HUMAN BIO 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 TT t tTt

Example 2 Possible Genotypes:  100% heterozygous Possible Pheontypes:  100% tall TT t tTt

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 tt T ttt Tt tt

Example 3 Possible Genotypes:  25% heterozygous  25% homozygous recessive Possible Phenotypes:  25% tall  25% short tt T ttt 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 Rr r rrr Rrrr Rr

Example 4 Possible phenotypes:  50% round seeds  50% wrinkled seeds Rr r rrr Rrrr 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