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Ms. Brelsford
Name:
Punnett Squares
 Recall that different forms of the same genes for a particular trait are called
alleles, and depending on the combination, the alleles can be described as
either:
o Homozygous :
o Heterozygous :
 The combination of alleles or genes that an individual has is called genotype
 The physical appearance is called the phenotype
 To determine the possible traits than an individual has, one uses a Punnett
square to show the combination of alleles for an offspring
Ex. In humans brown eye color (B) is dominant to blue eye color (b). A mother
and father, both having the brown eye phenotype, have a child. We know that
both parents carry the gene for blue eye color and therefore are heterozygous for
this trait.
♂
What is the probability of having a child with
blue eyes?
♀
What is the probability of having a child with
brown eyes?
What are the possible phenotypes?
What are the possible genotypes?
 The determination of traits is not always clearly defined as either dominant or
recessive. There are times where both traits can be shown by the individual
o Incomplete dominance :
o Co-dominance :
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Ms. Brelsford
Name:
Punnett Squares
Example:
Another characteristic that Mendel
observed in his pea plants was
pea shape. A pea was either
round or wrinkled. He found that
the dominant trait was round (R),
and the recessive trait was
wrinkled (r).
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Questions:
1.) What is the phenotype of a
plant whose genotype is heterozygous for pea shape?
2.) What is the phenotype if the genotype is homozygous recessive for pea shape?
3.) Complete the Punnett Square to show the cross between a heterozygous and a
homozygous recessive for pea shape (Rr x rr).
4.) Assume 40 pea plants were produced.
a.) How many plants have round peas? ______
b.) How many plants have wrinkled peas? ______
c.) What is the phenotypic ratio of round peas compared to wrinkled peas? ___ : ___
5.) Assume that 60 pea plants were produced.
a.) How many plants are heterozygous? ______
b.) How many plants are homozygous dominant? _______
c.) How many plants are homozygous recessive? _______
Ms. Brelsford
Name:
Punnett Squares
d.) What is the genotypic ratio of heterozygous plants compared to homozygous
recessive plants? ___ : ___