Dihybrid Crosses

The instructions or blueprints for making proteins are found in DNA The instructions or blueprints for making proteins are found in DNA. The information is carried in the sequence of the nitrogen bases. The sequences of DNA that help to create proteins are known as a genes.

The different versions of a gene are called alleles The different versions of a gene are called alleles. You would find the eye color gene in the same place for everyone, but some people have the allele for green eyes and some people have the allele for blue eyes.

Phenotype for Fur Color They are represented by a lowercase letter. Dominant alleles are always expressed. They are represented by a capital letter. Subject A Subject B Subject C Genotype for Fur Color GG Gg gg Phenotype for Fur Color Gray Fur White Fur Recessive alleles are only expressed in homozygous recessive organisms. They are represented by a lowercase letter.

gg The combination of alleles is known as the organism’s genotype. (e.g. heterozygous or homozygous dominant) gg Subject A Subject B Subject C Genotype for Fur Color GG Gg gg Phenotype for Fur Color Gray Fur White Fur The physical appearance caused by the expression of the alleles is the phenotype. (e.g. having gray fur or white fur)

Punnett Squares: Dihybrid Cross GGBb (male) × GgBb (female) A dihybrid cross involves making predictions about two traits. To do this, we will make a Punnett square for each trait. G G B b G g B b G G G G B B B b G g G g B b b b GGBb (male) × GgBb (female)

Punnett Squares: Dihybrid Cross GgBb (male) × ggBB (female) The most important rule is that there should be one box for each trait. Notice that one square is for fur pattern and the other is for eye color. These represent 2 traits of the same offspring. G g B b g G g g g B B B b B g g B b GgBb (male) × ggBB (female)

Solid Gray Fur What trait will appear in ALL of the offspring? Only 75% of the offspring will have green eyes. G G B b G g Solid Gray Fur Solid Gray Fur B b Green eyes Green eyes G G G G B B B b Solid Gray Fur Solid Gray Fur Green eyes Blue eyes G g G g B b b b GGBb (male) × GgBb (female)

6 different genotype combinations How many combinations of genotypes will appear in the offspring? We MULTIPLY the combinations from each square! 6 different genotype combinations 2 genotypes (GG, Gg) 3 genotypes (BB, Bb, bb) G G B b G g B b G G G G B B B b G g G g B b b b GGBb (male) × GgBb (female)

4 combinations of phenotypes How many combinations of phenotypes will appear in the offspring? We MULTIPLY the combinations from each square! 4 combinations of phenotypes 2 phenotypes 2 phenotypes G g B b G g Solid Gray Fur Solid Gray Fur B b Green eyes Green eyes G G G g B B B b Solid Gray Fur Striped Gray Fur Green eyes Blue eyes G g g g B b b b GgBb (male) × GgBb (female)

What fraction of the offspring will have striped fur and green eyes? ½ x ¾ = ⅜ fraction with striped fur: ½ fraction with green eyes: ¾ G g B b g B b G g g g B B B b G g g g B b b b GgBb (male) × GgBb (female)

What percentage of offspring will have solid gray fur and blue eyes? 50% x 50% = 25% Percent with solid gray fur: Percent with blue eyes: 50% 50% G g B b g b G g g g B b b b G g g g B b b b GgBb (male) × GgBb (female)