SUPPLEMENTARY MATERIAL Yale University: Science Education Outreach Program
I.A) Some Readily Observable Human Traits
Clearly human traits are most interesting to humans. A number of traits listed below exhibit Mendelian inheritance.
- Mid-digital hair
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 a hand lens to determine your phenotype. Even the slightest amount of hair indicates the dominant condition.
- Tongue rolling
The ability to roll one's tongue is due to a dominant allele, T. The homozygous-recessive condition (tt) results in inability to roll one's tongue.
- Widow's peak
Widow's peak describes a distinct downward point in the frontal hairline and is due to the dominant allele, W. The recessive allele, w, results in a continous hairline. (Omit study of this trait if baldness is affecting the hairline.)
- Earlobe attachment
Most individuals have free earlobes (FF, Ff). Homozygous recessives (ff) have earlobes attached directly to the head.
- Hitchhiker's thumb
Although considerable variation exists in this trait, we will consider those individuals who cannot extend their thumbs backward to approximately 45° to be carrying the dominant allele, H. Homozygous-recessive persons (hh) can bend their thumbs at least 45°, if not farther.
- Relative finger length
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 index finger (S) is dominant. In females, it is recessive. In rare cases each hand may be different. If one or both index fingers are greater than or equal to the length of the ring finger, the recessive genotype is present in males and the dominant present in females.