AP Syllabus focus:
‘Some traits show phenotypic ratios that differ from Mendel’s predictions, revealed through quantitative analysis of offspring.’
Mendelian genetics provides clear expected ratios for simple traits, but real inheritance data often deviate. AP Biology emphasises recognising these deviations and using quantitative comparisons of observed versus expected offspring outcomes.
Mendelian ratios and what they assume
Mendel’s classic ratios (for example, 3:1 phenotypes in a monohybrid cross) arise only when a trait fits a simplified genetic model.
Core assumptions behind Mendelian predictions
A single gene controls the trait.
There are two alleles in the cross.
Genotypes map cleanly to phenotypes (often taught as complete dominance).
Each offspring outcome is equally likely given parental gametes.
Practice Questions
FAQ
Use the phenotypes you can score reliably and consistently.
If phenotypes are ambiguous, combine categories only if justified by the genetic hypothesis.
Very small expected counts per category can break assumptions.
Non-independent observations (e.g., repeated measures of the same individual) also undermine the test.
Yes. The same observed ratio can be produced by different mechanisms.
Additional crosses or molecular evidence are often needed to distinguish causes.
Scoring error and misclassification of phenotypes can distort observed counts.
Unequal survival after birth but before counting can also bias the dataset.
They often measure trait values across many offspring and analyse patterns of variation.
Approaches include comparing distributions, estimating heritability, or mapping contributing loci with appropriate designs.
