Biology

EXPERIMENT 8: Phenotype and Genotype

Read the entire experiment and organize time, materials, and work space before beginning.

Remember to review the safety sections and wear goggles when working with chemicals.

Allow 40 to 60 minutes for this experiment

Objectives: To understand phenotype and genotype, and

To understand why you have certain characteristics

Materials: Student Provides: Self

Mirror

From LabPaq: PTC taste paper

Discussion and Review: Who you are is a product of both your physical nature and

your nurturing or training. Genetics is the study of your nature. By looking at some of

your physical features you can understand some of your genetics and why you have

certain physical features.

The study of genetics is one of the hottest areas of study in biology today. The recent

completion of the genome project, combined with growing interest in cloning and gene

replacement therapy, has generated great interest in genetics. In biology, genetics can

be divided into classical genetics (heredity) and molecular genetics. Heredity is

generally thought of as a study of parents and offspring and how their traits or

characteristics are related.

Two organisms can look alike but have different underlying gene combinations. The

way an organism looks and behaves is called its phenotype. The combination of genes

or alleles an organism contains for a particular trait is called its genotype. You cannot

always know an organism’s genotype simply by its appearance.

Mendel’s law of segregation explains the results of crosses between parents with similar

characteristics or phenotypes but whose offspring may have different characteristics.

He concluded that the two alleles for each trait must separate when gametes are

formed, and that a parent passes on at random only one allele for each trait to each

offspring. Each offspring therefore has two alleles, but one will dominate and determine

the phenotype. An organism is homozygous if its two alleles for a trait are identical, and

it is heterozygous for a trait if its two alleles for the trait differ from each other.

In this experiment, you will look at several human traits in order to observe Mendel’s law

of segregation. These traits are controlled by a single gene with two alleles and each

allele, if it is dominant in the pair, produces a distinct phenotype.

Hands-On Labs SM-1 Lab Manual

71

PROCEDURES: For each of the following traits determine your phenotype and if

possible your genotype. If you have a dominant phenotype, you may have either a

recessive allele and a dominant allele or two dominant alleles, whereas if your

phenotype is recessive, your genotype is recessive homozygous. Record your

observations in a table.

1. Dimpled chin: A cleft in the chin is a dominant trait.

2. Free ear lobe: For ear lobes to hang free is dependent on

a dominant allele. If two recessive alleles are present the

ear lobes are directly attached to the head.

3. Ability to taste PTC: Some people have the dominant gene that

allows them to detect the bitter taste of PTC while others do not. Put

a piece of PTC impregnated paper on your tongue for about five seconds. If you are

a taster you will know it.

4. Interlocking fingers: When the fingers are interlocked, some

people place the right thumb on top of the left (dominant allele);

others place the left thumb over the right (recessive allele).

5. Mid-digital hair: Some people have hair on the second (middle) joint of one or more

of the fingers, while others do not. The presence of hair is due to a

dominant allele and the absence of hair from all fingers is due to a

recessive allele.

6. Bent little finger: A dominant allele causes the last joint of the little

finger to bend inward toward the fourth finger.

7. Widow’s peak: The dominant allele produces a hairline with a point

in the center of the forehead (widow’s peak). The recessive allele

gives a continuous hairline.

8. Hitchhiker’s thumb: When you can bend the tip of the

thumb back 90 degrees in relation to the rest of the thumb

you are homozygous recessive for this allele.

9. Pigmented irises: When there is no pigment in the front part of the eyes and a blue

layer at the back of the iris shows through you are homozygous for the recessive

allele allowing blue eyes.

10. Long palmar muscle: Clench your fists tightly and examine the tendons running

over the inside of the wrists. If there are three tendons you have the recessive

alleles for the long palmar muscle, if there are only two tendons you do not.

Exercise: Refer to the previous experiment and construct a Punnett square showing

both the genotype and phenotype ratios possible if two heterozygous brown-eyed

individuals with dimpled chins were to have children. Your Punnett square will be 4 x 4

squares. Assume both independent assortment and segregation are occurring.

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