Survey of Human Heredity
Some variations in human beings are inheritable. These variations can be studied
using genetic principles. Some examples are given below:
Eye color is an example of Mendelian inheritance in man. Eye color is a
polygenic trait and is determined primarily by the amount and type of pigments
present in the eye’s iris. Humans and animals have many phenotypic variations
in eye color. In humans, these variations in color are attributed to varying ratios
of eumelanin produced by melanocytes in the iris. The brightly colored eyes of
many bird species are largely determined by other pigments, such as pteridines,
purines, and carotenoids.
Three main elements within the iris contribute to its color: the melanin
content of the iris pigment epithelium, the melanin content within the iris
stroma, and the cellular density of the iris stroma. In eyes of all colors, the iris
pigment epithelium contains the black pigment, eumelanin. Color variations
among different irises are typically attributed to the melanin content within the
iris stroma. The density of cells within the stroma affects how much light is
absorbed by the underlying pigment epithelium.
Darker colors like brown are dominant over blue and gray. In some albinos,
the iris is pink because the blood in the retinal layer is visible. The black circle in the center of the iris is the result of a sex-linked recessive gene.
Variations in the size and form of the ears and their position on the head
indicate multiple gene inheritances. A few of them correspond to variations in
a single gene.
Free Ear Lobes
Free ear lobes are dominant over attached ear lobes, but there is variation in
those that are not attached. On the outer margin of the pinna is a rolled rim;
there is a variation in its size. In some, it is almost lacking.
Nearly all persons have an enlarged portion of the cartilage that projects
inwards from the rolled rim at a distinct point. It is called Darwin’s point, which
is inherited as dominant. In some persons, it is expressed only 1 ear. The
inheritance of natural ear lobes is the expression of an autosomal dominant
gene. Many persons show this characteristic expressed on only 1 side.
A widow’s peak is a descending V-shaped point in the middle of the hairline
(above the forehead). The trait is inherited genetically and dominant. This is
one of the most important Mendelian traits found in human beings.
The shape and size of the tongue is the result of many different genes. Some
people have the ability to roll their tongue into a “U” shape. This is the
expression of a dominant gene. The gene gives a few individuals the ability to
fold the tongue.
The ability to hyperextend the thumb (extended backwards at the last joint) is
due to a recessive allele (n), and a straight thumb (N) is dominant to hitchhiker’s
The ability to taste phenylthio carbamide (PTC) is dominant (T) to the inability
to taste it (t). Your instructor will provide you with small pieces of paper that
have been previously soaked in this harmless chemical.
This characteristic is controlled by helandric genes. It is characterized by hair
on the pinna. It follows the linear pattern of inheritance that is transferred from
father to son, but never to daughter.