Roche A. Samy
When you study genetics, just like in a game of chance, gambling, lottery numbers or magic wheel, etc., you take chances to predict certain results. There is a definite formula, chart or calculation to predict these results. In Genetics, special charts resembling checker boards are used to find the possible results of various crosses. This type of chart is called Punnett Square and is named after the inventor-R.C. Punnett.
The
Chart A
below is an example of Punnett Square for an organism. The paired genes for a trait may be identical as with TT or tt. If both genes are same, the organism is
homozygous
, or pure for that trait.
(figure available)
(figure available)
In Punnet Square A, the possible gene pairs that result from crossing different pea plants are represented as
alleles
-T for tall t for short. (Alleles are contrasting effects on a trait).
The results of pollinating an egg of a homozygous tall pea plant is shown in the Punnett Square A. Everyone of the resulting pea plants will be heterozygous tall. In a similar process, other forms of crossing can be followed or calculated. (ex. crossing Tt x Tt or TT x Tt or Tt x tt).
These charts or Punnett Squares can also be used to show the probability of each resulting phenotype. For example, in the following chart B, the probability of producing tall plants from a cross between heterozygous tall pea plants are three out of four. (3:4) This means there is one chance in four of producing a short plant from such a cross.