Punnet squares

Punnet squares are a simple tool used to calculate the probability of obtaining a particular phenotype/genotype in the offspring based on the genotype of the parents.

Constructing a punnet square for looking at one trait on one gene

  1. Label your alleles
  2.  Cross the parental genotypes by separating each parental gamete into one cell.
  3. Cross the gametes together to form offspring genotypes.
  4. Examine the offspring and record the genotypic /phenotypic ratios.

Example cross:

Parental genotypes Aa x Aa

Punnet square 1

A- No albinism

a-Albinism

Genotypic ratio:

1 AA: 2 Aa : 1 aa

Phenotypic ratio :

3 Normal : 1 Albinism.

Example of an X linked punnet square

Parents XR Xr x XRY

punnet square 2
X
r– Colourblindness

XR– Normal

Y- Male allele

punnet square 3(Click to enlarge)

Be careful when answering probability questions regarding offspring ratios

For example:

What is the probability that an offspring from XR Xr x XRY is colourblind?

¼. (As there is one phenotype out of four resultant phenotypes that is colourblind.

What is the probability that XR Xr x XRY will produce a red-green colourblind male?

½ As there is two males and one of the males is red-green colourblind.

Punnet squares involving two genes

 When performing crosses involving two genes you need a larger punnet square for example if you are crossing individuals AaBb x AaBb

You must first cross the gametes.

dihybird

A- Yellow

a-Green

B- Round

b- wrinkled

Phenotypic ratios

9 Yellow round: 3 Yellow wrinkled: 3 Green round : 1 Green wrinkled.

Genotypic ratios

9 A-;B-               :  3 A-;bb                     : 3 aa; B-               : 1 aa; bb

Note the hyphen just means that the second allele could be an allele coding for a dominant phenotype or recessive phenotype regardless because there is presence of one allele coding for a the dominant phenotype those individuals will display the dominant phenotype for that gene.

See also