Kwadraty Punneta

The Punnet square, a simple diagram named after its inventor, is a graphical way to predict the likelihood that offspring may inherit a single gene trait, like lactose intolerance, given the parental genotypes.

Using this grid, the genetic contributions of one parent, who in this case, is homozygous recessive with alleles denoted as two lowercase L's are placed at the top. Those of the other, heterozygous with mixed alleles, represented by a dominant capital L along with a recessive lowercase L, are written on the left side.

Combining these row and column headers into the boxes, results in two possible genotypes and both phenotypes for a child. A 50% chance of being heterozygous and able to digest lactose. And a 50% chance of being homozygous recessive and lactose intolerant.