Biology · Introductory biology · Concept
Inheritance probabilities
Connect parental alleles, possible gametes and offspring probabilities without confusing genotype with phenotype.
Start with the model and its assumptions
A Punnett square follows allele copies from parents to offspring. Here we use one autosomal locus in a diploid organism, ordinary segregation, equal gamete probabilities and independent contributions from the two parents. These assumptions make the simple square appropriate; not every trait follows this model.
Distinguish an allele, a genotype and a phenotype
A and a are two alleles at the same locus. AA, Aa and aa describe genotypes—the allele pairs an offspring carries. A phenotype is the trait category expressed under the model. The genotype-to-phenotype relationship must be specified separately.
What does dominant actually mean?
With complete dominance, a heterozygote has the same phenotype category as the homozygote carrying the dominant allele. Dominant does not mean more common, stronger, healthier or more likely to be inherited. Each allele from an Aa parent still has probability ½.
List the gametes before filling the square
An Aa parent contributes A or a, each with probability ½. Write one parent’s gametes along the top and the other’s down the side. Each cell combines one contribution from each parent.
| Parent 1 ↓ / Parent 2 → | A (½) | a (½) |
|---|---|---|
| A (½) | AA (¼) | Aa (¼) |
| a (½) | Aa (¼) | aa (¼) |
Multiply within a cell, then add equivalent outcomes
A particular pair of independent gametes has probability ½ × ½ = ¼. Two cells produce the heterozygous genotype, because receiving A from the first parent and a from the second has the same genotype as receiving them the other way around.
Apply the phenotype rule after finding genotypes
Under complete dominance, AA and Aa share the dominant phenotype, so their probabilities add to ¾. The aa genotype has probability ¼ and gives the recessive phenotype. Incomplete dominance or codominance changes this grouping, not the allele probabilities in this same cross.
Interpret probability as a prediction over many outcomes
A ¼ probability of aa does not promise exactly one aa offspring in every set of four. Small samples vary by chance. When you compare observations with expectations, keep the expected probabilities separate from the actual counts you collected.
Common mistakes
- Treating dominant as more common or more beneficial.
- Counting Aa and aA as different genotypes.
- Applying an autosomal single-locus model to every trait.
Work through an example
Under complete dominance, what genotype and phenotype probabilities follow from two heterozygous parents?
Interpret an Aa × Aa cross →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.