Non Mendelian Genetics
Patterns of inheritance that aren’t simply dominant or recessive
I. Incomplete Dominance
A. The phenotype of a heterozygous individual is
intermediate between those of the two
homozygotes.
B. Examples
1. Snap Dragons – When red and white flowers are
crossed – all the F1 offspring will have pink flowers
2. Horses – When a chestnut colored horse is bred
with a cremello (white) colored horse; the offspring
are palomino color.
II. Co-dominance
A. The phenotypes of both homozygotes are produced
in the heterozygous individuals; both alleles are
expressed equally.
B. Examples
1. Short-horned cattle – red and white coats are codominant
2. Chickens – feather color; black and white are codominant
3. Four o’clock plants – red and white petals are codominant
Four o’clock plants
Short-horned cattle
Black and white feathers
III. Multiple Alleles
A. Traits controlled by more than 2 alleles.
B. Examples
1. ABO blood type in humans – A and B are
codominant, but dominant to O
2. Pigeons – feather color; BA, B and b; BA produces
ash red feathers, B produces wild-type (grey)
feathers, and b produces chocolate colored feathers
IV. Polygenic Inheritance
A. Many traits are controlled by 2 or more genes.
B. Examples:
1. Eye color
2. Hair color
3. Skin color
4. Height
Eye Color
Skin Color
V. Traits Influenced by the Environment
A. An individuals phenotype often depends on
conditions in the environment.
B. Such factors as temperature, amount of sunlight,
soil conditions and nutrition can affect the
phenotype of an organism.
C. Examples:
1. Arctic fox – brown in summer; white in winter
2. Hydrangea flower color – acidity of the soil
determines blue or reddish color
3. Siamese cats – cooler body areas are darker
4. Height in humans - early nutrition (or lack of) affects
the height of humans
The End!