Mendelian Genetics Ratios
1. MONOHYBRID CROSS (One Trait)
Law Involved:
Cross Example:
| Type | Ratio | Explanation |
|---|
| Phenotypic | 3 : 1 | Dominant : Recessive |
| Genotypic | 1 : 2 : 1 | AA : Aa : aa |
Key Point
- Alleles separate during gamete formation
2. DIHYBRID CROSS (Two Traits)
Law Involved:
- Law of Independent Assortment
Cross Example:
| Type | Ratio | Explanation |
|---|
| Phenotypic | 9 : 3 : 3 : 1 | Four trait combinations |
| Genotypic | 1:2:1:2:4:2:1:2:1 | Combination of alleles |
Key Point
- Traits assort independently
3. TEST CROSS
Definition:
- Cross between heterozygous × homozygous recessive
Cross Example:
| Type | Ratio |
|---|
| Phenotypic | 1 : 1 |
| Genotypic | 1 : 1 |
Key Point
- Used to identify unknown genotype
4. INCOMPLETE DOMINANCE
Example:
| Type | Ratio |
|---|
| Phenotypic | 1 : 2 : 1 |
| Genotypic | 1 : 2 : 1 |
Key Point
5. CODOMINANCE
Example:
| Type | Ratio |
|---|
| Phenotypic | 1 : 2 : 1 |
Key Point
- Both alleles expressed equally
SUPER SUMMARY TABLE
| Cross Type | Phenotypic Ratio | Genotypic Ratio | Law |
|---|
| Monohybrid | 3:1 | 1:2:1 | Segregation |
| Dihybrid | 9:3:3:1 | 1:2:1:2:4:2:1:2:1 | Independent Assortment |
| Test Cross | 1:1 | 1:1 | Segregation |
| Incomplete Dominance | 1:2:1 | 1:2:1 | Modified dominance |
| Codominance | 1:2:1 | 1:2:1 | Equal expression |