Biology
Punnett Squares
A Punnett square predicts offspring genotypes and phenotypes from parental crosses. Each parent's gametes label the rows and columns; each cell shows a possible offspring combination.
How to Approach Punnett Squares
Identify parent genotypes
Each parent has two alleles per trait (e.g., Aa). Write each parent's genotype clearly.
List each parent's gametes
Split each parent into the possible gametes they produce. An Aa parent makes 'A' and 'a' gametes.
Fill in the square
Combine each row gamete with each column gamete in every cell. Count phenotype ratios across all cells.
Frequently Asked Questions
What's the difference between genotype and phenotype?+
Genotype is the genetic code (Aa, AA, aa). Phenotype is what shows physically (dominant trait, recessive trait). AA and Aa often share the same phenotype because A is dominant.
How do I handle dihybrid crosses?+
Each parent contributes 4 gamete types (AB, Ab, aB, ab for AaBb). The square becomes 4x4 with 16 cells. Phenotype ratio is classically 9:3:3:1.
What about sex-linked traits?+
Traits on the X chromosome appear in different ratios in males vs. females. Set up the square using X and Y for sex, with allele superscripts (e.g., XᴬXᵃ × XᴬY).
Related Topics
More step-by-step guides in Biology and adjacent subjects.
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