Review What is an allele? What is a gene? Define phenotype. Define genotype. Each person inherits two Each person inherits two alleles for each gene, one allele from each parent. The basic unit of heredity. The physical appearance The physical appearance or visible traits displayed by offspring. The genetic makeup or The genetic makeup or allele combination present in offspring.

Punnett Square What is it? A graphical grid showing the potential offspring allele combinations from two parents.

Punnett Squares are grids that show the potential offspring of two parents. TT t t Tt PARENT PARENTPARENT OFFSPRING KEY: T = Tall t = short

Background Professor Reginald Punnett, ( ) a British geneticist created the Punnett square, a tool in genetics which is still used by biologists today to predict the probability of possible genotypes of offspring.

Constructing a Punnett square Begin by drawing a grid. Next, you put the genotype (allele combination) of one parent across the top and that of the other parent down the left side. Next, you put the genotype (allele combination) of one parent across the top and that of the other parent down the left side. 1 st parent: T T 1 st parent: T T 2 nd parent: t t 2 nd parent: t t TT t t TT x tt Note: Capital letters denote dominance KEY: T = Tall t = short

Constructing a Punnett square Next, all you have to do is fill in the boxes with the potential offspring combinations. BUT HOW??? Like a multiplication table, combine each column-head letter and row letter into the empty squares. Like a multiplication table, combine each column-head letter and row letter into the empty squares. TT t t & 3 2 & 41 & 4 2 & 3 Tt TT x tt KEY: T = Tall t = short

Interpreting a Punnett Square What does it tell us? T T t t Tt KEY: T = Tall t = short 1 st parent: TT Phenotype?_________ 2 nd parent: tt Phenotype?_________ Tall Short Offspring: ____ Phenotype?_______ All Tall Tt One parent dominant homozygous, other parent recessive homozygous. TT x tt

New Terms Homozygous:Heterozygous:Dominant:Recessive: having 2 identical alleles for a trait having 2 different alleles for a trait the term dominant gene refers to the allele that causes a phenotype that is seen a gene that does not produce its effect when it occurs with a dominant gene, but produces its effect only when there are two copies of it