1. Unit 8: Mendelian Genetics
8.3 Dihybrid Crosses and Independent Assortment

2. Independent Assortment
After his first round of experiments, Mendel wondered if the segregation of one pair of alleles affects another pair.
For example: does the alleles for seed color affect the alleles for seed shape?
To find out-Mendel followed two different traits as they passed from one generation to the next.
In the F1 generation, he crossed plants that produced round yellow peas (RRYY) with plants the produced wrinkled green peas (rryy). This generation produced the hybrids (RrYy) that he needed for the next generation.
Then he crossed the F1 generation to produce an F2 generation. The results were:
556 seeds total
315 round and yellow
32 wrinkled and green
209 combination of phenotypes not present in the original parent plants
This type of cross is called a dihybrid cross
Independent Assortment

3. Independent Assortment
Genes for different traits can segregate independently during the formation of gametes.
Example: your hair color does not determine your height.
Independent Assortment

4. Dihybrid Crosses
In a dihybrid cross, you are determining the genotype and phenotype for two parents that each have 2 traits.

5. Using a Punnett Square to Determine Offspring **See Page 10 in Notes**
1. Determine the parental genotypes.
The traits will follow the same dominant and recessive guidelines used in monohybrid crosses.
Ex: Heterozygous means a capital and lowercase letter, etc.
2. Once you determine the parental genotypes, you are going to “FOIL” the traits to determine what goes on the outsides of the Punnett square.
3. Draw a 4X4 Punnett square. Mom’s traits go along the top, dad’s along the left side.
4. Use the squares to determine the genotypes and phenotypes of the offspring.
Using a Punnett Square to Determine Offspring **See Page 10 in Notes**

6. Example
A heterozygous female mouse for black, short hair is crossed with a homozygous recessive male with white fur and long hair.
1. What are the genotypes of Mom and Dad?
2. What are the possible gametes after the “FOIL”?
3. What are the genotypic ratios for the offspring?
4. What are the phenotypic ratios for the offspring?

7. A Summary of Mendel’s Principles
The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents to offspring.
Where two or more forms (alleles) of the gene for a single trait exist, some alleles may be dominant and others may be recessive.
In most sexually reproducing organisms, each adult has two copies of each gene-one from each parent. These genes segregate from each other when gametes are formed.
Alleles for different genes usually segregate independently from each other when gametes are formed.
A Summary of Mendel’s Principles