Mendelian ratios
MCAT glossary · MCAT
Definition
Monohybrid cross 3:1 phenotypic, 1:2:1 genotypic; dihybrid 9:3:3:1; test cross reveals heterozygote vs homozygote.
Mendelian ratios in plain English
When two parents each carry two versions of a gene, their kids inherit predictable mixes — and those mixes show up as clean numeric ratios, which is what lets biologists reason about heredity. If you cross two parents that each carry one 'strong' (dominant) and one 'hidden' (recessive) version of a single trait, about 3 out of 4 offspring show the dominant look (a 3:1 ratio of appearances), while the underlying gene combos split 1:2:1. Track two traits at once and you get the classic 9:3:3:1 pattern. Think of it like flipping two coins per parent: certain combinations are just more likely, so the counts fall into these repeatable ratios. A 'test cross' is a detective trick — breed a mystery dominant individual with a purely recessive one, and the babies reveal whether the mystery parent secretly carried a hidden recessive version.
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