Download Human genetic traits can be used to illustrate a num

Human Genetics Lab
[adapted from Winchester, A. M. and Wejksnora, P. J. Laboratory
Manual of Genetics. 4th ed. New York. McGraw-Hill. 1996]
Human genetic traits can be used to illustrate genetic principles, such as complete dominance, incomplete
dominance, codominance, and sex-linkage. In this laboratory exercise, you will examine some human
genetic traits that illustrate basic genetic principles.
I. Single Gene Human Characteristics
Many human characteristics are determined by interactions among several loci (i.e. polygenic inheritance)
as well as by environmental factors. In this exercise, however, you will examine characteristics that
appear to be monogenetic, i.e. determined by different alleles at a single locus, and are not subject to
much environmental modification. A number of such characteristics are listed below. You should
determine your phenotype and your genotype for each, and write them on the data sheet. When you have
a dominant phenotype, you may not know if you carry the recessive allele. In such cases you should use a
dash (-) to represent the unknown second gene of your genotype. For instance, you probably do not have
cystic fibrosis, a recessive characteristic. Hence, you know that you carry at least one of the dominant
alleles (C), but you do not know whether the second allele of the pair is (C) or (c). You, therefore,
represent your genotype as (C-).
Attached earlobes. In most people the earlobes hang free, but when a person is homozygous for a
certain recessive gene (e), the earlobes are attached directly onto the side of the head.
Widow's peak. If your hairline forms a distinct point in the center of your forehead, you have what is
known as widow's peak. It results from the action of a dominant gene (W).
Tongue rolling.
A dominant gene (R) results in the ability to roll the tongue into a U-shape.
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 your muscles, and observe your little fingers.
Hitchhiker's thumb. This characteristic, caused by the recessive gene (h) is known more exactly as
"distal hyperextensibility of the thumb". Homozygotes for h can bend the distal joint of the thumb back
until there is almost (but not quite) a 90 degree angle between the two phalanges. There is some variation
in expressivity, because occasionally it is found in only one thumb. Also, the recessive genotype is only
95% penetrant-- that is, about one person in twenty with the genotype will not express the characteristic.
Long palmar muscle.
Homozygosity for the recessive gene (1), causes a long palmar muscle, which
can be detected by examination of the tendons that run over the inside of the wrists. Clench your fist
tightly and flex your hand backwards. Now feel the tendons. If there are three, you have the long palmar
muscle. If there are only two (the large middle one will be missing), you do not have this muscle.
Examine both wrists, as it sometimes is present in one and not the other because of variable expressivity.
If you find it in one or both wrists, you have the two recessive genes (ll).
Pigmented iris of the eyes.
Homozygosity for the recessive gene (p) results in a lack of pigment in
the front part of the eyes, therefore a blue layer at the back of the iris shows through. This gives blue eyes.
A dominant allele of this gene (P) causes pigment to be deposited in the front layer of the iris and masks
the blue to a varying degree. Other genes determine the nature and density of the pigments that produce
brown, hazel, violet, green, etc. eye colors. However, we only care about the presence or absence of such
pigment, i.e. blue vs. all other colors.
Curly hair.
Curly hair (C) is incompletely dominant over straight hair. Heterozygotes (Cc) have
wavy hair. This is due to cross-sectional shape of the hair. Flat hair  curly; round  straight.
Mid-digital hair. The complete absence of hair from the middle phalange of all fingers is recessive
(mm) to the presence of mid-digital hair on one or more fingers. There seems to be a number of M alleles
that determine whether hair grows on one, two, three, or four fingers.
PTC tasting.
Some people detect a distinct bitter taste from a chemical, phenylthiocarbamide (PTC),
while others do not taste it. A dominant gene (T) seems to confer the ability to taste low concentrations of
PTC. Place a paper strip impregnated with PTC on your tongue and allow it to remain there for about ten
seconds. If you are a taster, you will know it; if you’re not sure, then you are a non-taster (tt).
Index finger shorter than the ring finger. This characteristic appears to be sex-influenced. In males,
it is autosomal dominant; in females it is recessive. Use the symbol (SS) for a shorter second finger and
the symbol (SL) for a longer second finger. These should be tabulated according to sex.
Interlocking fingers. When the fingers are interlocked, some people place the left thumb on top of the
right, while others place the right thumb over the left. Studies suggest that placing the
left over the right is due to a dominant gene (F), while the right thumb on top is due to a recessive (f).
Cheek dimples.
Dimples (D) is dominant to no dimples (d). Variable expressivity can cause one
or both cheeks to have dimples.
Chin dimple (cleft chin).
Freckles.
A chin dimple (DC) is dominant over lack of a chin dimple (dC).
Freckles (F) is dominant over no freckles (f).
Single Gene Human Characteristics
Tabulate your phenotype for each of the human characteristics studied. Also give your possible genotype
and the total number of students in the class that show each alternative for a characteristic.
Characteristic
Attached/free earlobes
Widow's peak – yes, no
Can/can’t roll tongue
Bent/straight little finger
Hitchhiker's thumb – yes/no
Long palmar muscle – yes/no
Pigmented/non-pigmented iris
Curly/wavy/straight hair
PTC taster/non-taster
Cheek dimples – yes/no
Chin dimple – yes/no
Freckles – yes/no
Mid-digital/No mid-digital hair
Interlocking fingers – L/R
2nd finger shorter than 4th - y/n
female
2nd finger shorter than 4th - y/n
male
Your
Phenotype
Your
Genotype
Number of
Each in the
Class
Frequency of
Each in the
Class