The Punnett square is a tool used to determine the allele combinations that could be present in an individual.

In its simplest form, it is a two by two grid used to describe the possible genotypes and phenotypes of a single trait determined by a single gene. The maternal alleles are listed on the top of the columns, and the paternal alleles are ranked on the side of the rows. The boxes inside the square represent the offspring.

A Punnett square is a two way street – it can be used to predict the result of crosses between parents with known genotypes, or it can be used to determine the genetic makeup of parents from the characteristics of their offspring.

Punnett squares can be expanded to include multiple traits, though that soon becomes unwieldy. One that describes the inheritance of two traits will be a 4 x 4 grid with a total of 16 possible allele combinations in the offspring. The study of three traits will require an 8 x 8 Punnett square, and four traits produces a 16 x 16 grid.

Terms and Conditions
In diploid organisms there are at least two forms of each gene, called alleles. Alleles are located on homologous chromosomes – one on the maternal chromosome and the other on the paternal chromosome.

Alleles are usually written as follows:

A genotype is a list of the alleles present in an organism; it may refer just to the alleles involved in particular traits or to the complete genetic make-up of an individual. We cannot determine the complete genotype of most organisms but most of the time it is possible to figure out the genes that determine a particular characteristic.

A homozygous individual has two identical alleles for a particular trait. A heterozygous individual has two different alleles for a particular trait.

A phenotype is a description of the appearance or behavior or biochemistry of a trait or an individual.

The genome is a set of all the genes needed to specify an organism’s complete characteristics.