Non-Mendelian inheritance
Mendel’s model of inheritance assumes
that:
-each trait is controlled by a single gene
-each gene has only 2 alleles
-there is a clear dominant-recessive
relationship between the alleles
Most genes do not meet these criteria.
Polygenic inheritance
Polygenic inheritance occurs when multiple
genes are involved in controlling the
phenotype of a trait.
The phenotype is an accumulation of
contributions by multiple genes.
These traits show continuous variation and
are referred to as quantitative traits.
For example – human height
Continuous variation
• When multiple genes act
together to produce a
physical (phenotypic)
character, a gradation or
range of differences
occur.
• Examples: height,
weight in humans
• Referred to as polygenic
traits
Pleiotropy
Pleiotropy refers to an allele which has more
than one effect on the phenotype.
This can be seen in human diseases such as
cystic fibrosis or sickle cell anemia.
In these diseases, multiple symptoms can be
traced back to one defective allele.
Pleiotropic effects
• Occurs when an allele
has >1 effect on
phenotype
• Examples are:
– Sickle cell anemia
– Cystic fibrosis
Incomplete dominance
• Situation where 2 alleles
“blend” to form
hybridized phenotype
• Examples:
– Flower color in 4
o’clocks and snapdragons
Multiple alleles and Blood Groups
• ABO Blood groups
–
–
–
–
IAIA and IAi = type A
IBIB and IBi = Type B
IAIB and IBIA = Type AB
ii = Type O
• Rh blood factor
• Rh +
• Rh-
Human Blood types
Environmental effects
• Allele expression may be
affected by environmental
conditions
• Examples:
– Coat color in arctic foxes
– Coat color in Himalayan
rabbits and Siamese cats
• ch allele affected by temp
>33 C  tyrosinase enzyme
inactivates + reduces
melanin pigment production
Environmental effects on phenotype
Epistasis – genes acting together
• Situation whereby 1
gene pair affects the
expression of a 2nd gene
pair
• Examples:
– Anthocyanin (purple)
pigment in corn
– Animal coat colors
Epistasis (cont’d)
• Horse coloration involves
2 or more gene pairings..
• EE or Ee is for black
• ee is for red (sorrel)
• PLUS other genes can add
to base colors
• (Bay is AA, EE – black
with agouti gene;
Buckskin is AA, EE, CcrC
– bay with cremello gene,
Dun is AA, EE, Dd – bay
with dun gene; Palomino
is ee, CcrC – sorrel with
cremello gene)