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Human Genetics Activity
Several human traits are governed by a single gene pair and are readily assessed in people. Today you will
determine the phenotype of yourself and 2 others for several of these traits.
Use the table on the next page to keep record of the genotypes of you and 2 other people. If possible, use
siblings and parents to get the best explanation possible for your traits.
Hitchhikers thumb – “distal hyper-extensibility of the thumb” Some people will be able to bend the distal joint
back to form a nearly 90◦ angle with the rest of the thumb due to a recessive mutation, h. Those who cannot
bend their thumb back have the dominant allele H. Stick your thumb straight up and bend it backwards as far
as you can.
Attached ear lobes – Most people have free-hanging earlobes E, but those with a recessive allele, e, have ear
lobes that are attached to the side of the head. Using a mirror or another person determine whether your ear
lobes hang free or are attached.
Widow’s peak – A dominant allele, W, results in a distinct pattern of hair growth with hair growing to a point on
the forehead. If no point is present, then the allele for continuous hair, w, is present. Determine if you have
the widow’s peak or the continuous hairline. If the gene for baldness has had its effect on the front of the
head, then skip this analysis.
Bent little finger – The little finger (“pinky”) may be straight or bent. Lay your hand on the table and relax it. If
your little finger bends in towards the rest of your fingers, you have the dominant allele, B, if not you have the
recessive, b.
Hair on mid-digits – The presence of hair on the middle joints of the fingers is the result of a dominant allele,
M, while the complete absence of hair results from the recessive, m. Analyze your fingers to determine
whether any of them have hair on the middle joint. It is thought that additional gene(s) determine the number
of joints on which hair grows.
Tongue rolling – A dominant gene, R, provides the ability to roll the tongue into a U-shape while those carrying
the recessive, r, cannot roll their tongue. Determine whether you can roll your tongue or not.
Long palmar muscle – A recessive allele, p, allows growth of the long palmar muscle in the wrist. To
determine your phenotype, clench your fist and look at the inside of your wrist. If you can see three distinct
tendons, you have the long palmar muscle, if you have two (you are missing the middle one) you do not have
this muscle. You might have to check both arms.
Pigmented iris of eye – There are many different eye colors. The absence of pigment in the front of the iris is
determined by the recessive allele, u. This situation results in a blue iris because the back layer is blue and it
shows through if no pigment is present. The pigments can range in color from green to hazel to violet to a
whole array of brown colors with each of these pigments being controlled by separate genes.
Interlocking fingers – A dominant allele, I, predisposes an individual to place their left thumb over their right
when they interlock their hands where the recessive allele, i, predisposes the individual to place the right
thumb over the left. Non-genetic factors may also be involved in this phenotype.
Freckles – Freckles can be found in almost anyone, regardless of race, and is generally caused by the
presence of a dominant allele, F, which predisposes an individual to have deposits of concentrated melanin;
that is often more noticeable in people with fair complexions. The lack of this allele results in the lack of
genetically caused freckles; however there are additional non-genetic factors that may also be involved in this
Hallux length (toes) – A dominant allele, T, causes the second toe to be longer than the first. If the big toe is
longer, you have the recessive allele, t.
Cleft chin – A dominant allele, Y, creates a Y-shaped fissure on the chin due to incomplete fusion of jaw
halves during embryonic development. The absence of a cleft chin is due to the recessive allele, y.
Dimples – A dominant allele, D, causes underlying flesh to form on some people’s cheeks, especially when
they smile, creating indentions. Those having the recessive allele, d, don’t have these.
Fill in the following table for each of the following traits that you carry. Record your phenotype AND
genotype, as well as the phenotypes of the 2 others based on the above data. (Dominant genotype can be
recorded as appropriate upper case letter and an underscore ‘T_’ when unable to distinguish between TT and
Person’s name
Hitchhikers thumb
(yes or no)
Ear lobes
(free or attached)
Widow’s peak
(yes or no)
Bent little finger
(yes or no)
Hair on mid-digits
(yes or no)
Tongue rolling
(yes or no)
Long palmar muscle
(yes or no)
Pigmented iris
(yes or no)
Interlocking fingers
(left top or right top)
(yes or no)
Hallux (big toe) length
(longer or shorter)
Cleft chin
(yes or no)
(yes or no)
Person 1’s
Person 1’s
Person 2’s
Person 2’s