Download Determination of Genotypes from Phenotypes in Humans

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? ______________________
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3. What is a genotype and phenotype?
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4. What is a dominant trait?
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5.
What is a recessive trait?
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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
PTC taste
Phenotype (trait)
Possible Genotypes (alleles)
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?