Punnett Squares

 The work of Reginald C. Punnett, English mathematician  Predict possible offspring and their ratios from any given cross  Greater the number of results, the closer to the predicted outcome

More on R.C. Punnett

Punnett Squares  Individuals with identical alleles are called homozygous (TT, tt)  Individuals with different alleles are called heterozygous (Bb)  Phenotype – physical characteristics (brown eyes, can taste PTC, wrinkled seed coats, etc.  Genotype – the actual genes present/genetic makeup (homozygous recessive, homozygous dominant, heterozygous)

How to use Punnett Squares Choose a letter to represent the alleles in the cross. (first letter of dominant trait?) Write the genotypes of the parents. Determine the possible gametes (reproductive cells) that the parent can produce, containing one gene from each gene pair. Enter the possible gametes at the top (mom) and side (dad) of the Punnett square.

Naked Punnett Square

Bb (heterozygous brown eyed) X Bb

Bb (heterozygous blue eyed) X Bb B b

Bb (heterozygous blue eyed) X Bb B b B b

More Punnett Squares Complete the Punnett square by combining the alleles from the gametes in the appropriate boxes. Put pairs together (dominant first) in same order as parents Determine the genotypes/phenotypes (and their numbers) of the offspring. Write down the genotypic and phenotypic ratios in simplest form.

Dominant brown eye gene from each B b B b B

Dominant B from one, recessive b from the other……….(dominant first, please) B b B b BBbBb BbBb

Recessive blue eyed gene from each B b B b BBbBb BbBbb

Genotype ratio = 1BB:2Bb:1bb Phenotype ratio= 3 brown eyed:1 blue B b B b BBbBb BbBbb

Try these:  Tt X Tt, where T=taster of PTC, t=nontaster  RrYy X RrYy, where R=round peas, r=wrinkled peas, Y=yellow, y=green

Punnett Squares Monohybrid & Dihybrid Crosses

Now work on the worksheet problems!