Punnett Squares

Phenotype vs. Genotypes Phenotype: physical appearance of a trait Example: Brown vs. Blue Eyes Genotype: Genetic notation for a trait found on a chromosome Example: Brown eyes = BB Blue eyes: bb

Dominant vs. Recessive Dominant Trait: An allele that is fully expressed whenever the allele is present Brown eyes (B) Recessive Trait: An allele that is expressed only when there is no dominant allele present Blue eyes (b)

Heterozygous vs. Homozygous Homozygous: describes an individual that carries two identical alleles of a gene BB or bb Heterozygous: describes an individual that carries two different alleles of a gene Bb

How to Solve Punnett Squares: Determine the genotypes of the parents. Set up your Punnett square with the genotypes of each parent on each side. Fill in the squares to represent possible combinations that can occur during fertilization. Write out the possible genotypic ratio of the offspring. Determine the phenotypic ratio of the offspring.

Sample Problem #1 A heterozygous male black eyed mouse is crossed with a red eyed female mouse.

Sample Problem #2 A heterozygous smooth pea pod plant is crossed with a homozygous recessive wrinkled pea plant.

Sample Problem #3 In humans, acondroplasia, also known as dwarfism, is dominant (D) over normal height (d). A homozygous dominant person (DD) dies before the age of one. A heterozygous person is dwarfed. A homozygous recessive person is normal. If a homozygous recessive person crosses with a heterozygous person, what is the likelihood of them having a normal child?