Biology · Introductory biology · Concept
Punnett squares and inheritance probability
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.
Two genes at once
Genes on different chromosomes assort independently, so a cross involving two genes can be solved one gene at a time and the probabilities multiplied. Two double heterozygotes, RrYy × RrYy, give their four phenotypes in a 9:3:3:1 ratio. Genes close together on one chromosome are linked and do not follow this rule.
Reading a pedigree
A pedigree charts a trait through a family. Two unaffected parents with an affected child point to a recessive allele, and an affected daughter of an unaffected father rules out X-linked recessive inheritance. A probability for an unaffected relative counts only the outcomes that are still possible.
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.
Key terms
- Allele
- One version of a gene. A diploid organism usually carries two copies of each gene, one from each parent, and the two can be different alleles without looking different.
- Genotype
- The alleles an organism carries for the gene or genes being studied, such as Aa. It differs from the phenotype, the trait you can observe.
- Phenotype
- A trait you can observe or measure, such as flower color or blood type. It comes from genotype and environment, and one phenotype can come from different genotypes, such as AA and Aa.
- Allele segregation
- The separation of a gene’s two alleles into different gametes during meiosis. A heterozygote (Aa) passes A to half its gametes and a to the other half.
- Independent assortment
- Alleles of different genes go into gametes independently of each other, because each chromosome pair lines up on its own in meiosis. Genes close together on one chromosome usually don’t assort independently.
- Dihybrid cross
- A cross between two parents that are both heterozygous for two genes, such as RrYy × RrYy. With independent assortment and complete dominance, the offspring phenotypes come in a 9:3:3:1 ratio.
- Test cross
- Crossing an individual with the dominant phenotype to a homozygous recessive one. The recessive parent passes on only recessive alleles, so the offspring’s phenotypes reveal the tested parent’s genotype.
- Pedigree
- A family-tree diagram showing who has a trait across generations. It narrows down how the trait is inherited, but it can’t reveal unknown genotypes without assumptions.
- Genetic carrier
- Someone with one copy of the allele for a recessive condition who does not have the condition, such as Aa. A carrier can pass the allele to their children.
Work through an example
Under complete dominance, what genotype and phenotype probabilities follow from two heterozygous parents?
Punnett square for an Aa × Aa cross →Sources and scope
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