Punnett Practice
What does the Punnett square
represent?
• When written together, the letters on the
edges of the square represent the genotypes
of the parents (e.g. Tt, yy)
• BUT, when placed along the Punnett square,
they actually represent the alleles present in
the gametes of the parents (e.g. T, t, Y, y)
Explain to your partner why the second
statement makes sense.
What does the Punnett square
represent?
Gametes must have half
the normal number of
chromosomes that
body cells have or else
the offspring will have
too many
chromosomes!
Monohybrid Cross
T - allele for a tall plant
t - allele for a short plant
Show the possible offspring for the following cross:
TT x Tt
Monohybrid Cross
T - allele for a tall plant
t - allele for a short plant
Show the possible offspring for the following cross:
heterozygous X homozygous recessive
Test Cross
In dogs, there is a hereditary deafness caused by a
recessive gene, “d.”
A kennel owner has a male dog that she wants to use
for breeding purposes if possible. The dog can hear,
so the owner knows his genotype is either DD or
Dd. If the dog is heterozygous, the owner does not
wish to use him for breeding. (Why not?)
Test Cross
She can find out the genotype of
her dog by breeding him to a deaf
female (dd).
• Draw the Punnett squares to
illustrate these two possible
crosses.
• In each case, what percentage of
the offspring would be expected
to be hearing?
• How could you tell the genotype
of this male dog?
Test Cross
• Would you do a test cross if the
organism is homozygous
recessive?
• How can two hearing dogs
produce deaf offspring?
• What is a test cross?
Dihybrid Cross
• When you’re considering two sets of genes at the
same time
• For an online tutorial, go to:
http://www.biology.arizona.edu/mendelian_genetics/
problem_sets/dihybrid_cross/03t.html
Dihybrid Cross
A dihybrid cross with two heterozygous parents
• Gene 1: A-dominant, a-recessive
• Gene 2: C-dominant, c-recessive
Parents are heterozygous for both genes
To set up the Punnett square, think about what
combination of alleles can be in the parents’ gametes
Dihybrid Cross
Dihybrid Cross
AC
AC
Ac
aC
ac
Ac
aC
ac
Dihybrid Cross
AC
Ac
aC
ac
AC
AACC
AACc
AaCC
AaCc
Ac
AACc
AAcc
AaCc
Aacc
aC
AaCC
AaCc
aaCC
aaCc
ac
AaCc
Aacc
aaCc
aacc
More Dihybrid Crosses (Part 1)
Gene 1: A - dominant; a - recessive
Gene 2: C - dominant; c - recessive
1) heterozygous X homozygous recessive
2) heterozygous X homozygous dominant
More Dihybrid Crosses (Part 2)
• Read p. 320 Analyzing Data (blood types and
Rhesus factor). Answer questions 3 and 4, then
solve the problem below.
• Mrs. Xu, Mrs. Yang, and Mrs. Zhang all gave birth
to baby girls on the same day. Someone later
claimed that the hospital mixed up the babies. As a
hospital administrator, it is your job to make sure
that each pair of parents has the correct baby, so
you order blood typing to be done on all the
parents and all the babies.
More Dihybrid Crosses (Part 2)
Here are the results:
Parent
Mrs. Xu
Mr. Xu
Mrs. Yang
Mr. Yang
Mrs. Zhang
Blood Type
A+
B+
BO+
A+
Mr. Zhang
A-
Baby 1
Baby 2
Baby 3
O+
ABB-
More Dihybrid Crosses (Part 2)
First, for each parent, think about what possible genotype(s)
could give that phenotype.
If there is more than one possible genotype, then which of the
possible genotypes would give the most variation in terms of
possible children?
(If you want to make a Punnett square, this is a dihybrid cross.)
Which baby belongs to which set of parents? Prepare to defend
your answer.
The End