Introduction
Heredity is the process through which traits are passed from one generation to another. Gregor Mendel, the “Father of Genetics,” conducted experiments on pea plants and established fundamental laws of inheritance. Later, scientists extended these principles to other organisms, including guinea pigs. In this article, we will discuss Mendel’s dihybrid cross experiment in guinea pigs and his laws of inheritance.
Mendel’s Experiment on Dihybrid Cross in Guinea Pigs
A dihybrid cross is a genetic experiment that studies the inheritance of two different traits simultaneously. Mendel performed such crosses using pea plants, but similar studies were later conducted on guinea pigs to observe inheritance patterns.
Traits Considered in Guinea Pigs
For a dihybrid cross in guinea pigs, two contrasting pairs of traits are selected. For example:
- Coat Color:
- Black (dominant) – B
- White (recessive) – b
- Coat Texture:
- Rough (dominant) – R
- Smooth (recessive) – r
Mendel’s dihybrid cross experiment follows the principles of dominance, segregation, and independent assortment to determine how these traits are inherited.
Step-by-Step Explanation of the Dihybrid Cross
Step 1: Selection of Parental Generation (P Generation)
- Mendel selected purebred black, rough-coated guinea pigs (BBRR) and crossed them with purebred white, smooth-coated guinea pigs (bbrr).
Step 2: Formation of F₁ Generation
- The offspring of the parental cross (F₁ generation) were all black, rough-coated guinea pigs (BbRr), as the dominant traits (B and R) masked the recessive ones (b and r).
Step 3: Formation of F₂ Generation (Self-Crossing of F₁)
- The F₁ hybrids (BbRr) were then self-crossed (BbRr × BbRr) to observe inheritance patterns in the F₂ generation.
Step 4: Result of the F₂ Generation
Using the Punnett square, we can predict the possible genetic combinations:
| Gametes | BR | Br | bR | br |
| BR | BBRR (Black, Rough) | BBRr (Black, Rough) | BbRR (Black, Rough) | BbRr (Black, Rough) |
| Br | BBRr (Black, Rough) | BBrr (Black, Smooth) | BbRr (Black, Rough) | Bbrr (Black, Smooth) |
| bR | BbRR (Black, Rough) | BbRr (Black, Rough) | bbRR (White, Rough) | bbRr (White, Rough) |
| br | BbRr (Black, Rough) | Bbrr (Black, Smooth) | bbRr (White, Rough) | bbrr (White, Smooth) |
Step 5: Phenotypic Ratio in F₂ Generation
The phenotypic ratio obtained was 9:3:3:1, meaning:
- 9 Black, Rough-coated (BBRR, BBRr, BbRR, BbRr)
- 3 Black, Smooth-coated (BBrr, Bbrr)
- 3 White, Rough-coated (bbRR, bbRr)
- 1 White, Smooth-coated (bbrr)
This ratio confirmed Mendel’s Law of Independent Assortment, which states that the inheritance of one trait does not affect the inheritance of another.
Mendel’s Laws of Inheritance
1. Law of Dominance
- In a hybrid organism, the dominant allele is always expressed over the recessive allele.
- Example: In the F₁ generation, all guinea pigs had black coats (Bb) because black is dominant over white.
2. Law of Segregation
- During gamete formation, the two alleles for each trait separate independently, and each gamete carries only one allele from each pair.
- Example: In the F₂ generation, some guinea pigs had white coats (bb) because they received recessive alleles from both parents.
3. Law of Independent Assortment
- The inheritance of one trait is independent of the inheritance of another trait.
- Example: The coat color and coat texture genes assorted independently, producing the 9:3:3:1 ratio in the F₂ generation.
Mendel’s dihybrid cross in guinea pigs demonstrated that traits are inherited according to predictable patterns. His experiments laid the foundation for modern genetics, helping scientists understand how genes are passed from one generation to the next. The 9:3:3:1 ratio in dihybrid crosses remains a fundamental principle in genetics, illustrating how different traits assort independently.