I. SIMPLE AND MULTIPLE GENE INHERITANCE
Many normal human traits appear to have a genetic base because they "run in families." They do not, however, fit into a simple Mendelian mode of inheritance. Examples include body size and shape, skin color, facial features, and color of the eyes. These cannot easily be attributed to the action of a single gene locus.
When two or more gene loci contribute to a trait, that trait is known as polygenic. There are many traits, such as adult standing height, where environment interacts with the genes to produce the final result. This pattern of inheritance is known as multifactorial.
In subsequent units on variation, growth, and adaptation we will explore the influence of environment on humans, both short term and long term.
This unit is about a small number of frequently appearing non-metric human traits that do not appear to be affected by the external environment. The ones below are inherited in a simple Mendelian manner.
II. DARWIN TUBERCLE
Sometimes called Darwin's point, it is a projection on the helix of the ear resulting from a thickening of the cartilage. It varies in size. The tubercle is inherited as a dominant, but the expression (size) is quite variable. Its absence is inherited as a recessive. Some individuals may have it only on one ear. This feature has been known for a long time. A diagram of this landmark appears in Darwin's The Descent of Man (1871).
III. EARLOBE ATTACHMENT
The free-hanging earlobe is inherited as a dominant. The attached earlobe, or lack of a free-hanging earlobe is inherited as a recessive. Individuals with attached earlobes are characterized by the attachment of the lower part of the ear directly onto the head,
IV. EARWAX (CERUMEN)
Earwax occurs in two forms. The dominant type is sticky and yellow to brown in color. The recessive form is gray and brittle.
V. TONGUE ROLLING
This trait is the ability to roll the tongue longitudinally into a U-shape. Many people are not able to do this, even with practice. Some with the ability to do it need to practice a bit to perform the feat. The ability to roll the tongue is an autosomal dominant. Those who cannot are recessive homozygotes.
VI. HITCHHIKER'S THUMB
When the hand is placed in the hitchhiker's position, observe whether the thumb can be bent back 50 or more. Such distal hyperextensibility (ability to bend the thumb) is inherited as a recessive trait. The inability to do this is the dominant phenotype.
VII. PTC TASTING
For some people (and some chimpanzees also), the chemical phenylthiourea tastes very bitter. For others, it is tasteless. PTC is a harmless compound that tastes bitter to those who have the dominant phenotype. Those with the recessive do not taste the bitterness. PTC-like chemicals are found in the Brassica-family of vegetables, such as cabbages, brussel sprouts, and kale. Some researchers think that tasters have a lower incidence of deciduous dental caries, suggesting that there might be a substance in the saliva of tasters that inhibits bacterial destruction of the teeth. Others think that it may be in some way connected with thyroid function. PTC taste was a chance discovery in 1931.
VIII. MIDDIGITAL HAIR
It is easy to look for the presence or absence of hair on the middle segment of the fingers. If hair is present, that expression is a dominant. Even if just a small amount of hair is present, it qualifies as expression of the trait. The absence of hair is recessive. If hair is not present, the person is homozygous recessive.
IX. HAIR WHORL
This is observed by looking down at the top of a subject's head. A pattern which is clockwise is dominant. A pattern which is counterclockwise is recessive.
..... CJ '98
Bunney, S. The Cambridge Encyclopedia of Human Evolution. New York: Cambridge University Press, 1994.
Helmuth, H. A Laboratory Manual in Physical Anthropology. Toronto: Canadian Scholars. Press, 1993.
Stein, P. and Rowe, B. Physical Anthropology 6th ed. New York: McGraw-Hill, 1996.
Wolfe, L., Lieberman, L., and Hutchinson, D. Laboratory Textbook for Physical Anthropology 4th ed. Raleigh: Contemporary Publishing Company, 1994.