursor2
no product
may be white for different reasons; each Precursor 1
(white)
lacking a different necessary enzyme to
make color.
Strain 2:
enzyme 1
Precursor 1
enzyme 2
precursor2
no product
(white)
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
For example, two strains of white flowers
may be white for different reasons; each
lacking a different necessary enzyme to
make color.
So there must be a dominant gene at
both loci to produce color.
Genotype
Phenotype
aaB-
white
aabb
white
A-bb
white
A-B-
pigment
So, what’s the phenotypic
ratio from a cross:
AaBb x AaBb ?
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
For example, two strains of white flowers
may be white for different reasons; each
lacking a different necessary enzyme to
make color.
So there must be a dominant gene at
both loci to produce color.
Genotype
Phenotype
aaB-
white
aabb
white
A-bb
white
A-B-
pigment
So, what’s the phenotypic
ratio from a cross:
AaBb x AaBb ?
9/16 pigment (A-B-), 7/16 white
C. Interlocular Interactions:
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
For example, two strains of white flowers
may be white for different reasons; each
lacking a different necessary enzyme to
make color.
So there must be a dominant gene at
both loci to produce color.
Indeed, by mating two strains together,
we can determine whether the mutation
is the result of different alleles at the
same locus, or different GENES acting on
one PATHWAY. This is called a
complementation test.
Consider two strains that are wingless. Do these strains have different “loss of
function” mutations in the same gene, or mutations in different genes involved
in the same process (wing development)?
C. Interlocular Interactions
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
-example #3: Novel Phenotypes.
Comb shape in chickens is governed by 2
interacting genes that independently
produce “Rose” or “Pea” combs, but
together produce something completely
different (walnut).
Genotype
Phenotype
rrpp
single
R-pp
rose
rrP-
pea
R-P-
Walnut
C. Interlocular Interactions
1. Quantitative (Polygenic) Traits:
2. Epistasis:
-example #2: in a enzymatic process, all
enzymes may be needed to produce a
given phenotype. Absence of either may
produce the same alternative ‘null’.
-example #3: Novel Phenotypes.
Comb shape in chickens is governed by 2
interacting genes that independently
produce “Rose” or “Pea” combs, but
together produce something completely
different (walnut).
Fruit shape in summer squash is
influnced by two interacting loci, also.
Genotype
Phenotype
aabb
long
A-bb
sphere
aaB-
sphere
A-B-
disc
C. Interlocular Interactions
1. Quantitative (Polygenic) Traits:
2. Epistasis:
In all of these cases, the observed ratios are modifications of the basic Mendelian
Ratios.
A-B-