Mendel's Revolutionary Discovery
In 1866, Gregor Mendel published his experiments with pea plants that revealed the fundamental laws of inheritance. By tracking traits like seed color and plant height across generations, he discovered that hereditary factors (now called genes) come in pairs, segregate during gamete formation, and assort independently. His work, ignored for 35 years, laid the foundation for all of modern genetics.
The Punnett Square
The Punnett square is the simplest tool for predicting genetic crosses. Each parent contributes one allele per gamete. For a monohybrid cross between two heterozygotes (Aa × Aa), the four possible offspring genotypes are AA, Aa, aA, and aa — giving a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio under complete dominance. This simulator lets you visualize this process with any parental combination.
Complete vs Incomplete Dominance
Mendel observed complete dominance, where the heterozygote is indistinguishable from the homozygous dominant. But many real traits show incomplete dominance — the heterozygote has an intermediate phenotype. Snapdragon flower color is a classic example: red × white produces pink. This simulation lets you toggle between both modes and see how the phenotypic ratios change from 3:1 to 1:2:1.
Testing with Chi-Squared
Real experiments never produce perfect ratios. Random sampling means 100 offspring from an Aa × Aa cross might yield 78 dominant and 22 recessive instead of the expected 75:25. The chi-squared goodness-of-fit test quantifies whether the deviation is within statistical expectations or suggests something beyond simple Mendelian inheritance — such as linked genes, epistasis, or selection pressure.