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Inherited Traits

Middle schoolBiologyExact single-locus Mendelian cross probabilities with reproducible independent offspring sampling

Cross the parents to generate actual seeded offspring. Click a Punnett square to inspect its allele pairing, then compare observed counts with the expected probabilities.

AaAa

Each square has probability 1/4, including repeated allele pairings. Each dot is one generated offspring.

Measurement notebook

Change the setup, test a prediction and save the result. Up to 12 snapshots are kept.

About this experiment

Choose two parent genotypes, inspect every allele pairing and generate an actual finite family. Compare the expected genotype distribution with observed seeded offspring rather than treating a probability as a guaranteed family count.

Learning goals

  • Calculate genotype probabilities from parental allele segregation.
  • Distinguish genotype from the complete-dominance phenotype rule.
  • Explain finite-sample variation while keeping the cross probabilities fixed.

How to use it

  1. 1Cross two heterozygous parents and inspect all four Punnett pairings.
  2. 2Generate several batches and compare observed counts with one-to-two-to-one expectations.
  3. 3Cross opposite homozygotes and verify all offspring are heterozygous.
  4. 4Change the offspring seed and compare another family from the same parental cross.

The model

Both parents must contribute an A allele.
Either parent can supply the A allele.
The recessive phenotype requires two a alleles under complete dominance.

Assumptions

  • One autosomal locus has two alleles A and a. Each offspring receives one independent random allele copy from each parent.
  • A shows complete dominance for the displayed phenotype: AA and Aa share the dominant class and aa is recessive.
  • Each of the four chromosomal pairings has probability one quarter, including repeated allele symbols in homozygous parents.
  • A fixed seed produces a deterministic prefix of an independent-offspring sequence. Changing parental genotypes or seed clears the current family; the display is limited to 200 offspring.
  • This is not a prediction for an arbitrary real trait. Polygenic inheritance, penetrance, environment, sex linkage and viability selection are omitted.

Try this in class

  1. Q1Why does a family of four not necessarily contain exactly one AA, two Aa and one aa?
  2. Q2Why do repeated Punnett squares still carry separate probability weight?

Frequently asked questions

Are the generated offspring just the four Punnett squares repeated?

No. Each offspring is an independent seeded draw from parental gametes. The Punnett grid gives probabilities.

Does dominance mean an allele is more common?

No. Dominance describes the heterozygote phenotype rule, not population frequency or fitness.