What are incomplete dominance and co-dominance?
Incomplete Dominance: Usually if an allele is dominant over
another allele, the dominant trait is the only one that shows in
the offspring. Sometimes there is no completely dominant trait.
This type of inheritance is called incomplete dominance. In cases
of incomplete dominance, offspring will show a blending of traits
from each parent.
Incomplete dominance can be seen in many organisms. In four
o’clock flowers, red and white are possible flower colors. Neither
allele is completely dominant over the other. Homozygous red plants
are given the symbol RR. Homozygous white plants are given the
symbol WW. The capital letter R stands for red petal color. The
capital letter W stands for white petal color. Figure 3-20 shows
what can happen when homozygous red-flowering plants are crossed
with homozygous white-flowering plants.
Co-dominance: A different type of inheritance patter occurs when
both alleles of a certain trait appear in the offspring. This is
called co-dominance. As in the case of incomplete dominance,
neither allele is dominant over the other. However, co-dominance is
different from incomplete dominance because it does not involve
blending. The alleles from each parent both appear in the
offspring.
Co-dominance in Chickens: There is a kind of chicken that has a
gene for black feathers and a gene for white feathers. Neither of
these genes is dominant over the other. The capital letter B stands
for black feathers. The capital letter W stands for white feathers.
A chicken that is homozygous for black feathers has the symbol BB.
A chicken that is homozygous for white feathers has the symbol WW.
When a homozygous black chicken and a homozygous white chicken are
mated, all of the offspring have the gene combination BW. These
heterozygous offspring have both black and white feathers.
Heterozygous Crosses: Recessive genes do not disappear. They
show up again when heterozygous organisms are mated. If two
heterozygous black and white chickens are mated, there will be a 25
percent chance of the offspring being BB, or homozygous black.
There will be a 50 percent chance of the offspring being BW, or
heterozygous black and white. There will be a 25 percent chance of
the offspring being WW, or homozygous white.