Life Science Curriculum K-7
Submitted by: Sarah Sayovitz
Institution: Azusa Pacific University
Title of Experiment: Genetic Variation within a Population
- two coins that have a "heads" and a "tails" side
- a partner
- paper to record data
- a pen or pencil
Scientific Background of Experiment:
A gene is something that makes us who we are by determining what we act and look like. Genes are passed down from parents to their children. Some examples of genes are the color of our hair and eyes as well as how tall we will be. Different gene expressions, the outside appearance, like blonde hair or brown hair, happen because each gene has different alleles, which are the parts of genes that are passed down by each parent. There are two types of alleles &endash; one that gives a more common expression of the gene, which is called dominant, and one that occurs less often, the recessive allele. The dominant allele is usually shown as a capital letter, like B, and the recessive is shown as a lower-case letter, like b. Because one allele is given by each parent, every person has two alleles for every gene. For example: if B, the dominant allele, gives someone brown eyes, and b, the recessive allele, gives someone blue eyes, a person with the alleles "BB" or "Bb" would have brown eyes &endash; because the capital B is dominant over the lower-case b. Someone with the alleles "bb" would have the recessive expression of the gene being passed down. In this case, it might represent someone who has blue eyes. If you think about it, there are a lot less people with blue eyes because it is the less common expression due to its recessive alleles.
A way that scientists can predict &endash; or guess &endash; what genes someone will have is to look at the different alleles that they parents have. A Punnett Square is used to show the results if the two parents having children. If one parent has both dominant alleles and the other has both recessive, the cross of the parents would look like this:
RR X rr
Here, each of the offspring, or the new combinations of alleles that were made, would each have brown eyes because they have one of each allele, and the dominant allele, R, will determine the expression of the gene. When two of these offspring are tested, the following cross will result:
Rr X Rr
In this case, the result of the cross is slightly different. Can you guess how many of the offspring will have brown eyes? How about blue eyes? If you guessed that three out of the four would have brown eyes, you were right! The reason is that three of the four offspring have the dominant allele, R, as a part of the gene. This means that the one offspring left that did not have the dominant allele must be blue eyed because it produces a recessive gene. The Gene frequency is a number that tells how often the gene occurs or is expressed in the individuals of a population.
In a population, or a group of the same kind of animals or plants, there are certain numbers of individuals that will have the dominant and recessive genes that are described here. This is the concept of Probability. Probability is measuring the chance that something will happen, or in this case, the chance that a certain individual will have a certain gene. Scientists can use probability to determine how many organisms in a population will have the genes that they are studying. In this experiment, we will see how probability can be helpful to guess the number of organisms with the same gene or expression of a gene in a certain group. This will demonstrate the concept of the Punnett square as well as show the different possible combinations of alleles that can occur to produce different genes and gene expressions.
1.) Find a partner and give them a coin that has both a "heads" and a "tails" side.
2.) At the same time, you and your partner should flip your coins, noticing which side lands face up.
3.) Whenever you or your partner's coin lands "heads" side up, record it in the chart provided as the letter "B", the dominant allele. Whenever the coins land "tails" side up, record it as the letter "b", the recessive allele. Always record the letters in the same order, for example, if you record your toss first and your partner's second, always record the tosses in that order. Remember that a combination of "Bb" and "bB" is the same.
4.) Repeat steps 2 and 3 for forty tosses.
5.) When you have forty tosses completed and filled in the chart, count the number of each group of pairs that you recorded &endash; this will end up as three totals: one for number of "BB" tosses, one for "Bb" tosses, and one for "bb" tosses.
6.) Compare these results with the expected total of genes as shown in the second Punnett square above.
7.) Do these result's match up with the expected probability? Why or why not?
Misc. Helpful Information/ Hints/ Suggestions:
Recreate this chart to allow the students to keep track of their tosses:
Number of "BB" tosses (heads + heads) Number of "Bb" or "bB" tosses (heads + tails) Number of "bb" tosses
(tails + tails)
Total tosses: ---________ Total tosses: ________ Total tosses: ________
- Add the totals from the first two columns to find the total number of genes that would express the dominant gene.
- Divide these totals by forty to get the frequency of occurrence in the population.
Mader, Sylvia S. BIOLOGY Laboratory Manual. Fifth Edition. Boston: WCB McGraw-Hill. 1996. pp. 97-110
McFadden, Carol H. and William T. Keeton. BIOLOGY An Exploration of Life. New York: W.W. Norton & Company. 1995. pp. 277-281