# Test Cross Analyzer (determine homozygous vs heterozygous)

Determine genotype (AA vs Aa) by test cross with aa. Any recessive offspring proves heterozygous; zero recessive suggests homozygous with P = 0.5^n. Derived from Mendel 1866, Griffiths, Hartl & Clark 2007, StatPearls. Educational use only.

> Canonical page: https://elysiatools.com/en/tools/test-cross-analyzer

- **Category:** Education

- **Keywords:** test cross, test cross analyzer, backcross, homozygous, heterozygous, genotype determination, mendelian genetics, dominant phenotype, genetics, biology

## Overview

Test Cross Analyzer determines whether a dominant-phenotype parent is likely homozygous (AA) or heterozygous (Aa) from offspring counts in a cross with aa, using a selected significance level and decimal precision.

## Inputs

- **Dominant phenotype offspring count** (number): Number of offspring showing the dominant phenotype.
- **Recessive phenotype offspring count** (number): Number of offspring showing the recessive phenotype.
- **Significance Level** (select): Significance level for the heterozygosity test.
- **Decimal Places** (number)

## When to use

- Determine whether a dominant-phenotype organism is AA or Aa using test-cross offspring data.
- Confirm heterozygosity when at least one recessive-phenotype offspring is observed.
- Evaluate how strongly zero recessive offspring support a likely homozygous result.

## How it works

- Enter the numbers of dominant- and recessive-phenotype offspring.
- The analyzer treats any recessive offspring as evidence that the tested parent is heterozygous (Aa).
- When no recessive offspring are observed, it calculates P(0 recessive) = 0.5^n under the heterozygous hypothesis and compares it with the selected significance level.
- The JSON result reports the total offspring, observed ratio, conclusion, genotype, and relevant probability or confidence values.

## Use cases

- Teaching Mendelian genetics and test-cross logic in biology classes.
- Checking genotype-determination exercises with observed dominant and recessive offspring counts.
- Exploring how sample size changes confidence in a likely homozygous conclusion.

## Frequently asked questions

### What cross does this analyzer assume?

It assumes a test cross between a dominant-phenotype parent with an unknown genotype and a recessive homozygote, aa.

### What does a recessive offspring prove?

A recessive offspring shows that the tested parent contributed a recessive allele, so the parent is heterozygous (Aa).

### What does zero recessive offspring mean?

Zero recessive offspring suggests the parent is likely homozygous (AA), with strength depending on the total offspring count and selected significance level.

### Which significance levels are available?

You can select 0.05, 0.01, or 0.001.

### Is this tool intended for clinical or production decisions?

No. The analyzer is intended for educational use only.

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