Punnett square: Introduction, construction, Applications


The Punnett square is a table that show all the expected percentage of different genotypes in the offspring outcomes for a genetic cross between two individuals with known genotype. It is named after British geneticist, Reginald C. Punnett (1875–1967).

It consists of a square which is divided into four quadrants to predict the chances of an offspring to have its parents’ traits.

It is a helpful tool that helps to predict the variations and probabilities that can come from cross breeding used by genetic counsellors.

Punnett square  and way of construction of the square

Fig: Punnett square and way of construction of the square

How to construct Punnett squares

  • To create a Punnett square, start by drawing a box divided into four equal squares with each part equalling a 25% chance of the offspring receiving that set of genes.
  • Determine the genotypes of the parent organisms or the genetic contribution of parent. Identify the alleles that each parent possesses for a specific trait, which is commonly represented by letters. For example, while looking at height, one might use “T” for tall and “t” for dwarf.
  • Gently, draw a p-square and write down your “cross” (mating)
  • “split” the letters of each genotype for each parent & put them “outside” the p-square.
  • Then, fill in the p-square inside to determine the possible genotypes of offspring.
  • Summarize and interpret the outcomes of possible genotypes and phenotypes of offspring.

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