Page 143 - 86 human physiology part-2

Chapter 18

Polygenic Traits

Polygenic traits are traits determined by the combined effect of more than one pair of genes.
Human stature is an example of this trait. The size of all body parts from head to foot combined
determines height. The size of each individual body part are determined by numerous genes. Human
skin, eyes, and hair are also polygenic genes because they are determined by more than one allele at a
different location.

Intermediate Expressions

When there is incomplete dominance, blending can occur resulting in heterozygous individuals.
An example of intermediate expression is the pitch of a human male voice. Homozygous men have the
lowest and highest voice for this trait (AA and aa). The child killer Tay- Sachs is also characterized by
incomplete dominance.

Co-dominance

For some traits, two alleles can be co-dominant. Were both alleles are expressed in heterozygous
individuals. An example of that would be a person with AB blood. These people have the
characteristics of both A and B blood types when tested.

Multiple-Allele Series

There are some traits that are controlled by far more alleles. For example, the human HLA system,
which is responsible for accepting or rejecting foreign tissue in our bodies, can have as many as
30,000,000 different genotypes! The HLA system is what causes the rejection of organ transplants. The
multiple allele series is very common, as geneticists learn more about genetics, they realize that it is
more common than the simple two allele ones.

Modifying and Regulator Genes

Modifying and regulator genes are the two classes of genes that may have an effect on how the
other genes function. Modifying Genes alter how other genes are expressed in the phenotype. For
example, a dominant cataracts gene may impair vision at various degrees, depending on the presence of
a specific allele for a companion modifying gene. However, cataracts can also come from excessive
exposure to ultraviolet rays and diabetes. Regulator Genes also known as homoerotic genes, can either
initiate or block the expression of other genes. They also control a variety of chemicals in plants and
animals. For example, Regulator genes control the time of production of certain proteins that will be
new structural parts of our bodies. Regulator genes also work as a master switch starting the
development of our body parts right after conception and are also responsible for the changes in our
bodies as we get older. They control the aging processes and maturation.

358 | Human Physiology