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Test Cross

Determine unknown genotypes

1
Concept
2
Setup
3
Results
4
Analysis
5
Complete

What is a Test Cross?

A test cross helps determine the unknown genotype of an organism showing the dominant phenotype by crossing it with a homozygous recessive individual.

The Problem

When you see a plant with purple flowers, you know it has at least one dominant allele, but you don't know if it's:

PPHomozygous dominant
PpHeterozygous

Both genotypes produce the same purple phenotype due to dominance.

The Solution

Cross the unknown plant with a homozygous recessive plant (pp):

If unknown is PP: PP × pp → all Pp (all purple)
If unknown is Pp: Pp × pp → 50% Pp, 50% pp (1:1 ratio)

The offspring ratio reveals the unknown genotype!