11-2 Probability and Punnett Squares

11-2 Probability and Punnett Squares
Genetics and Probability
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Probability: the
likelihood that a
particular event will
occur
The probability of a
particular event
occurring from a coin
toss is ½ or 50%
Past outcomes do not
affect future ones
Principles of probability
can be used to predict
outcomes of genetic
crosses
Genetic
Terminology
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Homozygous:
organisms with two
identical alleles for a trait (ex. TT or tt)
Heterozygous: organisms with two different
alleles for a trait (ex. Tt)
Phenotype: the manner in which the genes
for a trait are expressed (what we physically
see)
Genotype: the actual genetic makeup of the
individual (the “code” we do not see)
For example in pea plants
T- Tall-Dominant Trait
t- Short- Recessive
Genotype
TT
Tt
tt
Phenotype
Tall
Tall (T is
dominant over t)
short
Back to the focus question. Is TT homozygous of heterozygous
Genetic Terminology

Punnett squares: a grid used for
determining the genetic combinations that
would result from a genetic cross
Reginald Punnett
parents
gametes
predicted
offspring
A heterozygous Tall Plant is cross-pollinated with a
homozygous tall plant. What are the possible
genotypes and phenotypes.
Step 1: Write down the cross in words (heterozygous tall x homozygous tall)
Step 2: Write down which trait is dominant and which is recessive Dominant-Tall
Recessive- Short
Step 3: Choose a logical letter to represent the gene. This is usually the dominant trait.
Dom -Tall
T
Rec - Short t
Step 4:Write down the possible genotype and phenotype combinations
Genotypes Phenotypes
TT
Tall
Tt
Tall
tt
Short
Step 5: Write down the cross in genetic symbols
Tt x TT
Step 6 : Use a punnett square to calculate the
probabilities of the offspring
t
T
T
TT
Tt
T
TT
Tt
Step 7: Write down the
genotype and phenotype
ratios
Genotypes Phenotypes
TT:Tt:tt
Tall:Short
2:2:0
4:0
2:2
100% tall
Step 8: Answer the question in a sentence
One half of the offspring will be TT and one half will be Tt, and all the plants will be tall.
(genotypes)
(phenotypes)
Round face is dominant over a square face. A homozygous
round faced man marries a square faced woman. What are
the possible genotypes and phenotypes of their children.
Round- dominant- R
Square- Recessive-r
R
R
Genotypes
r
r
Rr
Rr
Rr
Rr
all Rr
Phenotypes
All
Round Face
Probability and Segregation
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The principle of segregation (separation of
alleles (R or r) during gamete formation) is valid
since the phenotypic ratio of 3:1 is consistently
shown in each F2 generation
Probabilities Predict Averages
In order to get the predicted average for a
particular event, the event would have to
occur many times
 In a genetics cross,
you need many offspring
in a generation to come
close to the predicted ratio

Parents
First Generation
Second Generation
Long stems  short stems
All long
787 long: 277 short
Red flowers  white flowers
All red
705 red: 224 white
Green pods  yellow pods
All green
428 green: 152 yellow
Round seeds  wrinkled seeds All round
5474 round: 1850 wrinkled
Yellow seeds  green seeds
6022 yellow: 2001 green
All yellow
Results
All traits have a 3:1
ratio in F2 generation
Last photo of a moron!