Dihybrid Crosses Predict Independent Inheritance of Two Traits
- Gregor Mendel's experiments with pea plants demonstrated how traits are inherited independently when controlled by unlinked genes.
- But what about the inheritance of two traits simultaneously?
- These experiments reveal how the Principle of Independent Assortment results in unique phenotypic ratios in the offspring.
This is the question Gregor Mendel asked, and his experiments laid the foundation for our understanding of inheritance.
Constructing a Punnett Grid for Dihybrid Crosses
1. Determine Parental Genotypes:
- Identify the genotypes of the parent plants. In this example, both parents are heterozygous (RrYyRrYyRrYy).
2. Identify Gametes
- Each parent produces four types of gametes (RY,Ry,rY,ryRY, Ry, rY, ryRY,Ry,rY,ry), following the FOIL method:
- First (RYRYRY)
- Outer (RyRyRy)
- Inner (rYrYrY)
- Last (ryryry)
Always write gametes clearly at the top and side of the grid to avoid confusion.
3. Draw the Grid
- Construct a 4×4 grid, placing one parent's gametes across the top and the other along the side.
- Fill in each cell with the combination of alleles from the corresponding row and column.
| RY | Ry | rY | ry | |
|---|---|---|---|---|
| RY | RRYY | RRYy | RrYY | RrYy |
| Ry | RRYy | RRYy | RrYy | Rryy |
| rY | RrYY | RrYy | rrYY | rrYy |
| ry | RrYy | Rryy | rrYy | rryy |
Step 4: Determine Phenotypes
- The genotypes in the grid correspond to four phenotypes:
- Round, yellow (\( R\_Y\_ \)) — 9 combinations
- Round, green (\( R\_yy \)) — 3 combinations
- Wrinkled, yellow (\( rrY\_ \)) — 3 combinations
- Wrinkled, green (\( rryy \)) — 1 combination
