Mendel and the start of genetics

Gregor Mendel was a monk who bred pea plants in the 1850s and 1860s. His results showed how characteristics are inherited, long before anyone knew about genes or DNA.

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Mendel's experiments

Mendel crossed true-breeding tall pea plants with true-breeding dwarf plants.
All the offspring (the first generation) were tall.
When he bred these offspring together, the next generation had tall and dwarf plants in a ratio of about 3 : 1. In his stem-length cross he counted 787 tall and 277 dwarf plants.

What he concluded

Characteristics are passed on by separate 'units' (we now call them genes), one from each parent.
Some units are dominant (tall) and hide others that are recessive (dwarf).
The recessive characteristic can reappear in a later generation.
In modern terms: TT × tt gives all Tt (tall); Tt × Tt gives TT, Tt, Tt and tt, a 3 : 1 ratio.

Why his work was ignored

Mendel published his work in 1866, but it was not widely recognised until about 1900.
Reasons: chromosomes, genes and DNA had not been discovered, so there was no mechanism; most people believed characteristics blended together; he published in a little-read journal; and few biologists then used maths to explain results.
Worked example

Mendel counted 787 tall and 277 dwarf plants. Show that this is close to a 3 : 1 ratio.

  1. Divide the larger number by the smaller: 787 ÷ 277
  2. = 2.84 (to 2 decimal places)
  3. 2.84 : 1 is close to 3 : 1

Answer: 787 : 277 = 2.84 : 1, which is close to 3 : 1.

Key idea

Mendel bred pea plants and found a 3 : 1 ratio in the second generation. He said characteristics are carried by units (genes) in pairs, some dominant and some recessive. His work was ignored until 1900 because genes and chromosomes were unknown.

Quick check

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Extra practice

2 more questions

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  1. Mendel crossed true-breeding round-seeded and wrinkled-seeded peas. All offspring had round seeds. What does this show?

    1. Round is dominant
    2. The characteristics blended
    3. The offspring were clones
    4. Wrinkled is dominant
    Show the answer

    Answer: Round is dominant

    The dominant trait appears in the first generation.

  2. Why were pea plants a good choice for Mendel's experiments?

    1. They have clear contrasting characteristics and reproduce quickly
    2. They live for hundreds of years
    3. They cannot be pollinated
    4. They have no genes
    Show the answer

    Answer: They have clear contrasting characteristics and reproduce quickly

    Easy to control and observe.

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Sources and references

Checked by the GCSE-ready editorial team, 16 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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