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Heredity, GeneticsPunnett Squares & Probability
Heredity, GeneticsPunnett Squares & Probability
Mr. MillerMr. Miller
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Lets find out if the two pigeons on the left produce offspring that have the traits of the pigeon on the right
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To do this we must understand how to use a
Punnet Square
What is a Punnet Square?
• A chart that allows us to determine the different genotypes that offspring can inherit from their parents
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Lets see how to use it
• First we must get the genotype of each parent
Example:
T: allele for tallnesst: allele for shortness
tt – mothers genotype Tt – fathers genotype
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Now lets set up the Punnet Square
T t
t
t
T
T
t
t
t t
t t
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What are the number of genotypes for the traits the offspring can
inherit?
Tt ttTt tt
T t
t
t
•The offspring can inherit two sets of genes that will produce a tall phenotype
•The offspring can inherit two sets of genes that will produce a short phenotype
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Now lets talk about probability
What is probability?
Probability is the chance that an event will occur
How do you find the probability of an event?
You must know: • the number of specific events • the number of possible events
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Lets find the probability the child will be tall
How many genotypes will produce a tall offspring? Tt tt
Tt tt
2
What are number of possible genotypes? 4
T t
t
t
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Now lets find the probability!
Probability =
Number of specific event 2 1Number of possible events 4 2
The probability of having a tall offspring is ½, 1:2, or 50%
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Now try this one
Determine the types of genes an offspring can inherit from parents that have the following genotypes:
Zz - father’s genotypeZz - mother’s genotype
Z: gene for gray feathers
z: allele for white feathers
Z
Z
z
z
z
zz
Z
Z
Z
z
Z
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Now lets look at the possible genotypes and the phenotypes that will be expressed
Z
Z
z
z
z
zz
Z
Z
Z
z
Z
There is a single possibility of inheriting a homozygous (pure) dominant genotype which will produce a phenotype with grey feathers
There are two possibilities of inheriting heterozygous (hybrid) genotypes which will produce a phenotype with grey feathers
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Now lets look at the possible genotypes and the phenotypes that will be expressed
Z
Z
z
z
z
zz
Z
Z
Z
z
Z
There is a single possibility of inheriting a homozygous (pure) recessive genotype which will produce a phenotype with white feathers
So two grey birds can produce a pigeon with white feathers!
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Now lets find the probability
Lets find the probability the offspring will have grey feathers.
eventspossibleofnumber
eventsspecificofnumber4
3=
ZZ Zz
Zz zz
Z
Z
z
z
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Now lets find the probability
Lets find the probability the offspring will have white feathers.
eventspossibleofnumber
eventsspecificofnumber4
1=
ZZ Zz
Zz zz
Z
Z
z
z