1994 Woodrow Wilson Biology Institute
The Punnett square is a mathematical tool used by geneticists to show allelic combinations of gametes and to predict offspring ratios.
For example, what might be the gametes formed by a male fruit fly that is homozygous dominant for gray body color? What might be the gametes formed by a female fruit fly that is homozygous recessive for ebony body color? This will be the parental generation of our cross and the alleles would be: GG for the male and gg for the female.
When both the alleles of a gene pair are the same, the organism is said to be homozygous for that characteristic. It can be homozygous dominant (GG) or homozygous recessive (gg). When the two alleles in a gene pair are NOT the same, for example, when the genotype is Gg, the organism is heterozygous, or hybrid for that trait. When working with only one trait this condition is called a monohybrid. If we were to work with two traits, we would call the combination a dihybrid and so on. We use the term "genotype" to refer to the allele combinations of an organism.
Let's use the Punnett square to predict the results of a monohybrid cross between the homozygous dominant gray-bodied male with a homozygous recessive ebony-bodied female. Draw a square and divide it into four parts. Represent the alleles contributed by the homozygous dominant male parent by writing a capital letter G above each column (see diagram). Represent the alleles contributed by the homozygous recessive female parent by writing a lower case g next to each row (see diagram). These letters represent the alleles donated by the Parent (P) generation. The information that will be put inside each square represents the possible zygote allelic combination. We call this generation the first filial (F1) generation.
To fill in the squares bring the symbols down from the top of the columns and write that symbol in each square of the column. Also, bring the symbols across each row and write that symbol in each square of the row. These filled squares are the F1 generation's gene combinations. They are the result of the parent's donation of alleles. In this case, all the F1 generation is Gg or heterozygous.
Now predict the phenotypic ratios of the offspring. (Remember that phenotype is the outward appearance of a trait.)
Mendel performed a second series of experiments in which he crossed true-breeding plants that differed in two traits; such crosses are appropriately called dihybrid crosses. For example, he crossed tall plants with yellow seed color with short plants with green seed color. The F1 generation showed the dominant characteristics of being tall plants with yellow seeds. Mendel then allowed the F1 generation to self-pollinate. He found that the four factors segregated into the gametes independently. There were four phenotypes among the F2 plants - tall plants with yellow seeds (TY), tall plants with green seeds (Ty), short plants with yellow seeds (tY), and short plants with green seeds (ty). The dihybrid cross allowed Mendel to deduce the law of independent assortment, which states that the members of one pair of factors segregate independently from another pair. Therefore, all possible combinations of factors can occur in the offspring.
Punnett squares are used frequently in this unit because they are a valuable tool to show allelic combinations and predict offspring. When Punnett squares are practiced frequently the abstract concept of probability becomes accessible to students!