Slide 1
The examples we have looked at thus far, have only included genes
with two alleles.
Many genes, however, have more than two alleles in the population - a situation referred to as multiple alleles. Note - each diploid individual in the population is diploid can only have at most two different alleles - but in the entire population there can be more than two alleles.
A fairly straightforward example of this is the human ABO blood group.
There are 3 alleles - the A allele (indicated with a capital I superscript A),
the B allele (indicated with a capital I and a superscript B), and the O
allele indicated with a lower case i. Within this group there are two
different dominance relationships. We have already discussed how allele
A and allele B are codominant to each other. Both of these alleles (A
and B), are completely dominant to allele O. An individual who is
heterozygous for A and O has blood type A. Similarly for an individual
who is heterozygous for B and O has blood type B.
Slide 2
Sometimes a gene can have an effect on more than one aspect of an
organism's phenotype. This situation is referred to as pleiotropy.
Slide 3
An example of pleiotropy is the frizzle gene in chickens.
This gene is referred to as "frizzle" because it results in frizzy looking
feathers. The gene that causes this phenotype also causes the chicken to
have abnormal body temperature, higher metabolic and blood flow rates, and
greater digestive capacity.
It is a single gene, we will call it gene X, that has an impact on all of these aspects of the chicken's phenotype.
Slide 4
Epistasis refers to a relationship between at least two different
genes and one gene affects expression of another gene (or genes). A
common example of epistasis is coat color in mammals. The example we are
looking at here is coat color in horses. There are two different genes
that play a role in determining coat color. Gene B codes for pigment -
with the dominant B allele coding for black coat color and the recessive b
allele coding for a lighter brown coat color. Gene C is epistatic on
gene B (epistasis literally means "standing upon") - it determines whether or
not gene B gets expressed. The dominant C allele allows the pigment to
be deposited - so a horse that has at least one copy of the C allele will be
either black or brown. The recessive c allele, on the other hand, blocks
pigment deposition - any horse that has two copies of the c allele (is
homozygous recessive for c) - will be a whitish color. This is because
gene C is epistatic on gene B and controls whether or not it is expressed.
Slide 5
Epistasis and pleiotropy are often confused with each other but
they are quite different. One important difference is the number of
genes involved. Epistasis involves at least two genes but only one
aspect of the phenotype is affected. Coat color in horses is an example
- two genes but only one character - coat color.
Pleiotropy involves a singe gene - but the gene impacts several aspects of the phenotype - like frizzled chickens with their different body temperature and metabolism. A single gene caused all of these changes to the phenotype.
Slide 6
Most genetic traits are not determined by a single gene - most
traits are polygenic (the prefix poly means many). Skin color in humans,
for example, is a polygenic trait. Here at Penn State, Dr. Mark Shriver
in the department of anthropology is attempting to work out the exact number
of genes that are responsible for skin color in humans.
The model organism that he uses to learn more about our pigment genes is actually a small species of fish known as a zebrafish (zebrfish are common lab organisms). The gene that he is currently looking at is known as SLC24A5 - catch huh? This gene plays a role in pigment production in both zebrafish and humans. We know that there are other genes involved in skin color and other research groups around the world are looking at these genes. Some day we may know all of the genes that are responsible for skin color in humans.