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					                                           HUMAN SINGLE GENE TRAITS

Background Reading - Pages 575 –576 in Nelson Biology, Campbell - Biology

Purpose - To develop an understanding of basic Mendelian inheritance as it applies to human single
gene traits.


     All people are recognizably human, but no one is exactly like anyone else, not even an identical
twin. The basis for the similarity and the reasons fro the diversity that coexist in all species have
puzzled and intrigued people for thousands of years. Several human traits may be used to
demonstrate the individuality in humans. They are controlled by a single gene with two alleles; each
allele producing a distinct phenotype. Alleles are different expressions of the same gene. All can be
used to demonstrate Mendel's Law of Segregation.

Part I - Single Gene Traits

1. In this activity you will be examining NINE easily observable traits. Assume that each trait is
controlled by a single pair of alleles. For each trait described, record your phenotype and genotype in
TABLE I. Use the letter symbols given for each genotype. Indicate a dominant phenotype by a single
capital letter followed by a blank space(IE - A_). Indicate a recessive phenotype by the use of two
lower case letters (IE - aa). Pictures of these traits can be found in FIGURE I.
Q1 - Why is the dominant phenotype represented by a genotype with only one definite letter ?
2. Tongue rolling - The ability to roll the tongue is dominant (R), while non-rolling is recessive (r). it is
debatable whether this is genetic. Record your phenotype and genotype in TABLE I.
3. Free ear lobe - In most people the ear lobes hang free. This is the dominant trait (E). The attached
earlobe is recessive (e). Record your observations.
4. Hand clasping - Clasp your hands together. Notice whether your left or your right thumb is on top.
If the left thumb is on top you have the dominant trait (C), the right thumb is
recessive (c). Record your observations.
5. Hitchhiker's thumb - Hold out your hand and make a fist with the thumb extended. Bend the last
joint of the thumb back as far as possible. A straight thumb is dominant (S) while a bent thumb is
recessive (s). Record your observations.
6. Bent little finger - Place the palms of your hand gently together, side-by-side, with the palms facing
upward. The dominant condition is for the last two joints of the little fingers to bend away from each
other (B), while straight little fingers are recessive (b). Record your observations.
7. Widow's peak - The action of a dominant
gene (W) results in a hairline that forms a distinct point in the middle of the forehead. The straight
hairline is recessive (w). Record your observations.
8. Dimpled chin - A cleft in the chin is a dominant trait (D) while the absence of a cleft is recessive
(d). Record your observations.
9. Mid-digital hair - Each of your fingers is composed of three segments. If any hair grows on the
middle segments, you have the dominant allele for mid-digital hair (H). If you do not have any hair you
are recessive (h). record your observations.
10. Pigmented irises - When a person is homozygous for the recessive gene (p), there is no pigment
in the front part of the eyes and a blue layer at the back of the iris shows through, resulting in blue
eyes. A dominant allele of this gene (P) causes pigment to be deposited in the front of the iris, thus
masking the blue to various degrees. A dark iris pigment (green/brown/black) is dominant over the
light pigmentation. (gray/blue). record your observations.
11. Refer to TABLE II. Trace your probable genotype from trait 1 through trait 5 until you reach the
phenotype number.
                                         Mendelian Genetics Unit
Table I - Single Gene Trait Observations
        Trait Studied               Your             Your         Class Total         Percent Of
                                  Phenotype        Possible       With Trait          Class With
                                                   Genotype                              Trait

 Tongue Roller
                                                                  -----------------   ----------------
 Non Tongue Roller

 Free Ear Lobes
                                                                  -----------------   ----------------
 Attached Ear Lobes

 Hand Clasp (L Over R)
                                                                  -----------------   ----------------
 Hand Clasp (R Over L)

 Thumb - Straight
                                                                  -----------------   ----------------
 Thumb - Bent

 Bent Little Finger
                                                                  -----------------   ----------------
 Straight Little Finger

 Widow's Peak
                                                                  -----------------   ----------------
 Straight Hairline

 Dimpled Chin
                                                                  -----------------   ----------------
 Non Dimpled Chin

 Mid-Digital Hair
                                                                  -----------------   ----------------
 Non Mid-Digital Hair

 Dark Eye Pigment
                                                                  -----------------   ----------------
 Light Eye Pigment

Q2 - What is your phenotype number?
Q3 - How many different phenotypes show up?
Q4 - What percent is this of the possible number of phenotypes?
Q5 - What would happen to this percent if two more traits were added to the chart?
Q6 - How many phenotypes were duplicated?
Q7 - What percent is this of the possible total number of phenotypes?
Q8 - How many of your classmates have the same phenotype number as you?

                                      Mendelian Genetics Unit
Table II - Determination Of Your Phenotype Number
 TONGUE        HAND           BENT           EARLOBE         EYE       PHENOTYPE

                                             E_              P_        1
                              S_                             pp        2
                                             ee              P_        3
               C_                                            pp        4
                                             E_              P_        5
                              ss                             pp        6
                                             ee              P_        7
 R_                                                          pp        8
                                             E_              P_        9
                              S_                             pp        10
                                             ee              P_        11
               cc                                            pp        12
                                             E_              P_        13
                              ss                             pp        14
                                             ee              P_        15
                                                             pp        16
                                             E_              P_        17
                              S_                             pp        18
                                             ee              P_        19
               C_                                            pp        20
                                             E_              P_        21
                              ss                             pp        22
                                             ee              P_        23
 rr                                                          pp        24
                                             E_              P_        25
                              S_                             pp        26
                                             ee              P_        27
               cc                                            pp        28
                                             E_              P_        29
                              ss                             pp        30
                                             ee              P_        31
                                                             pp        32

                                   Mendelian Genetics Unit
Figure I: Single Gene Traits

               Tongue rolling                       Widow's peak

           Ear lobe attachment                      Hitchhiker's thumb

Bent little finger                              Mid-digital hair

Hand clasping

                                 Mendelian Genetics Unit