A Punnett square is a diagram used to calculate inheritance patterns. Resembling a checkerboard, this device makes it possible to figure out the exact gene combinations that an offspring will inherit from its two parents.
Understanding the Punnett square, which is a handy tool for predicting results in genetics, first involves having knowledge of alleles (pronounced uh-LEELZ). Alleles have to do with genes, which are the carriers of the information that determines all of the traits or characteristics of an individual organism. Every human being receives 23 gene-carrying chromosomes (coiled structures in a cell's nucleus that carries genetic material) from each parent, resulting in a full set of 46 chromosomes (and some 100,000 genes). When these chromosomes pair up at fertilization to form a new and unique individual, they do so in a way that related chromosomes always pair off (since the same trait is also located in the same place on each chromosome). Since the new individual receives information from both parents concerning a single trait, it always has two sets of directions for that trait. This pair is called "alleles." When the two sets of instructions are the same (such as both coding for brown hair), they are called "homozygous." When they are different (such as one coding brown hair and the other coding red hair), they are called "heterozygous." An allele is therefore a single member that makes up a gene pair. Two alleles are a kind of partnership, and in some cases, one partner is stronger than the other. When this is the case, the stronger one is called the "dominant allele." The other one in this relationship is called the "recessive allele."
Ever since Austrian monk and botanist (a person who studies plants) Gregor Mendel (1822–1884) began experimenting with pea plants and their traits in the 1860s, the rule concerning dominance has been that when two organisms showing different traits are crossed (like a tall and a short pea plant), the trait that shows up or is expressed in the first generation is considered the dominant trait. Just as an athlete may dominate a game to the point where the opponent has no chance to do anything, so a dominant allele expresses itself and suppresses, or masks, the activity of the recessive allele for that trait. However, the recessive allele does not go away just because it is masked. It is still part of the organism's inherited package called its "genotype." The word "phenotype" is the opposite of genotype and describes only the visible characteristics.
Since Mendel stated what became known as the laws of inheritance, biologists have been able to use these principles to predict what will happen when organisms with specific traits are crossed. The easiest way to do this is with the diagram named for the English geneticist, Reginald Crundall Punnett (1875–1967). Punnett devised a square that he divided into four equal parts. Across the outside top of the square are the symbols for the alleles from the male parent. The allele symbols from the female parent are written outside the left-hand side of the square. A capital letter is used for the dominant allele and a small letter stands for a recessive allele. The inside four squares show all the ways in which these alleles can combine. The answers are achieved much like a multiplication problem with the answer in each box being the product (Ll) of one top allele (L) being multiplied by one side allele (l).
A Punnett square is most useful when the results of one gene are considered. Calculating for two genes can be a difficult and complicated process. A Punnett square does not tell how many offspring will be produced, but it does predict what the genotype (genetic makeup including masked traits) and the phenotype (visible traits) will be.