1. Punnett Squares
Follow this step by step procedure EVERY time you set up a punnett square!

2. 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.

3. 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!!!

4. Step 1
Gene - eye color
Alleles – brown = B
blue = b

5. 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!

6. Step 2 Mother is heterozygous with brown eyesBb
Father HAS blue eyes which is recessive so his genotype MUST be bb
Write it like this: Bb x bb

7. 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.

8. Step 3
Bb x bb
Parent one on top row
Parent two on side

9. Step 3
Bb x bb
Parent one on top row B b
Parent two on side

10. Step 3
Bb x bb
Parent one on top row B b
Parent two on side

11. Step 4
Fill out the table
Go slow so you do not make silly mistakes!

12. Step 4
Bb x bb
Parent one on top row B b
Parent two on side

13. Step 4
Bb x bb
Parent one on top row B b Bb
Parent two on side

14. Step 4
Bb x bb
Parent one on top row B b Bb bb
Parent two on side

15. Step 4
Bb x bb
Parent one on top row B b Bb bb
Parent two on side

16. Step 4
Bb x bb
Parent one on top row B b Bb bb
Parent two on side

17. 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)

18. Step 4
2 out of 4, or 2/4 = ½ or 50%
Bb x bb
Parent one on top row B b Bb bb
Parent two on side
The question asked for the probability of blue eyes. How many offspring would have blue eyes? Out of how many?

19. 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.

20. 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?