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Unformatted text preview: Human Genetics Human genet ic traits are difficult to study because of the relat ively lo ng life span and the limited number of human offspring. In addit io n, the number of chro mosome pairs (23) increases the possible number of genet ic combinatio ns. It is possible, however, to take a sample fro m human populations to estimate the frequency o f a trait and the possible ways a given trait is inherited. Object ives:
· Invest igate the inheritance o f some human traits.
· Estimate the frequency o f selected traits in a given populat ion.
· Estimate the genet ic probabilit y of the occurrence of representative traits. Materials: PTC papers (phenylt hiocarbamide), Sodium Benzoate papers, Thiourea papers Forceps Colored Pencils Two Coins (to flip) Trait sketches and photos provided in lab Methods: Part I. Genet icists probably know more about blood types than any other inherited human trait. In the "ABO" blood group system the presence or absence of A and B ant igens (mucopolysaccharides on the cell surface) determines the four basic blood types. For laboratory purposes consider the ABO blood groups to be determined by a single pair of genes expressing codominance. Thus, the genotype of t ype A blood may be ho mozygous (IA IA) or heterozygous (IA i). The genotype of t ype B blood may be ho mozygous (IB IB ) or heterozygous (IB i). Type O blood must be ho mozygous (ii). Type AB blood must have both A and B antigens (IA IB ) with both being expressed through codominance. 1. Using the Punnett Square provided on Worksheet I determine the genotypes possible among the offspring of a cross between a heterozygous type A parent and a type AB parent. Use the informat ion provided above, and the results of the Punnett Square, to answer questions 1 through 5 on Worksheet I. 2. The lefthand column of Table 1 (page 42) provides a list of parental crosses based on their respect ive blood types. Determine: (1) all o f the genotypes possible for the respect ive pairs of parents (2) all o f the genotypes possible for the offspring of the respect ive pairs of parents (3) all of the blood types (phenotypes) possible for the offspring of the respect ive pairs of parents, and (4) all o f the blood types NOT POSSIBLE for the offspring of the respect ive pairs of parents. Enter your results in the spaces provided in Table 1. Part II. An inherited trait is symbo lized an assigned letter. Normally the dominant form of a trait is symbo lized by an upper case letter (e.g. E), and the recessive form is symbo lized as a lower case of the same letter (e.g. e). Read the descript ions of the traits
Page 1 of 17 ver. 1.6 listed below and determine whether you are dominant or recessive for each trait. Record the letter for each of your traits in the space provided in Table II. Use only a single letter since you don't know whether you are homozygous or heterozygous for each trait. One exception (assuming co mplete dominance) is if you are recessive for a given trait. In that case you must be homozygous recessive (e.g. ee) for that trait to be expressed. A. Attached Earlobe: The do minant gene (E) codes for the earlobe to hang free and is found in the majorit y of people. If a person is ho mozygous for the recessive trait (ee) the earlobes are attached direct ly to the side of the head. B. Widow's Peak: The dominant trait (W) is where the forehead hairline drops down and forms a dist inct point. This is known as a Widow's Peak. The recessive trait (ww) is expressed as the absence of the downward point. C. Tongue Rolling: The do minant gene (R) allows a person to roll their tongue into a "U" shape when extended out of the mouth. With the nonroller recessive trait (rr) a person only produce a slight downward curve of the tongue. D. Bent Little Finger: A dominant gene (B) causes the last jo int of the litt le finger to bend in toward the fourth finger. Lay both hands on the table, relax the muscles, and note whether you have bent or straight litt le fingers. Wit h the recessive trait (bb) the litt le fingers are somewhat straight. E. PTC Tasting: Place a piece o f PTC containing paper under your tongue. The dominant trait (T) allows one to taste the chemical on the paper. Individuals expressing the recessive trait (tt) will detect no obvious taste. F. Mouth Shape: The dominant trait (M) is expressed as a mouth that is relat ively lo ng across the face. The recessive trait (mm) is expressed as a mouth that is relat ively short across the face, or appears relat ively small. Once the class has collected all their results the instructor will co mpile the data for the ent ire class. Record the class data in the appropriate spaces in Table II (page 43). Genet ic Circle/ Worksheet 2 (page 44) 1. The Genetic Circle permit s you to visualize how human traits vary. In the center of the circle enter the number of individuals in your class. 2. Start from the middle of the circle, and using the data from Table II, work toward the outside circling your genotype in each of the respective spaces. 3. Connect the circled spaces with a line. 4. Use the informat ion fro m the Genet ic Circle to answer questions number 6 through 10 (page 45). Part III: Performing hypothetical genet ic crosses by flipping coins to represent random combinat ions o f the traits presented in Figure I. Flipping two coins at a time, each co in represents the gametes possible fro m either the mother or the father. Heads represents a Dominant gene contributed to the respective trait of the offspring, and Tails represents a Recessive gene contributed to the trait of the offspring. Proceed making the genet ic crosses as fo llows: 1. Choose a trait from Figure I (pp. 4755). 2. Select two people, one to represent one parent, the second the other parent. 3. Each person flips a coin to symbo lize a genet ic cross for the trait chosen. However the coin lands determines the gene contributed by the respect ive parent (Heads = Dominant, Tails = Recessive) Page 2 of 17 ver. 1.6 4. The pair of genes determined by the coin toss represents the genotype for that trait in the offspring (viz. Heads:Heads = ho mozygous dominant, Heads:tails =heterozygous, or tails:tails = ho mozygous recessive). 5. Record the offspring genotype and phenotype for the respect ive trait in Table III (p. 49). 6. Repeat steps 1 through 5 for all o f the traits presented in Figure I. Using the phenotypes fro m the hypothetical genet ic cross made above draw a sketch of the result ing offspring. Use the space below and colored pencils provided to draw the result ing offspring. Page 3 of 17 ver. 1.6 Worksheet I Use the Punnett Square below to set up the genet ic cross described in #1 (ABO blood groups) of Part 1 of this lab exercise. (Follow the numbered hint s to practice setting up a Punnett Square) 1. Fill in the parent genotypes (given) 2. Determine and fill in the gametes possible fro m each parent 3. Fill in all the genotypes possible fro m this cross, respect ively 4. Complete questions 1 through 5 below. Parent Cross and Punnett Square for ABO Blood Group Example ____________ X ____________ 1. What is the genotype of the parental heterozygote for type A blood? ___________ 2. What is the genotype for the type AB parent? ___________ 3. What are the genes possible in the gametes produced by a t ype A parent? __________ 4. What ABO gene combinations are possible in the offspring of this cross? __________ 5. What blood types (phenotypes) are possible in the offspring of this cross?__________ Page 4 of 17 ver. 1.6 Table I. ABO Blood Group Genotypes and Phenotypes For the parental genetic crosses given in co lum1 list all of the genotypes possible for the parents (column 2) and offspring (column 3). In column 4 list all of the blood types (phenotypes) possible for the offspring, and in Co lumn 5 list all o f the blood types NOT POSSIBLE for the offspring of each parental cross, respectively. Blood Types of Parents All Possible Genotypes of Parents All Possible Genotypes of Offspring All Blood Types POSSIBLE in Offspring All Blood Types NOT POSSIBLE in Offspring A and B A and O B and O AB and A AB and B AB and O O and O Table II. Personal and Class Results. Ratios and Frequencies of Representative Inheritable Human Traits. Data from the beginning of part 2 of this exercise Class Trait Your Genotype Dominant Recessive Ratio Frequency (%) of Your Trait Earlobes Widow's Peak Tongue Rolling Bent Little Finger PTC Tasting* Sodium Benzoate* Thiourea* Page 5 of 17 ver. 1.6 Page 6 of 17 ver. 1.6 Gene Circle Questions: 6. How would it help to extend the outer most section of the circle by adding another trait? ____________________________________________________________________ 7. If you had to keep extending the traits, how many more could you add? _____________________________________________________________________ 8. Why are there usually two choices when you must select a characterist ic for a trait? _____________________________________________________________________ 9. If two people ended up on the same sect ion on the outer rim o f the circle would they look alike? _____________________________________________________________________ 10. If you had two people with identical genetic traits in 100 sect ions of the genetic circle, would these two people look alike? _____________________________________________________________________ Results: From Part II, item “E” (page 40) Regarding the abilit y to taste (or not taste) PTC, sodium benzoate or thiourea, answer the fo llo wing questions in the spaces provided. 1. To what extent do the ratios obtained for the representative traits compare to Mendelian rations studied previously? _____________________________________ _____________________________________________________________________ Explain any variation noted ______________________________________________ _____________________________________________________________________ _____________________________________________________________________ 2. Could two parents who are "tasters" have a child who is a "nontaster"? __________ Explain your answer ___________________________________________________ ____________________________________________________________________ 3. The nat ional average is 65% for PTC tasting and tongue rolling and 35% for non tasters and nonrollers. How do the ratios obtained for the class co mpare to the national average?_____________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 4. How can you account for any variat ion from the national average? _____________________________________________________________________ _____________________________________________________________________ Page 7 of 17 ver. 1.6 Page 8 of 17 ver. 1.6 Page 9 of 17 ver. 1.6 Page 10 of 17 ver. 1.6 Page 11 of 17 ver. 1.6 Page 12 of 17 ver. 1.6 Page 13 of 17 ver. 1.6 Page 14 of 17 ver. 1.6 Page 15 of 17 ver. 1.6 Page 16 of 17 ver. 1.6 Table III. Hypothetical Genet ic Crosses. Record the genotypes result ing fro m the coin flips ("rando m selection") performed as described in Part 3 above. Based on the genotypes acquired for the "Mother" and the "Father", determine the genotype and phenotype of the offspring for each of the traits list ed. Parent 1 Father's Genes Parent 2 Mother's Genes Trait Child Offspring Genotype Offspring Phenotype Sex Ears Face Shape Chin Shape (a) Chin Shape (b) Cleft Chin Hair Body Widow's Peak Hair Color Eyebrows (a) Eyebrows (b) Eyebrow Color Eye Distance Eye Size Eye Shape Eye Slantedness Eye Color Eyelashes Mouth Size Lip Size Hapsburg Lip Dimples Nose Size Page 17 of 17 ver. 1.6 ...
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- Spring '06