Transcript Punnett Squares

Predicting
the Traits of
Offspring
By Reginald Punnett
Punnett Squares
• Punnett Squares are used to predict
the probability of certain traits by
showing the ways that different alleles
can combine
• In a Punnett square, capital letters
represent dominant alleles.
• Lower case letters represent recessive
alleles.
Genotype and Phenotype
• genotype- two letters that represent the two
alleles that an organism has for a trait.
Example: Tt would mean that it has one
dominant and one recessive allele.
• phenotype- the way an organism looks or
acts as a result of its genotype
• The two alleles for each trait come from the
organism’s parents.
Homos and Heteros
• Homozygous means that an
organism has two alleles that are
the same for a trait.
• Heterozygous means that an
organism has two different alleles
for a trait.
Making a Punnett Square
• 1. Write the letters representing the
alleles from one parent across the top
of the box.
• 2. Write the letters from the other
parent down the side of the box.
• 3. Fill in the boxes with the alleles from
each parent.
• The letters in the boxes represent the
genotypes of the possible offspring the
parents could produce.
Stuff To Remember
• The letter that is used to represent the
alleles will be the first letter of the
dominant trait. For example, if tall plants
are dominant over short plants, the letter
T is used, not the letter S. Lowercase t
would represent the short allele.
• If an organism is heterozygous, the
capital letter always comes first. For
example: Tt NOT tT.
Example Problem
• In pea plants, tall plants are dominant to
short plants. Show the genotypes and
phenotypes that are produced when two
heterozygous, tall plants are crossed with
each other.
• Write the genotype of
one parent across the
top.
• Write the genotype of
the other parent down
the side.
• Fill in each box with
the letter above and to
the left of it.
T
t
T
TT
Tt
t
Tt
tt
• What is the chance
that the offspring will
be tall?
• 75% (3 out of 4)
• What is the chance
that the offspring will
be short?
• 25% (1 out of 4)
T
t
T
TT
Tt
t
Tt
tt