Incomplete Dominance, Co-Dominance, Multiple

Alleles, & Polygenic Inheritance

Beyond Mendel’s Observations of

Inheritance

Incomplete Dominance

A condition in which neither allele for a gene

completely conceals the presence of the other; it

results in intermediate expression (blending) of a

trait.

Example: a RED flower crossed with a flower

results in a PINK flower.

PINK must be heterozygous because it is the

intermediate colour as a result of blending RED and

.

Therefore, RED and must be homozygous.

Incomplete Dominance

Co-Dominance

The condition in which both alleles for a trait are equally expressed in a heterozygote; both alleles are dominant.

Sickle Cell Anemia

Sickle cell anemia is

an example of co-

dominance.

This disease results in

red blood cells which

are sickle-shaped

and do not transport

oxygen effectively.

Heterozygous Advantage

Sickle cell anemia is considered co-

dominant because it provides a

“heterozygous advantage”, one in which a

heterozygous individual for this trait has a

survival advantage because having two

different alleles for this trait provides

more resistance to the life-threatening

blood disease malaria.

Sickle Cell Anemia

Multiple Alleles

Many traits in humans and other

species are the result of the

interaction of more than two

alleles for one gene.

A gene with more than two alleles

is said to have multiple alleles.

Multiple Alleles: Human Blood Groups

There are 4 different phenotypes and 6

different genotypes that are used to describe

human blood groups.

The letter I (i) is used when describing the

gene for blood groups.

Alleles “A” and “B” are both dominant and

exist co-dominantly.

Allele “o” is recessive to both “A” and “B”.

Multiple Alleles: Human Blood Groups

Phenotype Genotype

A IAIA – Homozygous

IAi – Heterozygous

B IBIB – Homozygous

IBi – Heterozygous

AB IAIB – Heterozygous

o ii - homozygous

Multiple Alleles: Human Blood Groups

Multiple Alleles: Rabbit Coat Colour

Another example of multiple alleles involves coat

colour in rabbits.

The gene that controls coat colour in rabbits has four

alleles: agouti (C), chinchilla (cch), Himalayan (ch),

and albino (c).

In that order, each allele is dominant to all the

alleles that follow.

The order of dominance sequence can be written as

C > cch > ch > c, where the symbol > means is

dominant to.

Multiple Alleles: Rabbit Coat Colour

Polygenic Inheritance

A polygenic trait is a

trait that is controlled by

more than one gene.

Polygenic traits often

time exhibit continuous

variation – a range of

variation in one trait

resulting from the

activity of many genes.