1. Punnett squares and probability
Predicting Genetic Outcomes
Punnett squares and probability

2. Patterns of inheritance
A dominant allele is always expressed
A recessive allele is only visible when paired with another recessive allele.

3. Alleles – Patterns of inheritance
To test for patterns of inheritance, Mendel crossed two organisms with different traits
Each had two distinct forms
Each allele is represented by a letter
CAPITAL = dominant
lower = recessive
Homozygous - if the two alleles for a trait are the same (AA or aa)
Heterozygous - if the two alleles for a trait are different (Aa)
TT Tt tt
Dominant Trait Recessive Trait

4. Predicting Inheritance
To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes.
UPPERCASE letters are used to represent dominant genes.
lowercase letters are used to represent recessive genes.

5. Predicting Inheritance
For example:
T = represents the gene for TALL in pea plants
t = represents the gene for short in pea plants
So:
TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive.
tt will result in a short plant.
Remember there are two alleles for every trait!

6. PUNNETT SQUARES A diagram that predicts the outcome of a genetic cross
Considers all the possible combinations of gametes
1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S
WHY E COLI?
What do you like to eat?
WHEN YOU EAT YOU NEED ENZYMES TO BREAK DOWN THE FOOD YOU EAT
EXPLAIN WHAT ENZYMES ARE
THEY BREAK DOWN FOOD
LIKE GLUCOSE AND LACTOSE
WHAT ARE GLUCOSE AND LACTOSE?

7. PUNNETT SQUARES The genes from one parent go here.
The genes from the other parent go here.

8. PUNNETT SQUARES T t

9. PUNNETT SQUARES T t

10. PUNNETT SQUARES T t

11. PUNNETT SQUARES T t Tt

12. PUNNETT SQUARES T t Tt

13. PUNNETT SQUARES T t Tt
F1 generation

14. Interpreting the Results
The genotype for all the offspring is Tt.
The genotype ratio is:
Tt – 4:4 = 100% heterozygous
The phenotype for all the offspring is tall.
The phenotype ratio is:
tall – 4:4 = 100% tall T t Tt

15. PUNNETT SQUARES T t TT Tt tt
F2 generation

16. Interpreting the Results
This time the ratios are different! Tt.
The genotype ratio is:
TT – 1:4
Tt – 2:4
tt – 1:4
1:2:1
2 – homozygous
2 – heterozygous
The phenotype ratio is:
TT, Tt, Tt = 3 tall
tt = 1 short
3:1 – tall : short T t TT Tt tt

17. PunnetT Squares
Simple dominance: one allele is completely dominant
P=Aa x Aa A=red, a=clear
A a A a
F1 Genotype=
AA: Aa: aa 1: 2: 1
F1 Phenotype=
A (red): a (clear) : 1 AA Aa aa

18. Laws of probability help explain genetic events
Genetic ratios are most properly expressed as probabilities:
Probabilities range from
0 - an event is certain NOT to happen
1.0 - an event is certain to happen

19. Predicting Outcomes-probability
The likelihood that a specific event will occur
Probability = # of one kind of possible outcome
total # of all possible outcomes
a coin lands on “heads”
1 outcome
Total possible outcomes
= 2
heads or tails
Possibility that the coin will land on heads = 1/2

20. Product law For simultaneous outcomes (this AND that)
What is the chance that you will roll snake eyes with two dice? (1 and 1)
Chance of rolling 1 with first die = 1/6
Chance of rolling 1 with second die = 1/6
Chance of rolling two 1’s = 1/6 X 1/6 = 1/36

21. What fraction would we expect to be
Probabilities
Multiplication Rule
The probability that two independent events, A and B, are realized simultaneously is given by the product of their separate probabilities
What fraction would we expect to be
Round AND Green
3/4 x 1/4 = 3/16

22. Sum law For outcomes that can occur more than one way (this OR that)
What is the chance that you will roll either a 1 or a 6 with one die?
Chance of rolling 1 = 1/6
Chance of rolling 6 = 1/6
Chance of rolling 1 or 6 = 1/6 + 1/6 = 2/6 = 1/3

23. What fraction would we expect to be
Probabilities
Addition Rule
The probability that one or the other of two mutually exclusive events, A or B, is the sum of their separate probabilities
What fraction would we expect to be
(Round and Green) OR
(wrinkled and yellow)
3/ /16 = 6/16

24. Dihybrid cross F1 produces equal amounts of 4 possible genotypes
F2 reveals even more genotypic possibilities (9:3:3:1)
Dihybrid cross is equivalent to two monohybrid crosses (12:4 or 3:1)
Illustrates the Law of Independent Assortment

25. Why we look the way we look...
Mendel and Heredity