Punnett Square

A Punnett square is a graphical diagram developed by Reginald C. Punnett to represent genetic crosses. It is used to predict all possible genotypes and their probabilities in the offspring by arranging the gametes of parents along the top row and left column and showing their combinations in a square format.

In simple terms: Think of a Punnett Square as a probability multiplication table for genes - just as a multiplication table maps two numbers to a product, a Punnett Square maps two parents' gametes to possible offspring genotypes.

Why is it important?

• It applies the rules of probability to heredity​
• It can predict both genotypic and phenotypic ratios​
• It is the foundation for understanding Mendel's Laws of Segregation and Independent Assortment​
• It works for all inheritance patterns - complete dominance, incomplete dominance, codominance, test crosses​

• Punnett Square was developed by British geneticist Reginald C. Punnett; it is a diagram used to show all possible genotypes of offspring in a genetic cross.
• In a Monohybrid Cross, Tall (TT) × Dwarf (tt) gives F₁ plants that are all Tt (tall) because T (tall) is dominant over t (dwarf).
• When F₁ (Tt) is self-pollinated, F₂ offspring are TT, Tt, Tt, tt, giving phenotypic ratio = 3:1 (tall: dwarf) and genotypic ratio = 1:2:1 (TT:Tt:tt).
• TT and Tt plants are phenotypically identical (both tall) but genotypically different because T allele dominates over t allele.
• Test Cross involves crossing an organism of dominant but unknown genotype with a homozygous recessive (tt) parent to determine its genotype.
• If all offspring are tall, the unknown parent is TT (homozygous dominant); if half tall and half dwarf, the unknown parent is Tt (heterozygous).
• The ratio 1/4 TT: 2/4 Tt: 1/4 tt can also be expressed using the binomial expansion (aT + bt)² formula.