The Genetics of Eye Color

The purpose of this project is to discover eye-color genotypes that could be present in a given population. This study will take into account dominant and recessive traits as well as the evolving understanding of the genetics of eye color.

Research Questions:

• How are genes passed down from parents to their offspring?
• How many eye-color alleles does each person have?
• How many eye-color alleles can one person pass to their offspring?
• Are dominant or recessive genes passed down more frequently?
• How can two brown-eyed parents have a blue-eyed baby?
• Why is it difficult for two blue-eyed parents to have a brown-eyed baby?
• What is the difference between genotype and phenotype?
• How can you determine possible genotypes?
• Are blue eyes or brown eyes more common?

Mendel first identified simple dominant and recessive traits in his famous experiments on pea plants. In human beings, a baby receives one set of genes from its father and another set from its mother. The combination of all of these genes, and the often complicated interactions between them, give the baby many of its physical characteristics, including its eye color. It was formerly thought that just two genes influenced eye color. However, it was recently discovered that the first gene is actually two distinct genes! Although eye color is much more complicated than we initially thought, for the purposes of this project we'll keep things simple and focus on more common cases and the interaction of dominant and recessive traits.

The following example shows the possible genetic outcomes for a father with Bb and a mother with Bb. 

Father’s Genotype

Bb (one gene for dominant BROWN and the other for recessive blue)

Father’s Phenotype

Because only one gene must be dominant to show the dominant trait, the father has brown eyes.

Mother’s Genotype

Bb (one gene for dominant BROWN and the other for recessive blue)

Mother’s Phenotype

Because only on gene must be dominant to show the dominant trait, the mother has brown eyes.

Materials:

• A number of willing participants who have living grandparents.
• Graph paper

Experimental Procedure:

1. Find a number of people willing to help you by providing data about their eye color for your study. It is helpful to have information for at least three generations, so try to find volunteers with living grandparents.
2. Record the eye-color of your volunteer on a chart. You will call this person Subject A and place them in the “Generation 0” line of the chart.
3. If the subject has brothers or sisters, collect data on them as well. Do not include the data if the siblings do not share both parents with the subject.
4. Collect data on the eye-color of Subject A’s parents. Place this information on Generation -1 of the chart.
5. If data is available for siblings of Subject A’s parents, collect this information as well. Again, do not include data if siblings do not share both parents.
6. Collect data on the eye-color of Subject A’s grandparents. Place this information on the Generation -2 line of the chart.
7. If data is available for siblings of Subject A’s grandparents, collect this information as well. Again, do not include data if siblings do not share both parents.
8. If data can be collected for any previous generations, collect this information as well, following the same parameters.
9. Deduce the possible genotypes for the people whose genetics are not obvious.
10. Check your answers by determining if the genotype you think they have can produce the eye colors found in their offspring.
11. You can take this further by determining the mathematical probability that your subject has one or another genotype.
12. It is also possible to repeat this experiment with multiple subjects. This will give you an idea of the likelihood of specific genotypes in a larger population.

Terms/Concepts: Mendel; Dominant gene; Recessive gene; Genetics; Inherited traits; Eye colors; Genotype; Phenotype; Allele

References: