Beyond Mendel – incomplete dominance, codominance, epistasis and the chi-squared test
BiologyGeneticsAges 17–18
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Sign in to playA cross simulator for nine inheritance patterns: complete and incomplete dominance, codominance, multiple alleles (ABO blood groups), dihybrid crosses, recessive (9 : 3 : 4) and dominant (12 : 3 : 1) epistasis, complementary genes (9 : 7) and linked genes with crossing over. Punnett squares give expected ratios, random offspring give counts, and a built-in chi-squared test reports degrees of freedom, the critical value at p = 0.05 and a conclusion. A polygenic screen shows Pascal's-triangle ratios and the effect of the environment.
Lesson: Extensions of Mendelian inheritance: incomplete dominance, codominance, multiple alleles, epistasis, linkage, polygenic traits; the chi-squared test
What it shows
This simulation lets students aged 16–18 breed offspring for nine inheritance patterns and test the results statistically. Punnett squares show gametes, genotypes and phenotypes for snapdragons, roan cattle, ABO blood groups, peas, Labrador retrievers, squash, sweet peas and fruit flies. A chi-squared test compares observed counts with any expected ratio, giving degrees of freedom, the critical value at p = 0.05, the p-value and a conclusion. Model: full penetrance, equal survival of all genotypes, no mutation; polygenic genes are unlinked with equal additive effects.
How to use
Choose an Inheritance pattern and the genotypes of Parent 1 and Parent 2, then press +10, +100 or +1000 offspring. Pick a ratio under Hypothesis to test and read the table and conclusion. For linked genes, set the Recombination frequency. On the Polygenic traits tab, set Number of genes n and Environmental effect σ, then breed offspring and compare the histogram with the expected curve.
Parameters you can change
- Starting screen Crosses and χ², Polygenic traits
- Inheritance pattern Complete dominance (3 : 1) – pea seed colour, Incomplete dominance (1 : 2 : 1) – snapdragon, Codominance (1 : 2 : 1) – roan cattle, Multiple alleles, codominance – ABO blood groups, Dihybrid cross (9 : 3 : 3 : 1) – pea seeds, Recessive epistasis (9 : 3 : 4) – Labrador coat, Dominant epistasis (12 : 3 : 1) – squash colour, Complementary genes (9 : 7) – sweet pea flowers, Linked genes and crossing over – fruit fly
- Recombination frequency (linked genes) 0–50 %
- Number of genes (Polygenic traits screen) 1–6 genes
- Environmental effect σ (Polygenic traits screen) 0–1.5 alleles
Questions to explore
- Why does a BbEe × BbEe cross in Labradors give a 9 : 3 : 4 ratio instead of 9 : 3 : 3 : 1?
- How does the chi-squared value change when you breed 1000 offspring instead of 100 from a linked-gene test cross?
- Why does adding an environmental effect turn several polygenic classes into one bell-shaped distribution?
