incomplete dominance, co-dominance, multiple alleles ...€¦ · polygenic inheritance a polygenic...
TRANSCRIPT
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.