Mendel and his peas
Gregor Mendel chose garden peas because they have clear either/or traits (purple or white flowers, round or wrinkled seeds), grow quickly and can be cross-pollinated by hand. He started with pure-breeding plants, crossed them, and counted the traits of the offspring over several generations. When he crossed purple-flowered and white-flowered plants, all the offspring were purple; but when those offspring were crossed together, white flowers reappeared in about one plant in four. He concluded that traits are passed on as separate “factors” that do not blend.
Genes, alleles, genotypes and phenotypes
Mendel’s factors are now called genes. A gene can come in different versions calledalleles. Body cells carry two alleles of each gene, one from each parent. The pair of alleles is thegenotype: homozygous if both are the same (PP or pp) and heterozygousif they differ (Pp). The characteristic you can observe, such as purple flowers, is the phenotype.
Dominant and recessive
A dominant allele, written with a capital letter, shows its effect even when only one copy is present. A recessive allele, written in lower case, only shows when an individual has two copies. So PP and Pp plants both have purple flowers, and only pp plants are white. A heterozygous individual is a carrier: it hides the recessive allele but can pass it on, which is how traits seem to skip a generation.
Using a Punnett square
Each parent makes gametes carrying just one allele of each gene, and each allele has an equal chance of being passed on. This is Mendel’s law of segregation, explained by the separation of homologous chromosomes in meiosis. A Punnett square lists one parent’s possible gametes across the top and the other’s down the side; each box shows one possible offspring, and all boxes are equally likely. For Pp × Pp the boxes are PP, Pp, Pp and pp: a genotype ratio of 1 : 2 : 1 and a phenotype ratio of 3 purple : 1 white.
Probability, not certainty
A Punnett square predicts probabilities, not exact outcomes. A ratio of 3 : 1 means each offspring has a 3 in 4 chance of being purple, not that every family of four will contain exactly one white plant. Each offspring is an independent event: two carrier parents who already have an affected child still have a 1 in 4 chance for the next one. With small numbers the results can differ a lot from the prediction; with large numbers they get closer, as the simulator’s graph shows.
The test cross
A purple plant could be PP or Pp. To find out, cross it with a homozygous recessive (pp) plant. If all offspring are purple, the plant is probably PP; if about half are white, it must be Pp.
Two traits at once
Mendel also followed two traits together, such as seed shape (round R, wrinkled r) and seed colour (yellow Y, green y). Crossing two RrYy plants gave about 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Each gene still gave 3 : 1, and the genes were inherited independently. This is the law of independent assortment, which holds for genes on different chromosomes.
Beyond Mendel
Many traits do not follow these simple patterns. In incomplete dominance heterozygotes are intermediate (red × white snapdragons give pink); in codominance both alleles show (the AB blood group). Genes close together on one chromosome are linked, and most human traits, such as height, depend on many genes and the environment.
Real-world applications
Plant and animal breeders use these rules to produce crops and livestock with useful traits. Genetic counsellors use them to work out the chance of inherited conditions such as cystic fibrosis, which is recessive.
Common misconceptions
- “Dominant means more common.” Dominance describes how alleles interact, not how frequent they are.
- “A 3 : 1 ratio means exactly 3 out of every 4.” It is a probability for each offspring.
- “Recessive alleles disappear.” They are hidden in carriers and can reappear in later generations.
Key takeaways
- Genes come in alleles; the genotype is the pair of alleles, the phenotype is the trait shown.
- Dominant alleles show with one copy; recessive alleles need two.
- Punnett squares give the probability of each genotype and phenotype for every offspring.
- Large samples approach the predicted ratios; small samples vary by chance.
Model note: the simulator assumes complete dominance, equal chances for each allele and, in the dihybrid cross, genes on different chromosomes. Real inheritance can involve linkage, several genes and environmental effects. This is a learning tool, not a genetic risk calculator.