Why do children resemble their parents — yet never copy them exactly? In a quiet monastery garden, Gregor Mendel grew thousands of pea plants and counted their offspring until the hidden rules of heredity appeared as clean numbers. From those ratios — 3:1, 9:3:3:1 — grew the whole science of genetics.
Genetics is the study of heredity (the passing of characters from parents to offspring) and of the variation between individuals. Its founder is Gregor Johann Mendel (1822–1884), an Austrian monk who worked on the garden pea, Pisum sativum, between 1856 and 1864 — which is why he is called the "Father of Genetics".
The pea was a brilliant choice. It is easy to grow, it has clear-cut contrasting characters (tall vs dwarf, round vs wrinkled seeds, green vs yellow seeds), it normally self-pollinates (so a line stays pure), yet it can be cross-pollinated by hand. Above all, Mendel did something new: he counted his offspring and treated the results as numbers.
Before the crosses, learn the vocabulary — every exam answer depends on using these terms precisely.
| Term | Meaning |
|---|---|
| Gene | A unit of heredity — a length of DNA that codes for one character (e.g. plant height). |
| Allele | One of the alternative forms of a gene (e.g. T = tall, t = dwarf). |
| Dominant | The allele that is expressed even when only one copy is present; written as a CAPITAL letter (T). |
| Recessive | The allele that is masked unless two copies are present; written as a small letter (t). |
| Homozygous | Both alleles the same — TT (homozygous dominant) or tt (homozygous recessive). "Pure-breeding." |
| Heterozygous | The two alleles different — Tt. A "hybrid"; it shows the dominant character but carries the recessive. |
| Genotype | The genetic make-up (the alleles present), e.g. Tt. |
| Phenotype | The observable character the genotype produces, e.g. tall. |
| P / F₁ / F₂ | The parental generation, their first filial offspring (F₁), and the second generation (F₂) from crossing the F₁. |
From his counted results Mendel drew two great generalisations, now called his laws.
Every organism carries two alleles for each character. During the formation of gametes (in meiosis) the two alleles separate, so that each gamete receives only one allele of the pair. At fertilisation the offspring again receives one allele from each parent.
When two or more pairs of characters are followed together, the alleles of one gene assort into gametes independently of the alleles of another gene. So the height gene does not "stick to" the seed-shape gene — every combination is possible. (This holds for genes on different chromosomes.)
A monohybrid cross follows one character. Mendel crossed a pure tall pea (TT) with a pure dwarf pea (tt).
The F₂ is worked out with a Punnett square — a grid that combines each parent's gametes:
| Tt × Tt | T | t |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
This famous 3:1 phenotypic ratio (genotypic ratio 1:2:1) is the signature of a monohybrid cross between two heterozygotes — and it is exactly what Mendel found: 787 tall : 277 dwarf (≈ 2.84 : 1).
A tall pea may be TT or Tt — identical to look at. To find out which, cross the unknown tall plant with a homozygous recessive (tt dwarf). This is the test cross.
A dihybrid cross follows two characters at once. Mendel crossed peas that were pure for round, yellow seeds (RRYY) with peas pure for wrinkled, green seeds (rryy). Round (R) and yellow (Y) are dominant.
The 9:3:3:1 ratio is the hallmark of a dihybrid cross. Notice that each single character on its own still gives 3:1 (12 round : 4 wrinkled, and 12 yellow : 4 green) — proof that the two genes are inherited independently. New combinations such as round-green and wrinkled-yellow (the recombinants) appear that were in neither original parent.
Not every gene has a simple dominant/recessive pair. Two important exceptions break the "one allele hides the other" rule.
Here the heterozygote shows a blended, intermediate phenotype. In the four-o'clock plant (Mirabilis) and in snapdragons, red (RR) × white (WW) gives an F₁ that is entirely pink (RW) — neither allele fully dominates. Crossing the pink F₁ gives an F₂ of 1 red : 2 pink : 1 white, so here the phenotypic ratio equals the genotypic ratio (1:2:1).
Here both alleles are fully and separately expressed in the heterozygote — there is no blending. In certain cattle, a red bull (CRCR) × white cow (CWCW) gives roan calves (CRCW) carrying both red and white hairs side by side. The human AB blood group (next section) is the classic example.
A gene may have multiple alleles — more than two forms existing in the population (though any one person still carries only two). The best example is human ABO blood grouping, controlled by three alleles: IA, IB and i.
| Blood group (phenotype) | Genotype(s) | Antigen on red cells |
|---|---|---|
| A | IAIA or IAi | A |
| B | IBIB or IBi | B |
| AB | IAIB | A and B (codominance) |
| O | ii | none |
This explains how a group-A parent and a group-B parent can have a child of any group (A, B, AB or O) — and why blood grouping is used in transfusion matching and as evidence in disputed-parentage cases.
Of the 23 pairs of human chromosomes, 22 are autosomes and one pair are the sex chromosomes. A female is XX and a male is XY. Eggs always carry an X; sperm carry either X or Y. So it is the father's sperm that decides the child's sex — an X-sperm → girl (XX), a Y-sperm → boy (XY) — giving a 1:1 (50:50) ratio of boys to girls.
Sex-linked characters are controlled by genes on the X chromosome (the small Y carries almost no matching genes). The key recessive examples in humans are red–green colour blindness and haemophilia (a blood-clotting disorder).
Because a male has only one X, a single recessive allele on it is not masked — so these conditions are far commoner in males. A female with one faulty allele is a healthy carrier (XHXh).
Take a carrier mother (XHXh) × normal father (XHY) for haemophilia:
| ♀ XHXh × ♂ XHY | XH | Y |
|---|---|---|
| XH | XHXH | XHY |
| Xh | XHXh | XhY |
A mutation is a sudden, heritable change in the DNA. A gene (point) mutation alters a single base — by substitution, insertion or deletion — which can change the protein made. Most are harmless or harmful, but a rare beneficial one supplies the variation on which evolution works. The classic medical example is sickle-cell anaemia, caused by a single base change in the haemoglobin gene. Mutations are the ultimate source of new alleles.
Mendel's tidy ratios underlie real life: genetic counselling uses Punnett squares to predict the risk of haemophilia or colour blindness in a family; blood-group genetics guides safe transfusion and settles parentage; and plant and animal breeders use the same crosses to fix useful characters. The single idea — that characters are carried by paired, separating alleles — turned heredity from folklore into an exact, predictive science.