Mendelian inheritance

In the mid-nineteenth century, Gregor Mendel conducted controlled crosses of pea plants and tracked the characteristics of their offspring over generations. Peas were useful for this work because their visible traits could be distinguished and their pollination could be managed. Mendel studied several pairs of contrasting traits and counted how often different forms appeared. His 1866 paper, based on work carried out over years, presented numerical patterns in hybrid offspring. Rather than assuming that parental traits simply blended, he proposed that hereditary factors were transmitted as discrete units. These factors are now understood in terms of genes and their alleles.

In a diploid organism, a cell typically has two copies of each autosomal gene, one inherited from each parent. During meiosis, the two homologous chromosomes—and the alleles they carry at a given locus—separate into different gametes. A gamete therefore receives one allele from each such pair. At fertilization, alleles from two gametes combine in the offspring. For a single gene with two alleles, if an individual has one of each and the alleles segregate normally, either allele can be transmitted; predicted offspring ratios depend on the parents' genotypes and on the number of offspring observed. Mendel's familiar three-to-one ratio describes the expected phenotype ratio in a particular cross of two heterozygotes when one allele is fully dominant, not a universal outcome for every trait.

Mendel's principle of independent assortment concerns how alleles of different genes enter gametes. Genes on different chromosomes generally assort independently as homologous chromosome pairs line up randomly during meiosis. Genes far apart on the same chromosome can also behave approximately as unlinked because recombination may separate them. Closely linked genes, however, tend to be inherited together more often than independent assortment predicts. The principle is therefore conditional: it is not a claim that every pair of genes always sorts independently.

A genotype is an organism's genetic constitution at one or more loci; a phenotype is an observable or measurable characteristic arising from genotype and other influences. In Mendel's classic pea examples, one allele could mask another in a heterozygote, producing a dominant–recessive phenotype relationship. Dominance describes how alleles affect a phenotype, not whether one allele is more common or more likely to be inherited. Many traits involve multiple genes, environmental influences, or allele interactions such as incomplete dominance and codominance. These patterns may yield phenotype ratios unlike Mendel's classic examples even when the underlying alleles still segregate according to meiosis.

The principles of segregation and assortment provide a foundation for predicting inheritance in families and experimental crosses. They also helped connect patterns in offspring with the behavior of chromosomes during meiosis, a mechanism that was not known in Mendel's time. Modern genetics uses this foundation alongside molecular and population-level evidence, while recognizing that linkage, chromosome changes, gene interactions, and other mechanisms can alter expected patterns. Mendelian models are useful starting points, not a complete description of all heredity.