Understanding Punnett Squares
A Punnett Square is a diagram used by biologists to determine the probability of an offspring having a particular genotype. The square is a graphical representation of all possible combinations of maternal and paternal alleles that can occur during fertilization for a specific trait.
Key Terminology
Before using the calculator, it helps to understand the terms used in Mendelian genetics:
- Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
- Genotype: The genetic constitution of an individual organism (e.g., "Aa" or "aa").
- Phenotype: The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment (e.g., "Brown eyes").
- Dominant: An allele that produces the same phenotype whether its paired allele is identical or different (represented by Uppercase letters, e.g., "A").
- Recessive: An allele that produces its characteristic phenotype only when its paired allele is identical (represented by lowercase letters, e.g., "a").
How to Use the Calculator
- Enter the genotype of Parent 1 (e.g., "Aa"). Ensure you use exactly two letters.
- Enter the genotype of Parent 2 (e.g., "Aa").
- The calculator will automatically generate the 2x2 square and calculate the ratios.
Common Monohybrid Crosses
Monohybrid crosses involve a single trait. Here are the three standard scenarios:
1. Heterozygous x Heterozygous (Aa x Aa)
This is often called a "Monohybrid Cross." Both parents carry one dominant and one recessive allele.
- Genotypes: 25% AA, 50% Aa, 25% aa (1:2:1 ratio).
- Phenotypes: 75% Dominant trait, 25% Recessive trait (3:1 ratio).
2. Homozygous Dominant x Homozygous Recessive (AA x aa)
One parent is purebred for the dominant trait, and the other is purebred for the recessive trait.
- Genotypes: 100% Aa.
- Phenotypes: 100% Dominant trait.
- Note: This is a "Test Cross" if you are testing if the AA parent is truly AA.
3. Heterozygous x Homozygous Recessive (Aa x aa)
Used to determine if an individual showing the dominant trait is heterozygous or homozygous.
- Genotypes: 50% Aa, 50% aa.
- Phenotypes: 50% Dominant, 50% Recessive (1:1 ratio).
Frequently Asked Questions (FAQ)
What if the trait is Co-Dominant?
This calculator follows standard Mendelian inheritance (Complete Dominance). In co-dominance (like blood types AB), neither allele is recessive. The phenotype is a mix (e.g., AB blood). The square is calculated the same way, but the phenotype interpretation differs.
Can I calculate multiple traits (Dihybrid)?
This tool is designed for Monohybrid (single trait) crosses. For two traits (e.g., AaBb x AaBb), you would need a 4x4 Punnett Square (16 combinations).
Why does the order of letters matter?
Biologically, the order usually doesn't matter (Aa is the same as aA). However, in genetics notation, we typically write the dominant (capital) letter first for consistency. This calculator automatically sorts the alleles.