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Name: _______________________
Date: ____
Period: ____
IDENTIFYING HUMAN MONOGENIC TRAITS
Human genetics can become very complicated because many characters are controlled by multiple
genes (polygenic). But, there are a number of monogenic characters, which follow Mendel’s
inheritance predictions. A monogenic character is controlled by a single gene, with two alternative
alleles (specific forms of a gene). One allele is dominant (expressed when present) and the
alternative is recessive (expressed only when the dominant allele is absent). Just because a trait is
dominant does not mean it occurs more frequently in a population. Some dominant alleles are very
rare in a population. Each individual inherits two alleles per character, one from mom and the other
from dad. This is known as your genotype, the specific combination of alleles you inherit from your
biological parents. With only two possible alleles per gene, there are only three possible combinations
of alleles or genotypes: two dominant alleles (homozygous dominant), two recessive alleles
(homozygous recessive) and one dominant and one recessive allele (heterozygous). Your specific
genotype dictates what you look like or your phenotype, your physical expression for a particular trait.
In this activity, we will identify our individual phenotypes for a handful of monogenic human traits.
From there, we will try to identify our genotypes. If you have the recessive phenotype, you can easily
identify your genotype, you must be homozygous recessive. But, if you have the dominant phenotype,
it is not as easy to identify genotype, because you might be homozygous dominant or heterozygous,
unless you analyze data from your biological parents. If one of your parents has the recessive
phenotype (homozygous recessive genotype), you know you must be heterozygous. Sometimes,
even with this information, you still can not identify your genotype.
MATERIALS
Untreated taste paper PTC taste paper
Mirror
Part 1 - Procedure
1) Place the untreated paper on your wet tongue, to see how it tastes. Dispose of this
paper in the trash when finished.
2) Next, place the PTC paper on your wet tongue to see if you are a PTC taster. Record your
results in the data table.
3) Using the description of each of the monogenic human traits in this activity, identify
your phenotype and record it in data table 1. You may need to use a mirror or ask your
classmates to help you identify some of these phenotypes. Some of the phenotypes may
be difficult to identify exactly, just use your best estimate.
Part 2 - Procedure
4) Identify your possible genotypes for each of these traits and record possible genotypes
in data table 1.
5) Using data table 2, record the phenotypic results for the entire class and calculate what
percent of the class expresses the dominant and recessive conditions.
6) Answer the analysis questions at the end of the activity.
DATA TABLE 1: Individual Phenotypes and Genotypes
Record your phenotype for each trait in the table below, then use the letters from
the key below to record your possible genotypes.
Trait
Dominant
Expression
Recessive
Expression
1) PTC testing
PTC taster (T)
PTC non-taster (t)
2) Tongue
curling
3) Widow’s
peak
4) Hair whorl
Tongue curls
(C)
Widow’s peak
present (W)
Clockwise whorl
(L)
5) Ear lobes
Unattached ear
lobes (E)
6) Ear bump
Bump present
(D)
7) Eye shape
Almond Eyes
(B)
8) Lip thickness Thick lips (K)
Tongue does not
curl (c)
Straight hair line
(w)
Counter-clockwise
whorl (l)
Attached ear lones
(e)
No bump present
(d)
Round eyes (b)
9) Eyelash
length
10) Freckles
Long eyelashes
(H)
Freckles
present (F)
Dimples present
(M)
Cleft chin
absent (N)
Hitchhiker’s
thumb present
(G)
Big toe shorter
than second (I)
Hair on fingers
(J)
Left thumb on
top of right (O)
Two tendons (P)
Short eyelashes (h)
Extra fingers or
toes (R)
Webbed fingers
or toes (V)
No extra fingers or
toes (r)
Non-webbed
fingers or toes (v)
11) Dimples
12) Cleft Chin
13) Hitchhiker’s
thumb
14) Big toe
15) Mid-digital
Hair
16) Interlocking fingers
17) Palmar
muscle
18) Polydactyly
19) Syndactyly
Thin lips (k)
Freckles absent (f)
Dimples absent (m)
Cleft chin present
(n)
Straight thumb (g)
Big toe longer than
second (i)
No hair on fingers
(j)
Right thumb on top
of left (o)
Three tendons (p)
Your
Your
phenotype possible
genotypes
DATA TABLE 2: Class Data of Phenotypic Expressions
Record how many students in your class expressed the dominant vs. the recessive
condition for each of the traits below. Calculate the percentage of students with the
dominant or recessive expression by dividing the number of students with the dominant
(or recessive) expression by the total number of students in your class and multiplying
by 100.
Trait
1) PTC testing
2) Tongue
curling
3) Widow’s peak
4) Hair whorl
5) Ear lobes
6) Ear bump
7) Eye shape
8) Lip thickness
9) Eyelash
length
10) Freckles
11) Dimples
12) Cleft Chin
13) Hitchhiker’s
thumb
14) Big toe
15) Mid-digital
Hair
16) Inter-locking
fingers
17) Palmar
muscle
18) Polydactyly
19) Syndactyly
Number of
students
expressing
the dominant
condition
Percent of
class
expressing
the dominant
condition
Number of
students
expressing
the recessive
condition
Percent of
class
expressing
the recessive
condition
ANALYSIS QUESTIONS
1) How many alleles does each individual contain per monogenic trait? ______
2) Where do these alleles come from? ________________________________
3) What are the 3 possible combinations of alleles for a monogenic trait and the
term to describe them?
A) _____________________________________________________________
B) _____________________________________________________________
C) _____________________________________________________________
4) What are the phenotypes associated with the three genotypes listed above?
A) _____________________________________________________________
B) _____________________________________________________________
C) _____________________________________________________________
5) Which phenotype were you able to easily identify the genotype for the trait?
Why? ______________
________________________________________________________________
6) Why is it difficult to identify the genotype for a trait that has a dominant
expression? ______________
________________________________________________________________
________________________________________________________________
7) If an expression is dominant does that mean it is the most common in a
population? Give some examples from your class data that show dominant
alleles not common in your classroom population.
________________________________________________________________
________________________________________________________________
________________________________________________________________
________________________________________________________________
8) How would knowing what your parents look like help you figure out your own
genotype?
________________________________________________________________
________________________________________________________________
10) Why did we use untreated paper first in the PTC taste test?
________________________________________________________________
________________________________________________________________