Genetic Crosses 3 Do Now 2 14 Objectives

Genetic Crosses 3. Do Now 2. 14 Objectives: 1. Define genotype, and distinguish it from phenotype. 2. 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?

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?

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…

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)

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?

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?

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

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

Genetic Crosses Yy YY Yy Y y yy x Yy 3: 1 1: 2: 1

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.