Readily Observable Human Traits
Clearly human traits are most interesting to humans. A number of traits listed below exhibit Mendelian
1. 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.
2. 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.
3. 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.)
4. Earlobe attachment
Most individuals have free earlobes (FF, Ff). Homozygous recessives (ff) have earlobes attached
directly to the head.
5. 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.
6. 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.
Taste Buds: The PROP Method
1. What is PROP?
PROP (6-n-propylthiouracil) is a medication used to treat Grave's Disease (hyperthyroidism). PROP is designed to reduce the thyroid's
ability to produce hormones. People taking this medication might take 50 mg (3-4 tablets each) daily. Each PROP paper contains
about 1.6 mg PROP. Follow the procedure below for preparing PROP papers.
2. Staining Fungiform Papillae
The bumps on the anterior surface of the tongue are papillae. The most numerous are the filiform papillae but these have no taste
function. The fungiform papillae look like tiny button mushrooms (hence their name). The fungiform papillae are the most densely
distributed on the tip of the tongue and thin out as you move toward the center of the tongue. The fungiform papillae are the structures
that contain taste buds. Although supertasters tend to have more taste buds on each papillae than nontasters, what really makes them
supertasters is that they have many more fungiform papillae. Counting fungiform papillae requires only a magnifying glass, a
flashlight, blue food coloring and Q-tips.
Swab the blue food coloring on the front of the tongue (cover the tip and about 1/2 inch back). Have the subject move the tongue
around in the mouth and swallow. This distributes the dye. Swallowing the dye is not hazardous. You will see pink circles emerge
from the blue background. The pink circles are the fungiform papillae. The fungiform papillae appear pink because they do not stain
as well as the filiform papillae.
For a complete description of the method used in the laboratory see: Bartoshuk, L.M., Duffy, V.B., Miller, I.J.,
PTC/PROP tasting : Anatomy, psychophysics, and sex effects. Physiology and Behavior (1994) 56:1165-1171
3. A simple classroom experiment.
This is an experiment that could be done in a science lab or classroom as well as used for research. You need a template that can be
placed on tongues in a consistent manner. For example, you can use a paper punch to punch a hole in a 1 inch square of wax paper or
you can use the reinforcements sold for notebook paper. Either of these wil give you a hole about 6-7 mm in diameter. Place the
template on the tongue so that the hole is in a standard position (see sketch below).
Swab blue food coloring on the tongue. Using a flashlight and a magnifying glass, count the number of fungiform papillae. The sketch
below shows examples of a supertaster and a nontaster.
The circles correspond approximately to the hole produced by a typical paper punch or to the hole inside a notebook paper reinforcer.
In the laboratory, we can look at the stained tongue under a microscope. We see small blue dots on most of the fungiform papillae.
These are taste pores, the conduits to tast buds. In size, the taste pores are to the fungiform papillae as a sesame seed is to a hamburger
4. Preparing PROP papers
Source of PROP (6-n-propylthiouracil) (pharmaceutical grade):
172 E. Aurora St.
Waterbury, CT 06708 (203-574-0075)
The PROP papers are made on filter paper (Whatman #1). Heat tap water to near boiling and then add 5 grams PROP to 500ml water.
The aim is to make a saturated solution. (As the solution cools, crystals will precipitate out of solution.) Dip the pieces of filter paper
in the PROP solution so that they are completely soaked. We use small pieces of filter paper (3 cm circles) and allow them to dry
individually on sheets of aluminum foil. The PROP crystallizes into the filter paper. Thuse the paper is a convenient way to deliver a
few crystals of PROP to a subject.
To taste the paper, put the whole piece in your mouth and let it get well moistened with saliva. The bitter taste may build slowly or
may be perceived immediately. When the bitter taste is at a maximum, the bitterness can be rated on a scale (see item 5).
This procedure was kindly provided by:
Linda M. Bartoshuk
Yale University School of Medicine
Department of Surgery
333 Cedar St.
New Haven, CT 06520-8041
Drosophila Life Cycle