Punnett squares are used to predict the chance of offspring inheriting a particular genotype and phenotype.
Do you remember what a genotype and phenotype is?
Step 1: Draw a 2x2 grid (a box and divide it into four smaller squares). Leave room above the box and to its left, so you can label it.
Step 2: Look at the genotype of one parent and write the alleles ABOVE the two columns. Look at the other parent and write the alleles NEXT TO the two rows.
I find it helpful to label the columns/rows with the genotype so I don't mixup the allele's.
Step 3: Fill in each of the boxes by writing the alleles (letters) that 'match up' in that box.
Step 4: Now you can calculate the probability!
From a Punnett Square all results will be out of 4.
Using the above example - 1 out of 4 offspring will have RR = one quarter = 25% 1 out of 4 offspring will have rr = one quarter = 25% 2 out of 4 offspring will have Rr = half = 50%
Step 5: Reporting the Phenotype
Remember the Phenotype is the physical expression (what you can see) of the Genotype.
If I told you that - R is the dominant allele for red hair r is the recessive allele for brown hair
Then we would report the results like this - 75% of offspring will have red hair (RR or Rr) 25% of offspring will have brown hair (rr)