Problem Set 3.11: An Inherited Trait (Human Genetics)
Read pages 384 to 386 in your text and read the text below. You will test yourself and at least one member of your family. Do not print pages 1 and 2 of this assignment.
Some Inherited Human Characteristics ...
Attached Ear Lobes:
In most people, the ear lobe hangs free, but when a person is homozygous for a certain recessive gene (ee), the ear lobes are attached directly to the side of your head so that there is no lobe hanging free. With the help of your classmate, determine your phenotype for this characteristic, and tabulate it on the Summary Sheet. You will find that there is considerable variation in the site and appearance of the lobes of those who have them, and you should concentrate only on the presence or absence of lobes in this study.
In some people the hairline drops downward and forward and forms a distinct point in the center of the forehead. This is known as the Widow's Peak. It results from the action of a certain dominant gene (W). Determine your phenotype by having your partner examine your front hairline for a widow's peak, or a continuous hairline. (You will have to skip this tabulation if a gene for baldness has had its effect at the front part of the head, but at your age this should not pose a problem).
A dominant gene (R) gives some people the ability to roll the tongue into a distinct U-shape when the tongue is extended from the mouth. Others, who do not have this gene, can do no more than produce a slight downward curve of the tongue, when it is extended from the mouth. Try this and record your phenotype on the Summary Sheet.
Bent Little Finger:
A dominant gene (B) causes the last joint of the little finger to bend inward toward the fourth finger. Lay both hands flat on the table, relax the muscles and note whether you have a bent or a straight little finger. and record your phenotype on the Summary Sheet
This characteristic, known more exactly as "distal hyper-extensibility of the thumb" can be determined by bending the distal joint of the thumb as far back as possible. While there tends to be some degree of continuous variation, it will be found that certain persons can bend it back until there is almost (but not quite) a 45 degree angle between the two joints. Evidence indicates that this characteristic is due to a recessive gene (hh). Study your own thumbs and tabulate your findings on the Summary Sheet
Long Palmar Muscle:
When a person is homozygous for a certain recessive gene (ll), he has a long palmar muscle, which can be detected by examining your tendons that run over the inside of your wrists. Clench your fist tightly and flex your hand. Now feel the tendons. If there are three, you have a long palmar muscle. If there are only two, (the large one in the middle will be missing) you do not have this muscle. Examine both wrists; if you find it in one or both wrists, you have two recessive genes. If it is not present in either wrist, you have the dominant gene (L). Record your phenotype on the Summary Sheet
Pigmented Iris of the Eyes:
When a person is homozygous for a certain recessive gene (pp), there is no pigment in the front of the eyes and a blue layer at the back of the iris shows through. This gives blue eyes. A dominant gene (P) causes pigment to be deposited in the front layer of the iris and makes the eye blue to a varying degree. Other genes determine the exact nature and density of this pigment and we have brown, hazel, violet, green and other eye colors. We will concern ourselves here, however, only with the presence or absence of such pigment. Determine your phenotype for pigmented or unpigmented iris.
(Note: ... sometimes the layer at the back of the iris is gray, and this should be counted as unpigmented).
Record your phenotype on the Summary Sheet
Some people have hair on the second, middle joint of their fingers while others do not. The complete absence of hair from all fingers is due to a recessive gene (mm); the presence of hair is due to a dominant allele (M). This hair may be very fine, and you should use a hand lens and look very carefully on all fingers before deciding whether this hair is absent from any one of your fingers. Tabulate the results on the Summary Sheet
When the fingers are interlocked, some people will almost invariably place the left thumb on top of the right and others will place the right over the left. Studies of family pedigrees indicate that the placing of the left over the right is due to a dominant gene (F), while the right thumb on top is due to a recessive gene (ff). Record your phenotype on the Summary Sheet
Human Genetics Summary Sheet
Some Inherited Human Characteristics
All features you record on the associated handout will depend upon you and your family characteristics. There are no correct or model answers.