1. BIOLOGY 12
Punnett Squares

2. the Punnett Square is a diagram that is used to predict an outcome of a particular cross or breeding experiment
it shows all the possible combinations of alleles in offspring
used by biologists to determine the probability of an offspring having a particular genotype

4. Sample Problem:
in pea plants, tall pea plants are dominant over short pea plants
using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous (purebred) tall pea plant and a homozygous (purebred) short pea plant.

5. Making Punnett Squares
Step 1: designate letters which will represent the genes/traits
capital letters represent dominant traits, and lowercase letters represent recessive traits
T = tall
t = short

6. Making Punnett Squares
Step 2: write down the genotypes (genes) of each parent
these are often given to you or are possible to determine
TT X tt

7. Making Punnett Squares
Step 3: draw a Punnett square - 4 small squares in the shape of a window
write the alleles of one parent across the top
write the alleles of the other parent along the left side of the Punnett square

9. Making Punnett Squares
Step 4: fill in each box of the Punnett square by transferring the letter above and to the left into each appropriate box
as a general rule, the capital letter goes first and a lowercase letter follows

12. Making Punnett Squares
Step 5: list the possible genotypes and phenotypes of the offspring for this cross

13. the letters inside the boxes indicate probable genotypes (genetic makeup) of offspring resulting from the cross of these particular parents
there are 4 boxes, and the genotypic results can be written either as fractions or percents
in this case, all 4 boxes out of the 4 are showing the Tt genotype; therefore, each of the offspring has a 4/4 or 100% chance of showing the Tt genotype

14. we also have to write the phenotype (physical appearance) in each box under the genotype
remember, T = tall and t = short (step 1)
since a capital letter indicates a dominant gene, T (tall) is dominant over t (short); therefore, each of the offspring has a 4/4 or 100% chance of being tall

15. Conclusion
punnett squares give us the probability of genotypes and phenotypes for each offspring

16. Single-Trait Inheritance Example:
cross between a pea plant that is heterozygous for round seeds and a pea plant that has wrinkled seeds. Round is dominant (R)
determine the genotypes and phenotypes of the offspring