 Reginald C. Punnett came up with the Punnett square.
 It is a square diagram which is used to predict the genotypes of a particular cross.
 During a breeding experiment, possible outcomes of offspring are studied using Punnett squares.
Consider these examples of a Punnett squares:
Example 1 : Eye colour

Parent 1

Homozygous Dominant Brown
colour eyes (
BB
).
The 2 alleles B and B are written on one side of the square. 
Parent 2

Homozygous Recessive Blue
colour eyes (
bb
).
The 2 alleles b and b are written on the other side of the square.  Corresponding alleles are written in each box, one from each parent to find possible combinations .
 The offsprings in the F _{ 1 } generation have all Heterozygous Brown colour eyes ( Bb )
Example 2: Pod colour of pea plant

Plant 1

Heterozygous Dominant Green
pod (
Gg
).
The 2 alleles G and g are written on one side of the square. 
Plant 2

Homozygous Recessive Yellow
pod (
gg
).
The 2 alleles g and g are written on the other side of the square.  Corresponding alleles are written in each box, one from each parent to find possible combinations.
 The offsprings have all Heterozygous Green pods ( Gg ) and Homozygous Recessive Yellow pods ( gg ) in the ratio 1:1