1. Unit 8: Mendelian Genetics 8.2 Probability and Punnett Squares

2. Probability The likelihood a particular event will occur
Example: coin flip
Probability of flipping a heads: ½ or 50%
Probability of flipping 3 heads in a row: ½ x ½ x ½ = 1/8 or 12.5%

3. Using Segregation to Predict Outcomes
We can use probability to predict the outcome of genetic matches because the way that alleles will segregate during gamete formation is just as random as flipping a coin.
Mendel’s Example:
If each F1 plant had a tall allele T and a short allele t than ½ the offspring would carry the short allele t
BUT the t allele is recessive so the only way to show the trait is to have two short alleles combine tt
In the F2 generation, each gamete also has a one in two (1/2) chance of having the short allele t.
Two gametes are needed to create a new plant so the probability of both gametes carrying the short allele t is ½ X ½ OR ¼.
This means that 3 of the 4 plants will be tall and one of the 4 will be short.
These ratios showed up consistently in Mendel’s experiments-proving that segregation occurs.
Using Segregation to Predict Outcomes

5. Combinations of Alleles
Not all organisms with the same trait have the same combination of alleles.
For example, in Mendel’s pea experiment-the Tt alleles and the TT alleles both produced tall pea plants.
When organisms have two identical alleles for a particular gene, we call them homozygous.
TT or tt
When organisms have two different alleles for a particular gene, we call them heterozygous. Tt
Combinations of Alleles

6. Every organism has a genetic makeup as well as a set of observable characteristics.
A genotype is the genetic makeup and is inherited
Example: TT, Tt, tt
A phenotype is the physical traits which are determined by the genotype and the environment
Example: Tt and TT = Tall phenotype
Two organisms may share the same phenotype but have different genotypes.
Genotype vs. Phenotype

7. Test cross: a cross between two organisms to determine the genotype and phenotype of the offspring.
Diagrams that predict the outcome of a genetic cross by considering all possible combinations of gametes in the cross.
Punnett Square

8. Steps to making a Punnett Square
** See page 10 of your notes **
Start with the parents
Figure out the gametes
Line them up on a Punnett square (2x2)
Write out the new Genotypes
Figure out the results
Steps to making a Punnett Square

9. The mother is homozygous recessive for a straight pinky finger and the father is homozygous dominant for a crooked pinky finger. Draw a Punnett Square to determine the possible:
a. genotypes
b. phenotypes
Monohybrid Example