Alleles and Inheritance: Mendel, Dominant and Recessive, and Why Traits Skip Generations

Before anyone knew what DNA was, Gregor Mendel — an Augustinian friar growing peas in a Brno monastery — deduced the fundamental rules of inheritance through nothing more than careful counting. His results, published in 1866 and ignored for thirty-four years, contain the entire logic of classical genetics. Understanding alleles and Mendel's laws is understanding why children resemble but do not replicate their parents.

Alleles: alternate versions of a gene

Every gene in the human genome exists at a specific location on a chromosome called a locus. Because humans are diploid — carrying two copies of each chromosome — every person carries two copies of each gene, one on each homologous chromosome. These two copies need not be identical: alternate versions of the same gene are called alleles. The two alleles a person carries at a given locus constitute their genotype. What those alleles actually produce — the observable characteristic — is the phenotype.

"Gregor Mendel really launches the book and the modern history of genes. He's a fascinating character — he was the first person to see that heredity was discrete, that it came in units."

Mendel's pea experiments

Mendel chose seven traits in pea plants that each existed in only two discrete forms: seed shape (round or wrinkled), seed colour (yellow or green), pod shape, pod colour, flower position, flower colour, and plant height. He cross-bred true-breeding plants with contrasting traits, counted the offspring across thousands of plants, and found a consistent pattern: in the first generation (F1) all offspring showed one version of the trait. In the second generation (F2), the hidden version reappeared in a ratio of approximately 3:1.

The two laws Mendel deduced

The Law of Segregation states that the two alleles for any trait separate during gamete formation — each gamete receives only one allele at each locus. This is the physical consequence of meiosis: when a parent cell divides to form eggs or sperm, homologous chromosomes separate, and each gamete receives only one copy of each chromosome pair.

The Law of Independent Assortment states that alleles for different traits are inherited independently of each other — the allele a child inherits at the eye-colour locus has no bearing on which allele they inherit at the blood-type locus. This holds true for genes on different chromosomes. Genes on the same chromosome violate independent assortment and are said to be linked — they tend to be inherited together unless crossing-over during meiosis separates them.

Mendel did not know what chromosomes were. He deduced the logic of inheritance from plant counts alone. When his papers were rediscovered in 1900 — sixteen years after his death — and when Thomas Hunt Morgan's work on fruit flies in the 1910s connected Mendel's abstract factors to physical chromosomes, the two halves of genetics clicked together.

Beyond simple dominance

Mendel's peas happen to exhibit clean dominant-recessive relationships. Biology is often messier. In incomplete dominance, neither allele fully masks the other — a cross between a red-flowered plant and a white-flowered plant produces pink offspring. In codominance, both alleles are fully expressed simultaneously — the AB blood type is produced by the simultaneous expression of both A and B antigens on red blood cells. Most human traits are polygenic — determined by many genes at once — which is why height, skin colour, and intelligence do not distribute in simple Mendelian ratios but in continuous bell curves.