GED Science Practice Test: Using Punnet Squares to Show Inheritance

While you may be following why Mendels’ pea plants ended up with different flower colors in different generations, it may still not be clear to you how the colors came out the way that they did.  One tool that geneticists use to demonstrate simple Mendelian inheritance is the Punnett Square.  In a Punnett Square for a cross showing one trait, a monohybrid cross, there are four boxes.  One parent’s alleles go on top of the box (one allele on top of each column), and one parent’s alleles go on the left side of the box (one allele on each row).  Then the 4 boxes are filled with combinations of the parents’ alleles.  The following diagram shows this basic Punnett Square set up:


Returning to Mendel’s experiment with the pea plant flower color, Punnett Squares can help to explain how the genotypes and phenotypes of the three different generations ended up the way that they did:


Notice in the diagram above that the parents of the F2 generation were had heterozygous genotypes (Pp), and that this explains how  Mendel could have ended up with a white flower in the F2 generation:  a recessive p allele from one heterozygous parent combined with a p allele from the other heterozygous parent.

Punnett squares are a useful tool for determining the genotype of a plant for which you only know the phenotype.  For example, the following diagram shows that a geneticist is trying to figure out the genotype of a purple flower.  He or she crosses it with a white flower.  If the genotype of the purple flower was PP (homozygous dominant), then one would get all purple flowers from the cross, as shown in the Punnett Square on the left.  If the genotype of the purple flower was Pp (heterozygous), then one would expect to get half purple flowered plants and half white-flowered plants, as shown in the Punnett Square on the right.


While Punnett Squares are valuable for understanding some simple crosses, people are misled to believe that Punnett Squares represent the exact outcome of a cross.  For example, in the Punnett Square on the right in the above picture, some people understand this to mean that out of four offspring pea plants, two will have purple flowers and two will have white flowers.  The Punnett Square represents a probability.  A better way of interpreting the Punnett Square on the right is to say “there will be a 50% chance that the offspring pea plant will have purple flowers.”  Just like the probability of getting heads on coin flip is 50/50, you could easily flip two heads in a row.


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