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Name _______________________________________ Date _________________ Period _____________
THE FPUNNET SQUARE
…slient p, invisible f…..
The Punnet Square was developed in 1917 by Reginald C. Punnet. It is a way to predict the probable
outcome of a cross between two organisms. The important thing to remember is that it only predicts the
probability of the offspring’s genotype or phenotype, not the actual outcome. For example, if an organism
gives birth to four offspring, the four offspring will not necessecelery look like those in the Punnet square.
For each cross below, do a Punnet square, and calculate the genotypic and phenotypic ratios. If there
are any other questions, make sure you answer them, as well. You must show your square to get credit for
the problem!
0. hWhat is the best practical joke ever?
1. A blue Smurf (bb) and a rare, green Smurf (Bb) mate. Assume that blue (b) is recessive to green (B).
Genotypic Ratio
Phenotypic Ratio
2. The gene for immunity to the poison ivy oil (E) is dominant to the gene for susceptibility to irritation
from the oil (e). Most people have two copies of the recessive gene, and can be irritated by the oil produced
by the leaf. Two people who are both susceptible to irritation mate. Can they have a child who can run
around in the yard without fear of the dreaded PI? Show your square!
Genotypes of parents: _________ X ________
Genotypic Ratio
Phenotypic Ratio
Will the child be immune?
3. Pea plants can have red or white flowers, and their seed pods can be either wrinkled or smooth. Assume
that red is dominant to white, and that wrinkled is dominant to smooth. Also assume that the traits assort
independently, that is, that the red- or white-ness of the flowers has nothing to do with whether or not the
seed pods are wrinkled or smooth. Cross a homozygous red-flowered, heterozygous wrinkled plant with a
white flowered, smooth plant.
Genotypes of parents: ________________ X ______________
Genotypic Ratio
Phenotypic Ratio
4. Friedreich's ataxia is a neurological disorder in which the myelin sheath of the spinal nerves degenerates,
causing impaired coordination in the arms and legs, various heart conditions, and other symptoms. It is
present in about 1 in 50,000 Americans. It is an autosomal recessive disorder. If two people who are carriers
of the recessive gene for this disorder, but who do not themselves suffer from it, what are the chances that
their offspring will be affected?
Genotypes of parents: ________________ X ______________
Genotypic Ratio
Phenotypic Ratio
% of children affected