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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. Introduction 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 Phenotype Your Possible Genotype 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 Class Total With Trait Percent Of Class With Trait ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- ----------------- ---------------- 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 ROLLING HAND CLASPING BENT THUMB EARLOBE ATTACH EYE PIGMENT PHENOTYPE NUMBER E_ P_ 1 pp 2 P_ 3 pp 4 P_ 5 pp 6 P_ 7 pp 8 P_ 9 pp 10 P_ 11 pp 12 P_ 13 pp 14 P_ 15 pp 16 P_ 17 pp 18 P_ 19 pp 20 P_ 21 pp 22 P_ 23 pp 24 P_ 25 pp 26 P_ 27 pp 28 P_ 29 pp 30 P_ 31 pp 32 S_ ee C_ E_ ss ee R_ E_ S_ ee cc E_ ss ee E_ S_ ee C_ E_ ss ee rr E_ S_ ee cc E_ ss ee Mendelian Genetics Unit Figure I: Single Gene Traits Tongue rolling Ear lobe attachment Bent little finger Widow's peak Hitchhiker's thumb Mid-digital hair Hand clasping Mendelian Genetics Unit