A dihybrid cross shows a 9:3:3:1 ratio. Explain its significance.
The 9:3:3:1 ratio in a dihybrid cross shows that two different traits are inherited independently of each other.
Key Idea
| Term | Meaning |
|---|---|
| Dihybrid Cross | Cross involving two traits |
| Independent Assortment | Genes of different traits assort separately |
Example Cross
| Parents | Genotype |
|---|---|
| Both | RrYy × RrYy |
Traits example:
R = Round, r = Wrinkled
Y = Yellow, y = Green
Resulting Ratio
| Phenotype | Ratio |
|---|---|
| Round Yellow | 9 |
| Round Green | 3 |
| Wrinkled Yellow | 3 |
| Wrinkled Green | 1 |
Stepwise Understanding
Two traits are considered together
Each parent produces four types of gametes (RY, Ry, rY, ry)
Gametes combine randomly
Different combinations form
Phenotypes appear in 9:3:3:1 ratio
Significance
| Point | Explanation |
|---|---|
| Independent Inheritance | Traits do not affect each other |
| Genetic Variation | More combinations possible |
| Mendel’s Law | Supports law of independent assortment |
Real Insight
This ratio is seen only when genes are not linked, if genes are on same chromosome, ratio may change, which is something students often miss.
So 9:3:3:1 ratio proves that two traits are inherited independently and not together.