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Heredity, GeneticsPunnett Squares & Probability
Heredity, GeneticsPunnett Squares & Probability
Mr. MillerMr. Miller
Lets find out if the two pigeons on the left produce offspring that have the traits of the pigeon on the right
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
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
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
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
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
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%
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
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
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!
Now lets find the probability
Lets find the probability the offspring will have grey feathers.
yprobabilit
eventspossibleofnumber
eventsspecificofnumber4
3=
ZZ Zz
Zz zz
Z
Z
z
z
Now lets find the probability
Lets find the probability the offspring will have white feathers.
yprobabilit
eventspossibleofnumber
eventsspecificofnumber4
1=
ZZ Zz
Zz zz
Z
Z
z
z
Vocabulary
• Probability: A number that describes how likely it is that an event will occur.
• Punnett Square: A chart that shows all the possible combination of alleles that can result from a genetic cross.
Vocabulary
• Phenotype: An organism’s physical appearance, or visible traits.
• Genotype: An organism’s genetic makeup or allele combination.
• Homozygous: An organism that has two identical alleles for a trait.