Exploring Mendelian Genetics

Chapter 11, Section 3

The Complexity of Inheritance

• Mendel studied simple characteristics in pea plants and was able to define some basic principles behind their inheritance patterns

• However, many characteristics visible in organisms have more complex inheritance patterns than those studied by Mendel

Incomplete Dominance

r r

w rw

Pink

rw

Pink

w rw

Pink

rw

Pink

Neither allele is dominant over the other, therefore the heterozygous phenotype is an intermediate (or BLEND) of the two homozygous phenotypes

Codominance

Both alleles are visible in the heterozygous phenotype

Ex: Roan Coat (both red and white hairs) AB blood (both A and B antigens)

R R

W RWRoan

RWRoan

W RWRoan

RWRoan

Multiple Alleles

When there are more than 2 alleles in the population

Ex: Blood Type (IA, IB, i)

Coat color in Rabbits

(C, cch, ch, c)

Phenotype Genotype

A IAIA, IAi

B IBIB, IBi

AB IAIB

O ii

Polygenic Inheritance

When two or more genes control the trait

The more genes that control the trait, the more variation in phenotypesEx: Height, eye color, skin color in

humans

Feather color in parakeets

Green Blue Yellow White

BBYY BByy bbYY bbyy

BbYY Bbyy bbYy

BBYy

BbYy

The Environmental Influence

Environmental factors (such as temperature, sun exposure, altitude, diet, exercise, etc.) can have significant effects on the phenotypes of certain organisms

In general, the product of a genotype is not typically a defined phenotype, but rather a range of possibilities influenced by the environment.