Download PPT Punnett Squares

Predictions in Genetics
Punnett Squares
Power-up:
1.
2.
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5.
If a woman’s sex cells are a mixture of Bs and bs? What
was her original genotype for eye color? What was her
phenotype?
If a man’s sex cells carry only bs? What was his original
genotype? What was his phenotype?
A woman’s genotype is Ww for widow’s peak. What alleles
will her eggs carry?
Dogs have 78 chromosomes in a body or somatic cell. How
many chromosomes would a dog’s sex cells have? How many
chromosome pairs are in a dog’s body cells?
For which genes below is the organism heterozygous? For
which genes below is the organism homozygous
I Can…

I can use a punnett square to predict
the outcome of genetic crosses
Probability
Probability is a
number that
describes how
likely it is that a
certain event will
appear.
 For example, the
probability of a
coin landings
heads up is 1 in 2.

Mendel and Probability

After repeating his
experiments many
times, and always
finding similar
ratios of dominant
to recessive traits
in the F2 generation,
Mendel concluded
that the principles
of probability
applied to genetics.
Mendel and Probability
Mendel didn’t always find that 3
out his 4 F2 plants were tall-but he found that this was
generally the case.
 That’s why he used the word
“probability” not words such as
“for sure”, “always”, or “certain”
to describe the ratios he
obtained.

Punnett Squares

A punnett square is a
chart that shows all
the possible
combinations of
alleles that can
result from a
genetic cross.
Step 1

Draw a box and divide it into 4
squares.
Step 2

Write the male parent’s alleles along
the TOP of the square and the female
parents alleles on the LEFT side.
B
B
b
b
Step 3

Copy the female parent’s alleles into
the boxes to the right.
B
b
B
B
B
b
b
b
Step 4

Copy the male parent’s alleles into the
boxes beneath them.
B
b
B
BB
Bb
b
bB
bb
Step 5:

Figure out what the phenotypes will
be depending on the allele
combination you see in each little box.
B
B
BB
b
Bb
Brown
eyes
Brown
eyes
bB
b
Brown
eyes
bb
Blue eyes
Step 6

Calculate the probability
for each allele
combination or phenotype:
– If the combination only
shows up 1 time, the
probability is 1/4 or 25%.
– If the combination shows up
2 times, then the
probability is 2/4 or 50%.
– If the combination shows up
3 times, then the
probability is 3/4 or 75%.
– If the combination shows up
4 times, then the
probability is 4/4 or 100%.
BB
Bb
Brown
eyes
Brown
eyes
bB
Brown
eyes
bb
Blue eyes
Meiosis and Punnett Squares
A punnett square is
actually a way to
show the events
that occur at
fertilization.
 The punnett square
shows the way
alleles are
seperated when
sex cells are
formed.

Practice
Complete the worksheet with your lab
group using the models provided.
 Do NOT write on the actual
worksheet. Each group member
should have their own piece of
notebook paper with

– 4 punnett squares (completed)
– Responses to Questions A and B
Practice 1

Build a punnett square for the
offspring of a heterozygous Browneyed Dad and a homozygous brown
B
b
eyed Mom.
B
B
BB
Bb
Brown
eyes
Brown
eyes
BB
Bb
Brown
eyes
Brown
eyes
Practice 2

Build a punnett square for the
offspring of a blue-eyed Dad and a
heterozygous brown eyed Mom.
B
b
b
b
Bb
Bb
Brown
eyes
Brown
eyes
bb
bb
Blue eyes
Blue eyes
Gummy Bear Practice: Part 1
Phenotype
Red
Orang e
Yell ow
Clear

Build Punnett Squares for each of the following sets of
Gummy Bear parents. Use the table above to determine
the phenotype of each offspring.
1.
2.
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6.

Genotype
RR or Rr
rr
YYor Yy
yy
Dad
Dad
Dad
Dad
Dad
Dad
(RR) x Mom (Rr)
(RR) x Mom (rr)
(heterozygous red) x Mom (orange)
(YY) x Mom (Yy)
(homozygous yellow) x Mom (clear)
(heterozygous yellow) x Mom (yy)
For each set of offspring calculate the probability of
having a red, orange, yellow, or clear offspring (Hint: Use
your punnett squares!!)
Gummy Bear Practice: Part 2
Open your bag. Build a table to record the
number of each color gummy bear you
have.
2. What percent of each color do you have?
(Should show percent for red, orange,
yellow, and clear)
3. What were the genotypes of the parents?
(Hint: Look at the punnett squares you
already completed in Part 1)
1.