Professor Dave again, let’s revisit Mendel.
When we looked at Mendelian genetics, we learned all about genotypes and phenotypes, alleles
and Punnett squares.
But even then we already began to see that the basic model of dominant and recessive
alleles does not explain certain phenomena.
We looked at incomplete dominance, such as when true-breeding red and and true-breeding
white snapdragons are hybridized to yield an entirely pink F1 generation.
Thus a third intermediate phenotype is involved, totally unlike complete dominance, where only
the dominant phenotype is expressed.
We also looked at codominance, where two phenotypes can be expressed simultaneously, as was the
case with the cows with both white and brown coloring.
Building on what we already know, let’s continue to extend this understanding into
still more interesting and complex patterns of inheritance.
First, it is important to understand that in cases of dominant and recessive alleles,
one allele is dominant simply because it is visible in the phenotype, and not because
it somehow subdues expression of the recessive allele.
With heterozygous individuals, both the dominant and recessive alleles are expressed, it is
simply that the product of the expression of the dominant allele produces the relevant
phenotype.
For example, when Mendel examined seed shape, he noticed that seeds could be round or wrinkled,
with round being dominant.
We now know that this is because the dominant allele codes for an enzyme that helps convert
unbranched starch into a branched form, while the recessive allele codes for a defective
form of that enzyme, thereby allowing unbranched starch to accumulate, which causes water to
enter the seed by osmosis, and when the seed dries, it wrinkles.
So if at least one dominant allele is present, in the form of the heterozygous or homozygous
dominant genotypes, the correct enzyme will be produced, and the seed will be round.
With the heterozygous organism, both the effective enzyme and the defective enzyme are produced,
but there is enough of the effective version to get the job done.
Only if homozygous recessive will there be no effective enzyme present at all, in which
case the seed will wrinkle.
Now with the understanding that all alleles will be expressed, we can better understand
situations involving multiple alleles, meaning more than simply one dominant and one recessive.
An example of this is blood type in humans.
With this, there are three alleles that are possible for a single gene.
These are IA, IB, and i, in lowercase, where the i stands for immunoglobulin.
The letters A and B refer to carbohydrates that can be found on the surface of red blood
cells, whereby expression of IA yields the A carbohydrate, expression of IB yields the
B carbohydrate, and expression of i does not result in any carbohydrate.
Therefore, different combinations of these alleles can yield blood of type A, B, AB,
or O, if neither of these carbohydrates are found.
We discussed the compatibility of blood types for transfusion in the anatomy and physiology
series, so head over there for more information in that context.
Now let’s step things up a notch.
Until now, even in situations with multiple alleles, we have only seen cases where a particular
gene influences only one phenotypic character.
Often times, however, a single gene can have multiple phenotypic effects.
This is a situation called pleiotropy.
In humans, pleiotropic alleles produce multiple symptoms associated with hereditary diseases
like cystic fibrosis and sickle-cell disease, whereas with pea plants, a single gene determines
both flower color as well as the color of the outer surface of the seed.
In contrast, there are other situations where multiple genes are involved in the expression
of a single phenotype.
When this happens because of the interaction of the products of gene expression, it is
called epistasis.
For example, in Labrador retrievers, the dominant coloring is black, while recessive is brown,
represented by a capital B and lower case B respectively.
This allele determines the pigment that produces the coloring of the dog.
However, there is another gene that determines whether or not the pigment will be deposited
onto the fur, with a dominant allele resulting in the pigment being deposited.
So if the dog is homozygous recessive for this trait, then the dog will end up yellow,
no matter which alleles are present for the pigment, as the pigment will not be deposited
onto the fur.
Only if the dog is heterozygous or homozygous dominant will the phenotype for color be expressed.
So the gene for pigment deposition is said to be epistatic to the gene that codes for
black or brown pigment.
Mating two dogs that are heterozygous for both traits to get a dihybrid cross clearly
shows the law of independent assortment in action, when examining the distribution of
dogs with different coloring.
An even more complex situation regarding multiple genes contributing to a single phenotype is
called polygenic inheritance.
In these cases, traits are not as simple as existing as one out of two or three discrete
options, instead they exist as gradations along a continuum.
An example of this is skin color in humans.
Most people have skin of one particular tone, and this is determined by a variety of genes,
but this tone can be very dark, very light, or any of a number of intermediate shades.
This is because all the genes working in tandem can have dominant or recessive alleles in
any combination, and thus produce darkness in an additive manner, and through a variety
of genotypes, given the incredible number of combinations possible when three or more
genes have alleles assorted independently.
And finally, there are even phenotypes that are subject to environmental factors, such
as flowers that can display different colors depending on the acidity of the soil, or other
such stimuli.
Such characteristics are referred to as multifactorial, which means that many factors, both genetic
and environmental, contribute to the presence of a particular phenotype.
So beyond our previous introduction to Mendelian genetics, as well as the degrees of dominance
we had already examined, we have now broadened our understanding of methods of inheritance
to include phenomena like pleiotropy, epistasis, polygenic inheritance, and environmental influence.
With all of these additional complications, it is incredible that Mendel was able to do
the work that he did.
But more importantly, we are now able to elevate our comprehension of inheritance such that
we may examine a variety of genetic disorders, and come up with strategies for treatment,
thanks to our modern understanding of molecular biology.