Use this Mendel's law of segregation simulator to see how alleles separate into gametes, recombine during fertilisation, and produce F1 or F2 offspring ratios. The tool helps students, teachers, and genetics learners connect meiosis with Punnett square probability.
Change parent genotypes, switch inheritance display, and move the offspring slider. The gamete diagram, Punnett square, probabilities, and simulated counts update instantly.
Start with a classic monohybrid cross, then adjust the parents and sample size to see sampling variation.
F₂ segregation cross
Shows the classic 1:2:1 genotype ratio and 3:1 phenotype ratio from two heterozygous F₁ parents.
Select the diploid genotype before meiosis separates the two alleles into gametes.
Choose the second parent, then compare the gamete contribution from each side of the cross.
Live segregation result
The genotype ratio is 1:2:1. The phenotype ratio is 3:1 under the selected inheritance display.
Aa gametes
50:50
Genotype
1:2:1
Sample
160
Each row and column contains one allele after meiosis. Fertilisation restores the diploid genotype.
AA
Dominant phenotype
Aa
Dominant phenotype
Aa
Dominant phenotype
aa
Recessive phenotype
Heterozygotes produce A and a gametes equally. Homozygotes produce only one allele type.
The expected column uses exact probability. The simulated column shows one reproducible random sample of the selected size.
| Genotype | Phenotype | Probability | Expected count | Simulated count |
|---|---|---|---|---|
| AA | Dominant phenotype | 25.0% | 40.0 | 34 |
| Aa | Dominant phenotype | 50.0% | 80.0 | 83 |
| aa | Recessive phenotype | 25.0% | 40.0 | 43 |
Mendel's law of segregation states that a diploid organism carries two alleles for a gene but passes only one allele into each gamete. Gregor Mendel inferred this rule from pea plant crosses in the 1860s, before scientists knew the structure of chromosomes. His monohybrid crosses showed that recessive traits can disappear in F1 offspring and return in F2 offspring.
Meiosis explains the mechanism. Homologous chromosomes separate during meiosis I, so an Aa parent produces A-bearing and a-bearing gametes. Random fertilisation then restores two alleles in each zygote.
OpenStax describes the same physical basis: allele segregation follows chromosome separation in the first meiotic division. Read the OpenStax inheritance section.
Select Aa × Aa, AA × aa, Aa × aa, or a visible heterozygote cross from the preset buttons.
Pick AA, Aa, or aa for each parent to control which alleles enter the gamete pool.
Move the sample-size slider to compare small-family variation with large-sample Mendelian expectations.
Compare the Punnett square, expected probabilities, and simulated counts to see segregation in action.
These buttons load common crosses such as Aa × Aa, AA × aa, and Aa × aa. They help you compare F1, F2, and test-cross patterns without typing values.
Each card shows the two alleles carried by one parent. The selector controls whether that parent can make A gametes, a gametes, or both.
Complete dominance groups AA and Aa into one phenotype. The visible heterozygote option keeps AA, Aa, and aa as three separate phenotype classes.
The slider changes the number of simulated offspring. It shows why small samples drift from ratios such as 3:1, while larger samples approach expected probabilities.
The diagram turns each genotype into gametes. A heterozygote creates A and a gametes in a 50:50 split, which directly demonstrates segregation.
The tables compare exact expectations with simulated counts. They make the difference between a probability model and one random sample visible.
A true-breeding AA × aa cross produces F1 offspring with genotype Aa. Every offspring receives A from one parent and a from the other parent. Complete dominance makes every F1individual show the dominant phenotype.
An Aa × Aa F2 cross produces a 1:2:1 genotype ratio. The four genotype boxes are AA, Aa, Aa, and aa. Complete dominance collapses AA and Aa into the same visible class, so the phenotype ratio becomes 3:1.
A test cross changes the pattern. Aa × aa produces Aa and aa offspring in a 1:1 ratio. Breeders use that logic to infer whether a dominant-looking parent carries a recessive allele.
| Cross | Gametes | Genotype ratio | Complete-dominance phenotype ratio |
|---|---|---|---|
| AA × aa | A only and a only | 0:1:0 | 1:0 |
| Aa × Aa | A or a from both parents | 1:2:1 | 3:1 |
| Aa × aa | A or a from parent 1, a from parent 2 | 0:1:1 | 1:1 |
Each parent makes A gametes 50% of the time and a gametes 50% of the time. The expected genotype counts from 160 offspring are 40 AA, 80 Aa, and 40 aa. Complete dominance predicts 120 dominant phenotype offspring and 40 recessive phenotype offspring.
The heterozygous parent contributes A to half of its gametes and a to half. The recessive parent contributes only a. The expected offspring counts are 60 Aa and 60 aa, so the complete-dominance phenotype ratio equals 1:1.
A Punnett square reports probability. It does not promise that every four offspring will include exactly one AA, two Aa, and one aa. Random fertilisation can produce short runs of the same genotype, just as coin flips can produce several heads in a row.
Sample size controls how noisy the result looks. In small families, chance can hide the expected 3:1 ratio. In large seed counts, the observed proportion usually moves closer to 75% dominant and 25% recessive.
Nature Education describes Mendel's pea work as a foundation for discrete inheritance and later genetic principles. Read the Nature Education overview.
Genetics courses use segregation to connect meiosis, gametes, Punnett squares, and probability. The same idea supports test crosses in plant and animal breeding. A recessive phenotype can reveal whether a dominant-looking parent carries a hidden allele.
Segregation also prepares students for advanced topics. Linkage, epistasis, penetrance, and selection all make more sense after the basic monohybrid model feels clear. Each advanced model changes one assumption in the simple Aa × Aa cross.
This simulator models one gene with two alleles. It does not estimate linkage, epistasis, environmental influence, penetrance, or polygenic inheritance. Those topics require extra genetic assumptions and different calculator logic.
The simulated counts show one reproducible random sample. Real laboratory data need careful phenotype scoring and, often, a chi-square test. Use this page for education and planning, not for medical diagnosis or formal breeding certification.
Use these tools to move from allele segregation to Punnett square prediction and ratio testing.