1. Probability and Punnett Squares
Chapter 11-2
What are the chances?

2. What are the chances of snow?

3. And getting out of school?

4. Probability
The likelihood that a particular event will occur.

5. Probability
What number will you get when rolling a dice?

6. Probability
When flipping a coin, what are the chances of getting heads verses tails?

7. Genetics and Probability
Example
Event- flipping a quarter
Event outcome- quarter lands on heads
- quarter lands on tails
Event outcome
Question
Event
Heads or tails?
The flip
Heads

8. Probability example (contd.)
Question
Event
Event outcome
Heads
The flip
Heads or tails?
Probabilities
How?
Of the 2 outcomes only, 1 heads, is turned over (1/2)
Each event is independent of the other (1/2 X 1/2 =1/4)
3 independent events
(1/2 X 1/2 X 1/2 = 1/8)
Event outcome
Probability of the event outcome
Heads on single flip
1 out of 2 = 1/2
Both heads on 2 flips
1 out of 4 = 1/4
All heads on st 3 flips
1 out of 8 = 1/8

9. Genetics and probability
How is probability associated with genetics?
Much like the flipping of a quarter, the segregation of alleles is completely random.
Mendel realized this. He was then able to explain the results of his genetic crosses by probability and math.

10. The Punnett Square
The principles of probability can be used to predict the outcomes of genetic crosses
Punnett square – a way to show which genes can combine when egg and sperm join
Alleles are represented by letters
Capitol letters = Dominant genes T
Lower case = Recessive genes t

11. The Punnett Square Genotype – all possible combinations of genes
Represented by letters T or t
Phenotype – physical appearance of organism
Description of characteristic
Tall or short

12. The Punnett Square
Parent’s gamete possibilities go out side of the square
Male across the top
Female down the side
Sperm =>
<= Egg

13. The Punnett Square If we cross two heterozygous tall Tt x
Sperm possibilities
Egg possibilities Tt T t
Sperm =>
<= Egg T t

14. The Punnett Square
Offspring (diploid zygote) possibilities go inside the boxes
Bring 1 letter down
Bring 1 letter over
Capital letter first
Lower case letter second T t
=> T
=> T T T t t
T t
t t

15. Probability Can be determine by completing a Punnett Square . Example:XX XY
What are the chances of having a boy?
Probability of first offspring being a male:
1/2

16. Probability
Probability does not change after traits show up in an offspring.
Same chance still exists for every possibility of being expressed. X Y XX XY
Probability of first offspring being a male: 1/2
Probability of second offspring being a male:
1/2
Probability of third offspring being a male:
1/2

17. Probability Ratios
Probabilities predict the average outcome of a large number of events
Probability can be expressed as a ratio, fraction, or percent
A ratio is a proportion
It compares quantities relative to each other
The larger the sampling of individuals, the closer the ratio will be to the predicted ratio

18. Genetic Ratios
Comparing the number of genotypes or phenotypes of the possible offspring
Genotypic Ratio
Comparing the amount of offspring that express each allele combination in a given cross
Phenotypic Ratio
Comparing the amount of offspring that express each physical trait in a given cross

19. Example Genotypic Ratio Phenotypic Ratio brown green
There is one homozygous dominant, two heterozygous, and one homozygous recessive.
The ratio is 1:2:1
1BB:2Bb:1bb
Phenotypic Ratio
There are three browns and one green.
The ratio is 3:1
3brown:1green

20. The Punnett Square Lets do a punnett square
Trait- skin of pea seeds (smooth or wrinkled)
Alleles- S= smooth and s= wrinkled
Ss X Ss SS Ss
Smooth
Smooth Ss ss
Smooth
Wrinkled

21. Punnett Square Example
Mendel’s Traits in Pea Plants
Pure Smooth X Pure Dented
2 Heterozygous Smooth Crossed

22. Mendel’s Work Observed several traits
Height: Tall (T) and Short (t)
Mated Pure Tall (TT) plants with short (tt) plants
Called Parent “P” generation
Offspring were always tall
Called offspring “F1” generation
F = Filius = T T Tt Tt t
SON! t Tt Tt

23. Mendel’s Work
Are offspring pure or heterozygous?
__________

24. Mendel’s Work
Took F1 offspring and mated with each other to produce F2 generation
About ¾ were tall; ¼ were short
Mendel concluded his plants were heterozygous after seeing the results T t TT Tt T t Tt tt

25. Working with Punnett Squares
Genetic Problems
Working with
Punnett Squares

26. The Punnett Square
Recall that for each trait, there are two genes (one from the mother, one from the father)
Let’s use earlobes as an example
Free earlobes are dominant to attached earlobes
FF = pure dominant; free earlobes
Ff = heterozygous; free earlobes
ff = pure recessive; attached earlobes

27. The Punnett Square
Complete the Punnett square to show the cross between two heterozygous parents
Father’s Genes
Ff x Ff F f FF Ff
Mother’s Genes F ff f Ff

28. The Punnett Square
What are the genotypes of offspring with free earlobes?
If the genotype is ff, what will the phenotype be?
Genotype Ratio:
FF and Ff
Attached earlobes
1FF : 2Ff : 1ff F f FF Ff F ff f Ff

29. The Punnett Square How many offspring will have free earlobes?
How many will have attached earlobes? 3 1
Mother’s Genes F f FF Ff
Father’s Genes F ff f Ff

30. The Punnett Square
The type of chin a person has is also a genetic trait
A person with a cleft chin has a small indentation in the middle of their chin
Cleft chin = dominant trait
Smooth chin = recessive trait

31. The Punnett Square I i Ii i ii ii Ii i Ii, ii
I = cleft chin i = smooth chin
What are all possible genotypes if the father is Ii and mother is ii?
What phenotypes will the offspring have and how many will there be?
2/4  cleft chin
2/4  smooth chin
Ii, ii I i Ii i ii ii Ii i