Modeling Monohybrid Genetic Crosses 2018


     Can a simulation help demonstrate how probability and chance affect the outcome of a genetic cross?  Genetic crosses come in a variety of different styles.  One such cross that involves two parents (P1’s) being homozygous or heterozygous for a single trait is called a monohybrid cross.   The offspring (F1’s) or zygotes of such a cross could also exhibit various genotypes and phenotypes depending on what the parents contribute.   The Monohybrid Punnett’s square is a graphical means to provide a visual understanding of all the possible outcomes between two parent contributions of a single trait (ie P1’s = Aa x AA); however, a Dihybrid Punnett’s square would involve two traits. (ie P1’s = AaBB x aaBb).

     Probability is defined as the likelihood of an event taking place. If one were to toss a coin in the air 100 times there is a likelihood that 70 tosses may turn out to be heads and 30 tosses may turn out to be tails even though mathematically one would expect the coin tosses to be nearly 50/50 and that those expected results would only be better supported with a large amount of well documented data. Nature does not always follow a strict pattern of probability due to environmental or genetic influences.  However, if one was to collect and correctly utilize a significantly large amount of data, then what one observes (i.e. naturally or as a simulation) should be statistically close to what one would expect (i.e. found in a Punnett’s square). 

     Chance can be defined as the probability that a subsequent event will not be influenced by an earlier event.  Many times, it is assumed that if one were to play a “game” long enough, there is a likelihood that one might win (ie. a lottery).  However, like some games, just because a couple gives birth to their first child having dimples, does not mean that their second child will have dimples as well.  In nature, each cross between two parents is unique and future crosses will not be influenced by past results.  If one were to simulate several crosses, then expected phenotypic and genotypic results between two parents that are heterozygous will be observed.


Monohybrid Simulation




(2) Blue Colored Macaroni Shells

(2) Red Colored Macaroni Shells

(2) Petri Dishes

Plastic Bowl

Blindfold / Modified Sunglasses / Closed Eyes




Instructor: Graphic/Spreadsheet Program (ie. Excel)



You will model the "random" pairing of alleles by choosing shells from Petri dishes. The shells will represent the alleles for having Blue shell color (dominant "B" = Blue shell) and having red shell color (recessive "b" = red shell).   

1.  Note Table #1 on your answer sheet and understand how one is to record the results.

2.  Place 1 blue shell and 1 red shell in each Petri dish. This will simulate each parent being heterozygous for having a fictitious blue shell; thus, each parent will be genotypically “Bb”

     and phenotypically, “Blue”. One dish will represent the testis of a male where sperm are created and the other dish will represent the ovary for the female where oocytes are created.

3. With their eyes covered, have one partner pick up only 1 shell from each dish in a similar manner and place them in the plastic bowl simultaneously.  The partner’s hands will represent male and female gametes (sperm and oocyte) traveling, fertilizing and eventually

     fusing together within the oviducts (or uterus) simulated by the plastic bowl to form a zygote and thus, an offspring.

4.  After the shells have been chosen, have the other partner accurately record the results on Table #1. After recording the results (ie using “tic” marks (i.e. (/)), have the same partner place the shells back into the dishes. To maintain randomness, move the dishes a little to slightly mix up   

     the simulated alleles. When you and your partner are ready, repeat the steps. Remember, the more you do, the more data, the better the results. When ~10 minutes is up, everyone is to total your results at the bottom of each column.

5.  Have the recorder record the results on the spreadsheet as directed. Then answer all questions on your Answer Sheet. A class discussion will proceed after everyone is finished.

6.  Clean and return all materials as directed.