Download Punnett Squares

Punnett Squares
Follow this step by step
procedure EVERY time you
set up a punnett square!
Genetics Word Problem
A brown eyed woman who is heterozygous
for the trait has a child with a man who has
blue eyes. The brown eye allele is
dominant over blue eyes. What is the
probability that their child will have blue
eyes.
Step 1
Assign a letter to the alleles involved.
– I always try to pick letters that look different
from capital to lowercase
– It does not matter what letter you choose as
long as the same letter is used for the same
gene
– Assign a capital letter for dominant alleles,
lowercase for recessive
– WRITE IT OUT! EVERYTIME!!!
Step 1
Gene - eye color
Alleles – brown = B
blue = b
Step 2
Using the information given in the word
problem, determine the parental
genotypes (genetic make-up)
– Sometimes you are given the parents’
phenotype (physical appearance), genotype
(hetero/homozygous), or even the offspring
genotype or phenotype
– The important thing to remember is you can
always find the parents’ genotype from given
info!
Step 2
Mother is heterozygous with brown eyes
– Bb
Father HAS blue eyes which is recessive
so his genotype MUST be
– bb
Write it like this:
Bb x bb
Step 3
Determine the types of gametes that each
parent will contribute
– Remember each parent can only pass one of
the alleles to the offspring
– Which one does it pass??? We don’t know, so
all genetic possibilities have to be tested
– It will be helpful if we construct our actual
square here.
Step 3
Parent one on top row
Bb x bb
Parent two on side
Step 3
Bb x bb
Parent one on top row
B
b
Parent two on side
Step 3
Bb x bb
Parent one on top row
B
Parent two on side
b
b
b
Step 4
Fill out the table
Go slow so you do not make silly
mistakes!
Step 4
Bb x bb
Parent one on top row
B
Parent two on side
b
b
b
Step 4
Bb x bb
Parent one on top row
B
Parent two on side
b
b
Bb
b
Step 4
Bb x bb
Parent one on top row
Parent two on side
b
b
B
b
Bb
bb
Step 4
Bb x bb
Parent one on top row
Parent two on side
B
b
b
Bb
bb
b
Bb
Step 4
Bb x bb
Parent one on top row
Parent two on side
B
b
b
Bb
bb
b
Bb
bb
Step 5
Determine the ratio or probability!
– Give the answer in whatever form the
question asks for
– In this example it asks for the probability (that
is a fraction or percentage)
2 out of 4, or 2/4 = ½ or 50%
Step 4
Bb x bb
Parent one on top row
Parent two on side
B
b
b
Bb
bb
b
Bb
bb
The question asked for the probability of blue
eyes. How many offspring would have blue
eyes? Out of how many?
You Try!
You are an expert in genetics called to
testify at a divorce case. A man with
brown eyes is questioning the paternity of
a child who was born by his estranged
wife who also has brown eyes. The man
claims it cannot be his baby because it
has blue eyes.
Give your expert testimony!
What parental genotypes would cause this
scenario?
Show your punnett square that explains
how or if this can happen.
Does this PROVE that the baby is his?
What would be the next step in granting or
refusing the man a divorce based on
infidelity?