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Understanding Genetics and Evolution » Mendel and the Discovery of Genes

What you'll learn this session

Study time: 30 minutes

AQA spec: 4.6.3.3

What Gregor Mendel found out by breeding plants
Why nobody paid attention to his work until after he died
How chromosomes and DNA were linked to Mendel's 'units'
How many scientists built up the gene theory

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Mendel and his breeding experiments

Today we know that characteristics are controlled by genes. It was not always this way. In the mid-19th century nobody knew what genes were, or even that they existed. The first big step came from a monk and teacher called Gregor Mendel.

Mendel carried out breeding experiments on plants. He worked with pea plants, which grow quickly and have clear, easy-to-spot differences such as tall or short stems. He carefully chose which plants to cross, grew the offspring, and counted how many of each type he got. He did this with thousands of plants, so his results could be trusted.

One of Mendel's key observations was this: the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged. A 'unit' for height, for example, was handed from parent to offspring without being mixed up or watered down.

Key terms:

Mendel's 'units': the name Mendel used for the factors that control each characteristic. They are passed on unchanged from parents to offspring.
Gene theory: the modern idea, built up by many scientists, that characteristics are controlled by genes located on chromosomes.

Why was Mendel's work not recognised?

You might expect such an important discovery to be famous straight away. It was not. The importance of Mendel's discovery was not recognised until after his death. Here is why.

🔍 Nothing to link it to

In Mendel's time, scientists had not seen what happens to chromosomes when cells divide. Nobody knew where his 'units' could be found inside a cell.

💬 An idea on its own

Without a link to chromosomes, his 'units' were just an idea from plant breeding. They could not be linked to anything else scientists knew, so the importance of his work was not recognised.

The ideas of one person, however clever, are rarely accepted until other evidence backs them up. Science moves forward when new findings fit together. Mendel's work only made sense once other discoveries had been made.

Common mistakes

Do not write that Mendel's work was 'wrong' or that it was 'not recognised because he was not clever'. His results were correct. They were not recognised because other scientists did not yet have the knowledge to see how important they were.

The next steps: chromosomes

The story now moves forward a few decades, one step at a time.

Late 19th century: scientists, using better microscopes, observed the behaviour of chromosomes during cell division. They saw chromosomes being copied and shared out between the new cells.
Early 20th century: scientists noticed that chromosomes and Mendel's 'units' behaved in similar ways. This led to the idea that the 'units', now called genes, were located on chromosomes.

At this point Mendel's work was finally recognised, and people realised he had been right all along.

Remember from earlier lessons that a gene is a small section of DNA on a chromosome. That idea, which we now take for granted, took many decades to put together.

The mid-20th century: DNA

Once scientists knew genes were on chromosomes, the next question was: what are genes made of, and how do they work?

In the mid-20th century, the structure of DNA was determined. Soon after, the mechanism of gene function was worked out. In other words, scientists figured out how a section of DNA acts as a code that makes a protein. You can find the details of DNA structure and protein making in the lessons on DNA Structure and on Protein Synthesis and Mutations.

Putting the timeline together

Mid-19th century

Mendel breeds plants and finds that 'units' are passed on unchanged.

Late 19th to early 20th century

Chromosomes are seen in cell division. Chromosomes and 'units' are found to behave similarly, so genes are placed on chromosomes.

Mid-20th century

The structure of DNA is determined and the way genes work is explained.

This scientific work by many scientists led to the gene theory being developed. No single person worked out genetics alone. Each scientist built on what had come before, and each new piece of evidence made the idea stronger.

Why this matters

This is a good example of how scientific ideas change over time. A theory is accepted when many scientists, working separately, find evidence that supports it. Our understanding is still developing today.

Worked example: ordering the story

Put these in order: (A) the structure of DNA is determined, (B) Mendel breeds pea plants, (C) chromosomes are seen behaving in cell division, (D) genes are placed on chromosomes.
Answer: B, C, D, A. This follows the mid-19th century, late 19th century, early 20th century, then mid-20th century.

Exam-style question

(a) Describe what Mendel found out from his breeding experiments on plants. [2 marks]

(b) Explain why the importance of Mendel's work was not recognised until after his death. [2 marks]

Model answer

(a) The inheritance of each characteristic is determined by 'units' (1) that are passed on to descendants unchanged (1).
(b) Scientists at the time had not yet observed chromosomes behaving during cell division (1), so they could not see how his 'units' fitted with anything else they knew / they did not understand the importance of his findings (1).
(c) They observed that chromosomes and Mendel's 'units' behaved in similar ways (1), which led to the idea that the 'units', now called genes, are located on chromosomes (1).

Exam tip

For 'explain why' questions, write a reason and then say what it led to. Here: no link to chromosomes was known, so his work was not understood.

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