NON-MENDELIAN GENETICS
Definition
Genetic interactions that do not follow
complete dominance, are expressed together,
or influence each other’s expression.
Incomplete Dominance
Definition
The phenotype of the heterozygote is intermediate
between two homozygotes.
Experimental
Parental Monohybrid Cross
Design
True-breeding red (R) flowered snapdragons were
crossed with true-breeding white (r) flowered.
Incomplete Dominance
F1 Generation
Genotype:Rr
Phenotype: Pink
Incomplete Dominance
F2 Generation
Genotypes: RR, Rr, rr Phenotypes: 1 Red, 2 Pink, 1 White
Incomplete Dominance
Interpretation
1) Red snapdragons have two alleles of a gene for red
pigment.
Expression of both alleles gives the deep red color
2) White snapdragons have two alleles that specify "no
pigment".
Expression of both alleles gives no color
3) Pink snapdragons are heterozygous with one red allele; its
expression results in only enough pigment molecules to make
flowers pink.
Codominance
Definition
Two nonidentical alleles of a pair express two different
phenotype in the heterozygote.
Example
1) Blood groups are controlled by three alleles
(multiple allele system).
a) IA, IB, i
2) Possible Outcomes
a) ii
=
O blood group
b) IAIA or IAi =
A blood group
c) IBIB or IBi =
B blood group
d) IAIB
=
AB blood group (Codominance)
Codominance
Epistasis
Definition
Interaction of two or more gene pairs at different loci
influence the same trait, but one allele has an overriding
effect on the phenotype.
Example
1) Hair color in Labrador dogs is determined by two sets of
gene pairs.
2) Possible Alleles
Quantity of Pigment (Melanin) Produced
Black – B
Brown – b
Pigment (Melanin) Deposition
Turned On – E
Turned Off – e
Epistasis
Black
Brown
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
Yellow
BBee
Bbee
bbee
Pleiotropy
Definition
A single gene affects more than one
phenotype
Example
1) White Mohan Tiger
a) Same allele for absence of melanin
production gives rise to crossed eyes.
2) Phenylketonuria (PKU)
a) Same allele for toxic substances gives
rise to light colored hair.
Penetrance (Expressivity)
Definition
Differing expressions of a gene.
Example
1) Polydactyly
Penetrance (Expressivity)
Environmental Effects
Definition
The environment can affect the expression of a
gene.
Example
1) Himalayan rabbits
a) Homozygous for ch allele coding for
tyrosinase expressing melanin production
b) Tyrosinase is not active above 33° C
c) Hair appears light due to lack of melanin
Continuous Variation
(Polygenic Inheritance)
Definition
Continuous range of small differences in a given
trait among all the individuals of a population
due to inheritance of multiple alleles that affect
the same trait.
Examples
1) Height
2) Eye color
(4 Genes and 8 Alleles)
POLYGENIC INHERITANCE
EYE COLOR
Continuous Variation
(Polygenic Inheritance)
Eye Color
Light Blue
Medium Blue
Dark Blue
Gray
Green
Hazel
Light Brown
Medium Brown
Dark Brown
Alleles
aabbccdd
aabbccDd
aabbCcDd
aaBbCcDd
AaBbCcDd
AaBbCcDD
AaBbCCDD
AaBBCCDD
AABBCCDD
Alleles
0
1
2
3
4
5
6
7
8