Do Now: In rats, black fur color is dominant to white fur color. If a black rat has one

copy of the black fur allele (B) and one copy of the white fur allele (b), what % of the

gametes it produces will carry the white fur allele?

Phenotype is the characteristic an organism has because of the alleles it has.◦ Examples: brown fur, white flowers, dwarfism, blue

eyes, etc…

Genotype is the different alleles an organism has, represented by letters◦ Examples: Bb, pp, Dd, bb, etc…

What is the phenotype of the rat in the do now?

What is it’s genotype?

The genotype of an organism can be heterozygous or homozygous

Heterozygous individuals have two different alleles for a gene – like the rat in the do now. (Hetero = different)◦ Example: Bb = one brown eye allele, one blue

eye allele

Homozygous individuals have two of the same allele. (Homo = same)◦ Example: BB, bb, PP, pp, etc…

Since a homozygous individual has two identical alleles, their phenotype will be whatever those alleles are◦ Genotype = BB; Phenotype = brown eyes◦ Genotype = bb; Phenotype = blue eyes

A heterozygous individual will have the dominant trait.

A dominant allele is on that masks or hides the presence of a recessive one.◦ Genotype = Bb; Phenotype = brown eyes◦ Brown eye allele (B) is dominant to blue eye allele (b)

In cows, having spots (S) is dominant to no spots (s)◦ What is the phenotype of a cow that has spots?◦ What is the genotype of a heterozygous cow?◦ Will a heterozygous cow have spots?◦ What is the genotype of a cow with no spots?◦ If a cow has spots, what 2 genotypes might it

have?

In pea plants, yellow seed color (Y) is dominant to green seed color (y).◦ What is the genotype of a plant with green seeds?◦ What is the phenotype of a Yy plant?◦ Is a YY plant homozygous or heterozygous?◦ Is a yy plant homozygous or heterozygous?◦ If a YY plant self-fertilizes, what color will the

seeds of its offspring be?

A diagram that helps predict the outcome of a genetic cross.

The offspring produced by a genetic cross are described in two ways:◦ Phenotype ratio: the ratio of one phenotype to the other.◦ Example: 2 black rats, 2 white rats = 1:1 phenotype ratio

◦ Genotype ratio: the ratio of all of the different genotypes of the offspring

◦ Example: 1 BB, 2 Bb, 1 bb = 1:2:1 genotype ratio

Genetic Crosses

Yy x Y yYy Y yYy Y y

Y Y

y y

Y

Y

y

y

3:1

1:2:1

The example problems on your worksheet will let you know if you’ve picked up the skills you need to be a future geneticist!

Work on them now, so that I can help before you go.

DON’T FORGET THE MITOSIS LAB!!!