In the preceding problems, we examined several
human traits that are fairly simple and that follow the
Mendelian pattern of inheritance. Most of our traits are much
more complex, involving many genes of interactions between
genes. As an example, hair color is determined by at least four
genes, each one coding for the production of melanin, a brown
pigment. Because the effect of these genes is cumulative, hair
color can range form (little melanin) to very dark brown (much
melanin).
Clearly, human traits are most interesting to
humans. A number of traits listed below exhibit Mendelian
inheritance. For each, examine your phenotype and fill in
Table 11-1. List your possible genotype(s) for each trait. When
convenient, examine your parents’ phenotypes and attempt to
determine your actual genotype.
1) Mid-digital hair (Figure 11-2a).
Examine the joint of your fingers for the presence of hair, the
dominant condition (MM, Mm). Complete absence of hair is due to
the homozygous recessive condition (mm). You may need hand lens
to determine your phenotype. Even the slightest amount of hair
indicates the dominant condition.
2) Tongue rolling (Figure 11-2b). The
ability to roll one’s tongue is due to dominant allele, T. The
homozygous recessive condition, t, results in the inability to
roll one’s tongue.
3) Widow’s peak (Figure 11-2c).
Widow’s peak describes a distinct downward pint in the frontal
hairline and is due to the dominant allele, W. The recessive
allele, w, results in a continuous hairline. (Omit study of this
trait if baldness is affecting the hairline.)
4) Earlobe attachment (Figure
11-2d). Most individuals have free earlobes (FF, Ff). Homozygous
recessives (ff) have earlobes attached directly to the head.
5) Hitchhiker’s thumb (Figure
11-2e). Although considerable variation exists in this trait, we
will consider those individuals who cannot extend their thumbs
backwards 45 degrees to be carrying the dominant allele, H.
Homozygous recessive persons (hh) can bend their thumbs at least
45 degrees, if not farther.
6) Relative finger length (Figure 11-2f).
An interesting sex-influenced (not sex-linked) trait relates to
the relative lengths of the index and ring finger. In males, the
allele for a short (S) index finger is dominant. In females, it
is recessive. In rare cases each hand may be different. If one
or both index fingers are greater or equal to the length of the
ring finger, the recessive genotype is present in males, and the
dominant present in females.
