Lesson Four:
Extending Mendelian Genetics
Incomplete Dominance
In Mendel’s experiments, each trait
observed exhibited complete dominance
of one allele over another. It doesn’t
always happen that neatly.
In some instances, the F1 hybrids have
the appearance somewhere in between
the appearances of the parents. This is
called incomplete dominance.
Incomplete Dominance
In the case of the
snapdragon, breeding a red
colored flower to a white
colored flower produces
pink offspring.
From these, offspring
have the ratio of one red,
two pink and one white
flowers.
This shows that the Law of
Segregation still stands.
Codominance
Another situation which Mendel’s experiments
failed to grasp was codominance. Codominance is
the situation in which both alleles are separately
manifested.
This differs from incomplete dominance in that the
hybrid is not an intermediate between the two
homozygous phenotypes, but rather, each
phenotype is expressed individually in the
heterozygote.
Codominance
An example of codominance is hair color in short horn
cattle. Red (RR) cattle bred to white (WW) cattle
sometimes birth roan (RW) cattle.
At first glance, it looks
like incomplete
dominance.
However, a closer look reveals
that roan coloring is a result of
having both red and white hairs.
Multiple Alleles
In some cases, there are more than two alleles controlling one
trait. This phenomenon is referred to as multiple alleles.
An example of this is ABO blood types, which refer to
carbohydrates (A substance or B substance) that may be found
on the surface of red blood cells.
There are four phenotypes for this blood group: type A, type B,
type AB or type O.
The alleles for these are symbolized: IA, IB, or i. While IA and
IB are dominant over i, they are codominant to each other.
There are six possible genotypes for the four possible
phenotypes.
Type A: IAIA or IAi
Type AB: IAIB
Type B: IBIB or IBi
Type O: ii
Other Cases
• Pleiotropy –when one gene has multiple effects
– Example: the same allele that causes a cross-eyed
condition causes abnormal pigmentation in tigers
• Epistasis –when one gene affects the expression of a
second gene
– Example: mice have have one gene that regulates black
(B) and brown (b) hair color. They also have a gene for
color (C) or no color (c). No matter the combination of
the B genes, being cc will render a colorless mouse
• Polygenic Inheritance – when more than two or
more genes have an additive effect on phenotype
– Example – skin color has at least three genes affecting it,
which cause gradual change, or quantitative variation.