H U M A N B I O 11

Genetics: Punnet Squares

Mendelian Genetics

Alleles a form of the gene

Phenotype physical appearance

Genotype the alleles a person has for a trait

Mendelian Genetics

Homozygous two same alleles

Heterozygous two different alleles

Mendelian Genetics

Dominant show through if homozygous dominant or if heterozygous

Recessive are masked only show if homozygous recessive

Think About It:

What would happen if a BB brown eyed woman had a child with a bb blue eyed man? 100% chance the child will have brown eyes.

What would happen if a Bb brown eyed woman had a child with a Bb brown eyed man? 75% chance the child will have brown eyes. 25% chance the child will have blue eyes.

Punnet Squares

Tables used to figure out the probability of offspring having a certain trait.

They allow us to cross (breed) the genes of mother and father to determine possible genotypes and phenotypes.

Using Punnet Squares

Step 1: Make a legend for the letters used to represent dominant and

recessive alleles Alleles for the same gene use the same letter Dominant are capital and recessive are lowercase

Step 2: Write the mother’s and father’s genotype

Using Punnet Squares

Step 3: For a single trait use a 2 by 2 square Write one parent’s genotype along top and other’s along left

side

Using Punnet Squares

Step 4: Fill in each small box by transferring the letter above and the

letter to the left Capital letter always goes first in the pair

Step 5: Answer the question or list the possible genotypes and

phenotypes with their percentages or probabilities

Example 1

Brown or blue eyes?

Possible Phenotypes: 75% (3 out of 4) will have

brown eyes 25% (1 out of 4) will have

blue eyes

Possible Genotypes: 25% (1 out of 4) will be homozygous dominant 50% (2 out of 4) will be heterozygous 25% (1 out of 4) will be homozygous recessive

Example 2

In pea plants, tall pea plants are dominant over short pea plants. Predict the genotypes and phenotypes of the offspring of a cross between a homozygous tall pea plant and a homozygous short pea plant.

Example 2

Legend: T = tall

t = short

Parents: TT x tt

T T

t

t Tt

Tt Tt

Tt

Example 2

Possible Genotypes: 100% heterozygous

Possible Pheontypes: 100% tall

T T

t

t Tt

Tt Tt

Tt

Example 3

In pea plants, tall pea plants are dominant over short pea plants. Predict the genotypes and phenotypes of the offspring of a cross between a homozygous recessive pea plant and a heterozygous pea plant.

Example 3

Legend: T = tall

t = short

Parents: tt x Tt

t t

T

t tt

Tt Tt

tt

Example 3

Possible Genotypes: 25% heterozygous 25% homozygous recessive

Possible Phenotypes: 25% tall 25% short

t t

T

t tt

Tt Tt

tt

Example 4

Determine the possible phenotypes of a cross between a pea plant that is heterozygous for round seeds and a pea plant that has wrinkled seeds.

Example 4

Legend: R = round

r = wrinkled

Parents: Rr x rr

R r

r

r rr

Rr rr

Rr

Example 4

Possible phenotypes: 50% round seeds 50% wrinkled seeds

R r

r

r rr

Rr rr

Rr

Practice 1

Curly hair is dominant over straight hair. If a homozygous recessive man and a heterozygous dominant woman have a child, what is the probability that the child will have curly hair?

50% chance the child will have curly hair.

Practice 2

Curly hair is dominant over straight hair. If a homozygous recessive man and a homozygous recessive woman have a child, what is the probability that the child will have straight hair?

0% chance the child will have straight hair.

Practice 3

A father is homozygous dominant for tongue rolling, while a mother is heterozygous for tongue rolling. Determine the possible genotypes and phenotypes for their next child.

50% homozygous dominant 50% heterozygous

100% tongue rolling

Practice 4

The child from practice 3 ended up being heterozygous for tongue rolling. When he became an adult, he had a child with a homozygous recessive woman. What are the possible genotypes and phenotypes of their child?

50% heterozygous 50% homozygous recessive

50% tongue rolling 50% non tongue rolling