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Punnett Squares
Objective
• SWBAT predict possible combinations of alleles
in a zygote from the genetic make up of the
parents
DO NOW: Look at this picture.
What do you think it is saying?
What’s the word…?
1. Rats have one allele from its mother that
is R and one from its father that is r, what
term would describe it’s genotype?
2. aa can be described as this
____________ while AA can be
described as ______________.
3. Different forms of genes are called
________________ (like brown or blonde
hair)
4. A section of DNA coding for a specific trait
Some Vocabulary
• Heterozygous: Has two different forms of
an allele
• Example: Bb
• Homozygous: Contains the same form of
an allele
• Example: bb, BB
• Homozygous Dominant:
• Homozygous Recessive:
Practice
A man is heterozygous for the hair color gene, and he has black
hair. He marries a woman who is homozygous recessive and
has blonde hair. Write the genotype and phenotype for each
person AND tell me what the dominant and recessive traits are.
Man
Genotype
Phenotype
Woman
Dominant Trait =
Recessive Trait =
Can we predict phenotypes of
children?
Mendel’s Experiment
• Mendel bred a green pea plant with a
yellow pea plant (Parental Generation).
He found that all of the offspring (F1
generation) were also green.
• When he bred these offspring together though, he got 3
green pea plants and one yellow plant. (F2 generation)
How can we test this ourselves?
• Mendel realized that there was a mathematical
way to predict the genotype and phenotype of
an offspring
• This mathematical technique is known as a
punnett square
Punnett Squares Show Possible Zygote
Formations
Sperm
A
A
a
a
AA
Aa
zygote
zygote
Aa
aa
zygote
zygote
Mendel’s Punnett Squares
• In Mendel’s first experiment, he crossed: a GG
(green pea plant) and a gg (yellow pea plant)
together
G
G
g
g
Gg
Gg

Parents: GG and gg

Offspring:
Gg

Genotypes: all Gg

Phenotypes: all Green
Gg
Then he took 2 of the offspring from the F1
generation and mated them together
G
G
g
• Parents’ Genotypes: Gg, Gg
GG
Gg
• Parents’ Phenotype: Green
• Offspring’s Genotypes:
GG, Gg, gg
g
Gg
gg
• Offspring’s Phenotypes:
Green and Yellow
Now YOU try Understanding
G
G
GG
G
GG
Check for
Understanding
• Parents:
• Genotypes GG and Gg
• Phenotypes all green
• Offspring:
g
Gg
Gg
G = green
g = yellow
• Genotypes: GG and Gg
• Phenotypes: all green
Determining Probability
•
Use the information from your last punnett square to answer
the questions.
•
What is the probability that the offspring will have green
peas?
•
•
What is the probability that the offspring will have yellow
peas?
•
•
4/ 4 = 100%
0
/4 =
0
%
What is the ratio of green peas to yellow peas?
4:0
More Vocab
• Genotypic Ratio: Number of times a
genotype would appear in the offspring
after a test cross
• Example: 2 GG: 2 Gg
• Phenotypic Ratio: Number of times a
phenotype would appear in the offspring
after a test cross
• Example: 4 green: 0 yellow
More Practice -
Check for
Understanding
A woman homozygous dominant for a BIG tongue mates
with heterozygous man. What are the genotypes and
phenotypes of their offspring?
B
B
B
BB
BB
B = Big tongue
b = Small tongue
• Parents: BB and Bb
• Offspring:
b
Bb
Bb
• Genotypes: BB and Bb
• Phenotypes: all BIG
Huntington’s disease is an autosomal dominant disorder.
Symptoms appear in mid- adulthood and cause neurological
cell death. Using the following Punnett square, determine the
chance that someone with Huntington’s disease will have
children with the disease if mated with a homozygous recessive
individual. H
H
h
Hh
Hh
h
Hh
Hh
H = Huntington’s Disease
h = No disease
What’s the probability that one of her children will
have Huntington’s Disease?
100%
Practice
SpongeBob’s aunt is famous around town for her itty, bitty stubby nose!
She recently met a cute squarepants fellow who also has a stubby
nose, which is a recessive trait. Would it be possible for them to have a
child with a regular long nose? Why or why not? Create a Punnett
square to help you answer this question.
l
l
ll
l
ll
Recessive = stubby nose = l
Dominant = long nose = L
• Parents: ll and ll
• Offspring:
l
ll
ll
• Genotypes: all ll
• Phenotypes:
all stubby nose
Do Now
1. The genetic makeup of a cell is known as
_______________________.
2. The physical characteristic of something
(like having dimples)
3. Green colored skin is more likely to be
expressed over red skin. The green
colored skin is known as a
_______________ allele.
4. If my alien had red skin (see #4), it would
be expressing the ________________
trait.
Law of Segregation
• If a person is heterozygous
(Rr), what are the possible
alleles that his/her gametes will
have?
• Either R OR r
• Think- Pair- Share: what does
this have to do with meiosis?
• Because the two alleles are on
homologous chromosomes and
they will separate in meiosis
Law of Independent
Assortment
• Occurs because chromosomes go through
“random segregation” when they make gametes
• Mother and father chromosomes DO NOT
need to move together into new cells
• Example: if my dad has blonde hair (bb) and
dimples (dd), what gametes can he possibly
make?
• Example: If my mom is heterozygous for brown
hair (Bb) and dimples (Dd), what gametes can
she possibly make?