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Mendel Carefully
Accumulated Data And
Realized That The
Principles Of Probability
Could Be Used To Explain
The Results.
Probability=
The Likelihood That
A Particular Event
Will Occur.
Coin Flip
Two Possible Outcomes
Chance of Heads Is
1 in 2 or 50%
Probability of 3 Heads In A Row
Each Toss Is An Independent Event
Chance of 1 Head = ½
Chance of 3 Heads In Row =
½ x ½ x ½ = 1/8
Past Outcomes Do Not Influence
Future Events !
Key Concept:
The Principles Of Probability Can
Be Used To Predict The
Outcomes Of Genetic Crosses,
Because Alleles Segregate As
Completely Random Events.
1.
2.
3.
Define The Gametes
That Will Be
Produced By The
Parents.
Place The Gametes
Of One Parent
Across The Top.
Place The Gametes
Of The Other Parent
Down The Side.

The Gametes
Represent The
Alleles.

Indicate The Genetic
Makeup Of The F1
Offspring By
Combining The Allele
From Each Parent In
The Appropriate
Location In The Box


Capital Letters
Indicate Dominant
Alleles
Lower Case Letters
Indicate Recessive
Alleles
T = Tall
t = short

Genes Containing The Same Alleles For A
Trait Are Said To Be: Homozygous
TTHomozygous Dominant
tt
Homozygous Recessive

Genes Containing Different Alleles For A
Trait Are Said To Be: Heterozygous
Tt

Homozygous Organisms Are Said To Be:
True Breeding

Heterozygous Organisms Are Said To Be:
Hybrid
Phenotype
› The Physical Appearance or
Characteristics Of An Organism.
Genotype
› The Genetic Makeup (Alleles) Of
An Organism
TT or Tt
Tt or TT (Tall Plants)
One (1) Phenotype (Tall)
Two (2) Genotypes
 True Breeding (TT)
 Hybrid (Tt)
tt (Short Plants)
One (1) Phenotype
One (1) Genotype

Look At Fig. 11-7
› 25% (1/4) F2 Plants Are (TT)
› 50% (1/2) F2 Plants Are (Tt)
Therefore,
75% F2 Plants Are Tall
› 25% (1/4) F2 Plants Are (tt)
Therefore,
25% F2 Plants Are Short

If Mendel Was Correct, The Ratio Of Tall
To Short Will Be 3-to-1.
› Which It Is!
› Therefore, Mendel Was Correct.
› Therefore, Segregation Is Supported By The
Data.
Probabilities Predict The
Average Outcome For A Large
Number Of Events.
Probability Does NOT Predict The
Precise Outcome Of A Single
Event.
This Is True In Genetics And Coin
Tosses.
The Larger The Sample Size –
The Closer The Results To
Expectations.