Punnett squares are diagrams that help student assess genotypic and phenotypic probabilities of the offspring generation. The simplest Punnett squares look at just a single trait, or gene. Genes, many times, have two varieties, called alleles. Diploid individuals (like us!) carry two copies of the gene. Sometimes an individual can carry two dominant alleles for a single gene. Sometimes they can carry two recessive alleles and sometimes an individual can carry both a dominant and recessive allele. For example lets look at the gene for flower color in plants. We will call this gene “purple,” because the dominant allele yields purple colored flowers. The dominant allele is designated with the uppercase “D” symbol. The recessive allele is designated with a lowercase “d” symbol. Since each flower contains the gene for color and each flower carries two copies of this gene, we designate individuals as DD, Dd, or dd, depending on whether they are dominant, recessive or heterozygous genotype.
Need some help on definitions? Let’s take a step back and define genotype and phenotype. The genotype of the individual is what actually exists at the molecular level in an individual. When we name the genotype of an individual, we are actually writing out the alleles that exist. For example, the genotype of this particular flower is DD, Dd, or dd. Phenotype is what we actually see. When we write the phenotype of the flower, we are not writing the allele combination but rather the color that we see. For example, purple or white would be the phenotype of these flowers.
Dominant and recessive? These terms are commonly used to describe how one allele affects the other allele at the phenotypic level. For example, if the purple flower color allele is dominant, than we will see purple. In the following genotypes, DD and Dd, there are at least one dominant allele. Therefore either allele combination will yield a purple flower. In the genotype Dd, the lowercase “d” is recessive to the big “D”. This means that even though a white allele exists – the little “d” – the dominant purple allele will cover it up and the flower will be purple. A white flower can only exist if there is no dominant D allele present. Therefore we need two recessive alleles in combination, or dd, to produce a white flower.
So what is a Punnett square? These diagrams are helpful for understanding Mendel’s Law of Segregation. This law states that when gametes (sperm or egg) form, the alleles separate, so that each sperm or egg receives just one allele. An individual with the “Dd” genotype will make gametes carrying either D or d. A parent carrying “DD” will make either D or D gametes.
When setting up the Punnett square, the gametes from one parent is located on the top of the box and the gametes from the other parent is located to the left of the box. We separate the alleles into their own column or row, because of the Law of Segregation.
Now its easy to figure out what genotypes and hence phenotypes might be present in the F1 (or offspring) generation. Simply start at the top of the box and place the allele into each column or row. The offspring need to have two alleles (they are diploid afterall) therefore each box will carry two alleles, one from each parent.