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

Now lets set up the Punnet Square

T t

t

t

T

T

t

t

t t

t t

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.

Vocabulary

• Heterozygous: An organism that has different alleles for a trait.

• Codominance: When alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring.