Objectives

• Simulate a monohybrid cross
• Collect and analyze data
• Explore the role of probability in genetics
• Design a simulation of a dihybrid cross (optional)

Materials

• 1 penny per student

Background

A number of genetic activities can be simulated with coins. In this example students simulate a monohybrid cross. They use the head of a penny to represent a dominant allele for a trait and the tail to represent the corresponding recessive allele. It will be more interesting if students simulate a cross that they have actually made in lab. For example, heads could represent the allele for normal (wild type) wings and tails could represent the allele for vestigial wings in Drosophila. If students have done crosses with Wisconsin Fast Plants®, heads could represent the allele for purple stem and tails could represent the allele for non-purple stems, etc. Alternately, this activity can be used to introduce students to the role of probability in genetics. Since each penny has a head and a tail surface, a penny represents a diploid, heterozygous cell. Flipping the penny and letting it land on either heads or tails represents the segregation of alleles that occurs during meiosis in the formation of gametes. Flipping two pennies and combining the results (HH, HT, or TT) represents the recombination of alleles that occurs in fertilization, which establishes the genotype of the next generation.

Procedure

Begin by asking, “If you toss the penny, what are the chances that it will land on heads?” Most students should know that the answer is 50% or half the time or that the chances of a head or a tail are equal.

Now ask, “If you toss two pennies, what are the chances you will get two heads?” This should provoke more discussion and disagreement. Some will argue that the chances are half that of getting a head with the toss of one penny; that is, ¼ or 25%. The class may not agree on an answer, and some pairs will probably begin tossing their pennies and comparing the results. You may wish to summarize their ideas and reasons on the board or overhead for this and the following questions. Next ask, “If you toss two pennies are the chances of getting two tails the same as getting two heads?” Most of the students will probably agree with this, although it may take some discussion.

Finally ask, “What are the chances of getting a head and a tail from tossing two pennies?” Unless your class has reached agreement on the preceding questions, there likely will be strong discussion and disagreement on this one. Bring the discussion to an end. If you have been summarizing their answers on the board, have them copy these into their notebooks. Tell them to come back to class tomorrow with answers to the questions you have asked. You might also require them to diagram all possible results of tossing two pennies (HH, HT, TH, and TT). They could do this in the form of a Punnett Square, do rubbings of the pennies, etc. This will help them understand that the chances are 25% for tossing two heads, 50% for tossing a combination of a head and a tail, and 25% for tossing two tails.

At the next class, introduce (or review if using a cross they have completed) the monohybrid cross they will simulate.

Let heads = the dominant allele (A or use the appropriate allele symbol)

And tails = the recessive allele (a or use the appropriate allele symbol)

Since each penny has a head and a tail, they will simulate the crossing of a heterozygous F₁ to produce an F₂: A/a x A/a

Ask them to look at their diagrams and answers to the questions from the previous class period, and using their knowledge of genetics, predict the phenotype ratio of the F₂ of this cross. They should record this prediction.

Students work in teams of two to simulate the cross. Students in a team simultaneously toss their pennies. They record the results in a table similar to the one shown here. Teams repeat this for one hundred tosses. Each team totals their results, and all teams pool their data for the Class Totals.

Phenotypes of the F₂ and Analysis of Results

Total F₂ with Dominant Phenotype = Class Total A/A + Class Total A/a = ___

Total F₂ with Recessive Phenotype = Class Total a/a = ___

Predicted Phenotype Ratio 3:1

Actual Phenotype Ratio = Total F₂ with Dominant Phenotype/ Total F₂ with Recessive Phenotype : Total F₂ with Recessive Phenotype/ Total F₂ with Recessive Phenotype = ___ : ___

Class discussion can focus on why the Actual Phenotype Ratio varied from the Predicted (or ideal) Phenotype Ratio. This should help them see that their results were determined by chance and that the observed variation from their Predicted Ratio can also be attributed to chance. (They could do an X² analysis of their data to test this.) Students should discuss what (if anything) they could do to make the actual ratio a better match for the predicted ratio (larger data set).

Advanced students could be challenged to design a coin simulation of a dihybrid cross. (Hint: a penny could represent one pair of alleles and a nickel could represent a second set.)