Punnett Squares

Punnett SquaresThe work of Reginald C. Punnett, English mathematicianPredict possible offspring and their ratios from any given crossGreater the number of results, the closer to the predicted outcome

More on R.C. Punnett

Punnett SquaresIndividuals 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 SquaresChoose 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 bBb

More Punnett SquaresComplete the Punnett square by combining the alleles from the gametes in the appropriate boxes.Put pairs together (dominant first) in same order as parentsDetermine 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 bBbBB

Dominant B from one, recessive b from the other.(dominant first, please) B bBbBBBbBb

Recessive blue eyed gene from each B bBbBBBbBbbb

Genotype ratio = 1BB:2Bb:1bbPhenotype ratio= 3 brown eyed:1 blue B bBbBBBbBbbb

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!