Punnett Square (Grid)
Phenotype Distribution
Genotype & Phenotype Statistics
| Phenotype | Genotype | Count (/16) | Probability (%) | Ratio |
|---|
Dihybrid Cross Punnett Square
Understanding the Dihybrid Cross
A dihybrid cross is a breeding experiment between two organisms that are identically hybrid for two traits. This type of genetic cross allows biologists to determine the genotypic and phenotypic ratios of offspring for two separate characteristics. The concept was famously demonstrated by Gregor Mendel using pea plants, observing traits such as seed color (Yellow vs. Green) and seed shape (Round vs. Wrinkled).
Mendel's Law of Independent Assortment
The calculation of a dihybrid cross relies on Mendel's Law of Independent Assortment. This law states that the alleles for two different traits segregate independently of one another during the formation of gametes. Consequently, the inheritance of one trait does not influence the inheritance of the other.
For a standard dihybrid cross where both parents are heterozygous for both traits (e.g., AaBb x AaBb), the expected phenotypic ratio in the offspring is typically 9:3:3:1:
- 9: Dominant for both traits
- 3: Dominant for first trait, Recessive for second
- 3: Recessive for first trait, Dominant for second
- 1: Recessive for both traits
How to Use the Punnett Square
A Punnett Square for a dihybrid cross consists of a 4x4 grid, resulting in 16 possible combinations. To determine the probability of a specific genotype, we consider the total number of squares containing that genotype divided by the total number of squares.
The probability formula for a specific outcome is:
\(P = \frac{\text{Number of favorable outcomes}}{16}\)
To calculate the combined probability of two independent events occurring together (e.g., getting a Yellow seed AND a Round seed), you can also multiply their individual probabilities:
\(P(A \text{ and } B) = P(A) \times P(B)\)
Calculating Gametes
Before filling the square, you must determine the gametes produced by each parent. This is done using the FOIL method (First, Outer, Inner, Last). For a parent with genotype AaBb, the gametes are:
- First: AB
- Outer: Ab
- Inner: aB
- Last: ab
These four gametes are placed along the top and side of the 4x4 grid to generate the potential offspring genotypes.