Use an Inheritance Model Without Overclaiming

Connect allele combinations with probabilities in a simplified dominant-recessive model.

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In this model, each parent contributes one allele independently. A is completely dominant to a, so AA and Aa share the modeled dominant phenotype. Probability predicts proportions over many outcomes, not an exact sequence in a small family.

Worked example

For Aa x aa, possible offspring genotypes are Aa and aa with equal probability. Two offspring are not guaranteed to include exactly one of each.

Model Assumptions

Consider a fictional plant characteristic with alleles A and a, complete dominance of A, independent allele transmission, and no survival differences among genotypes. Cross two heterozygous parents, Aa x Aa.

1. For Aa x Aa, what is the probability of aa? 1/2 1/4 3/4

2. What fraction is expected to show the dominant phenotype in this model? 1/4 1/2 3/4

3. Must four offspring include exactly one aa individual? No, random outcomes can differ from expected proportions Yes, every group of four follows a fixed order Yes, inheritance counts cannot vary

4. Does dominant mean an allele is necessarily common in a population? Yes, dominant always means most common No, dominance does not determine frequency Yes, recessive alleles cannot persist

5. Can this single-gene model describe every human characteristic? Yes, every trait is a single A/a pair Yes, environment never matters No, many traits involve multiple genes and environmental influences

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