Determination of Genotypes from Phenotypes in Humans NAME DATE An organism can be thought of as a large collection of phenotypes. A phenotype is the appearance of a trait and it determined by genes (genotype). Alleles of a genotype determine the phenotype. Chromosomes are long strands of DNA which contain many alleles. Human somatic (body) cells have 23 pair (46) chromosomes. In meiosis, gametes with haploid number of chromosomes form. Alleles will independently segregate, only passing one of the pair for the trait along to an offspring. When two haploid gametes fuse in conception, a diploid cell is formed (embryo), which will develop into a human exhibiting the phenotypic traits from the combinations of alleles in the genotypes. One of Mendel s conclusions is the Law of Dominance, which states that where two alleles pair up to make a trait, one will be dominant over the other. Thus, the phenotype will be that of the dominant allele. However, certain cases you will encounter in lab today show Incomplete Dominance, where two alleles pair and an intermediate of the two traits is seen in the phenotype. You will be looking 3 specific traits and attempt to determine genotype from the phenotypes shown. Be sure to know which traits are completely dominant and incompletely dominance. You will be performing a test to determine whether or not you have the ability to taste phenylthiocarbamide (PTC). Tasters / Nontaster phenotypes are controlled by one gene with two alleles dominant and recessive. Tasters detect a very bitter taste when exposed to PTC, nontasters do not. Bitter foods tend to be poisonous. Tasting is the dominant phenotype (T) and non tasting is recessive (t). Pre-Lab Questions: 1. How many alleles can a parent pass on for a certain trait? 2. How does a diploid cell become haploid? What is the difference? 3. What is a genotype and phenotype? 4. What is a dominant trait? 5. What is a recessive trait?
Materials: - Untreated taste control paper - PTC taste paper Procedure: 1. Obtain 1 strip of PTC paper and 1 untreated taste control strip from teacher. Place untreated control strip on tongue and determine the taste. Then, dispose of strip in waste container. Place the PTC paper on your wet tongue to see if you can taste phenylthiocarbamide. 2. Since PTC is bitter, you will notice quickly if you are a taster or non taster. Chart your data in Table 1 under phenotype (taster/non taster). Discard the PTC strip in the waste container. 3. Determine your possible genotype(s) for this phenotype in the table. 4. For the remainder of the two traits, you will be visually observing your lab partner: Observe your lab partner and help them determine if they have the following: a. Hairline: Presence of Widows peak Dominant (W); Absence of widow s peak - Recessive (w) b. Hair Texture: Curly Hair Homozygous Dominant (H); Wavy Hair Heterozygous (Hh); Straight Hair- Homozygous Recessive (h) c. thumb A thumb tip that bends backward more than 30 degrees (hitchhiker s thumb) is dominant (BB or Bb) to a straight thumb (bb). d. Eye lashes - Long eyelashes (EE or Ee) are dominant to short eyelashes (ee).
e. Tongue rolling - This trait is dominant. (Use upper & lower case R s to represent a genotypes) f. Clasped hands/ thumb up - Clasp your hands interlacing your fingers as shown below. Record which thumb is up. Right thumb up is recessive. (Use L s and l s for genotypes) g. Earlobes Attached earlobes (d) are recessive; detached (D) are dominant. 5. Log the phenotypes for both traits in Table 1. Determine possible genotype(s) based on the phenotypes expressed in the Table 1.
TABLE 1 Phenotype and Genotype traits Trait Phenotype (trait) Possible Genotypes (alleles) PTC taste Hairline Hair Texture Thumb Eye Lashes Tongue Rolling Clasp hands Ear Lobe QUESTIONS 1. Which traits do you have that are dominant? 2. Which traits do you have that are recessive? 3. Which of your traits do you share with one or more of your classmates? 4. What genotypes are possible for your parents?
5. What phenotypes are possible for your parents? 6.. If you are dominant for the trait, does one of your parents have to have the trait? Explain. 7. If you are recessive for the trait, does one of your parents have to have the trait? Explain. 8. Imagine you are a non taster. Your mother is a taster and your father is a non taster. What are the genotypes of your parents? 9. Why might PTC be such a prevalent dominant allele in the population? 10. If you and a particular classmate shared all of the same traits examined what traits could you describe to prove your uniqueness? 11. What determines your traits?
12. How can a person s genotype for a trait be determined from his or her phenotype for the trait? 13. Why was untreated paper used in the PTC taste test?