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:
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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?