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Punnett Squares
Punnett Squares
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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
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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
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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
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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
BB
b
Dominant B from one, recessive b from
the other……….(dominant first, please)
B
B
BB
b
Bb
b
Bb
Recessive blue eyed gene from each
B
b
B
BB
Bb
b
Bb
bb
Genotype ratio = 1BB:2Bb:1bb
Phenotype ratio= 3 brown eyed:1 blue
B
b
B
BB
Bb
b
Bb
bb
Try these:
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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!