Slide 1
The classic Mendelian genetic crosses we have looked at thus far
in the course only included genes with two alleles and those two alleles had a
complete dominant-recessive relationship. This means that one allele -
for example the purple flower allele in pea plants - is dominant to the
alternative allele - the white flower allele.
However,as we shall see, relationships between alleles are not always dominant-recessive.
Slide 2
There are 3 basic "Flavors" of dominance relationships between
alleles.
Complete Dominance - "Mendelian inheritance"
One allele (dominant)
completely masks other (recessive)
Incomplete dominance
Both alleles contribute to phenotype, but
heterozygote is intermediate to either homozygote (i.e., in monohybrid cross,
three phenotypes are observed).
Co-dominance
Both alleles contribute equally to phenotype-no masking
Next we will look at some examples.
Slide 3
Incomplete dominance occurs when both alleles contribute to the
organisms phenotype and the heterozygous individuals have an intermediate
phenotype.
An example is flower color in snapdragons. In snapdragons there are two alleles for color - red and white. Notice that the way these alleles are written is slightly different than the way completely dominant/recessive alleles are written. Because neither allele is completely dominant it is not appropriate to use upper and lower case letters. Instead, the gene is named after the character - in this case gene C is for flower color. And the alleles are superscripted - R for the red allele and W for the white allele.
Red flowered snapdragons are homozygous for the red allele and white flowered snapdragons are homozygous for the white allele. Pink flowered snapdragons are heterozygous - pink is a color that is intermediate between red and white.
Slide 4
In a codominant relationship - both alleles are equally expressed
in the phenotype and the heterozygous individuals have a unique
phenotype. We will look at the human ABO blood group as an
example. For now - we will just look at alleles A and B (we will talk
about allele O later).
The A allele codes for an A sugar that sits on the surface of human red blood cells. The B allele codes for a B sugar that sits on the surface of human red blood cells. If someone has both the A allele and the B allele - they will have both A and B sugars on the surface of their red blood cells. The two alleles are codominant and the heterozygous phenotype is unique.
Slide 5
To summarize the relationships between alleles we can look at this
dominance continuum. The only way to determine the relationship between
alleles is by analyzing the heterozygotes - the homozygous individuals don't
tell us anything about how two different alleles interact.
In complete dominance, the heterozygote is indistinguishable from the homozygous dominant individuals.
In incomplete dominance, the heterozygote has a phenotype that is intermediate beween both homozygotes.
In codominance, the heterozygote has a unique phenotype because both alleles are fully expressed.