1. Genetic Crosses Do Now 2.14 Objectives:
Define genotype, and distinguish it from phenotype.
Define homozygous and heterozygous.
Use a Punnett square to predict the outcome of a genetic cross for an autosomal dominant trait.
Task: 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?

2. Phenotype & Genotype
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?

3. Heterozygous & Homozygous
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…

4. Dominant & Recessive alleles: The relationship of Genotype & Phenotype
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 one 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)

5. Practice 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?

6. More Practice
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?

7. Punnett Square
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

8. Monohybrid Cross Step 1: draw a 2 x 2 square
Step 2: put the possible alleles from parent number one on top of the two squares
Step 3: put the possible alleles from parent number 2 on the left
Step 4: Write the alleles at top down the column, and the alleles at left across the row
Step 5: Check

9. Genetic Crosses Y Y Y y y y x Y Y Y y y y Y Y Y y Y y y y
3:1
1:2:1

10. Practice
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.