Study of Mendelian Traits
Some variations in human beings are inheritable. These variations can be studied using genetic principles. Some examples are given below:
Eye Color
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.
Ears
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.
Widow’s Peak
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.
Tongue
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.
Hitchhikers Thumb
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 thumb.
PTC Tasting
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.
Hypertrichesis
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.
Support our developers
