4.6.3.3 - Development of Genetics

Mendel and inherited units

In the mid-19th century, Gregor Mendel carried out breeding experiments on plants. He crossed plants with different characteristics and looked carefully at the characteristics shown by their descendants.

Mendel's important observation was that the inheritance of each characteristic is determined by units passed on to descendants unchanged. This was a powerful idea because it treated inheritance as something carried by separate units, rather than as a general blending of parental characteristics.

Today, Mendel's units are called genes. That name was not available to Mendel, and he did not know what the units were made of or where they were in cells. His work was still important because it gave scientists a testable pattern for inheritance.

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Use the timeline as a route through the lesson. The key skill is to link each new piece of evidence to the next improvement in scientific understanding.

Why Mendel was not recognised

AQA expects you to understand why the importance of Mendel's discovery was not recognised until after his death. The simplest answer is that Mendel had a strong pattern but scientists did not yet have the cell evidence needed to explain it physically.

Mendel's units were abstract. In the mid-19th century, scientists had not yet connected those units to chromosomes, genes or DNA. Without that link, his work was easier to treat as an unusual plant-breeding result rather than as a general explanation of inheritance.

There were also communication and timing problems. Mendel's work was not immediately taken up by the wider scientific community, and the word gene had not yet been coined. His results became much more meaningful once later scientists could compare them with chromosome behaviour.

In an answer, connect the delay to the missing explanation: Mendel had evidence for inherited units, but later scientists supplied the chromosome and DNA context.

Chromosomes gave units a place

In the late 19th century, scientists observed the behaviour of chromosomes during cell division. This was a different line of evidence from Mendel's plant breeding, but it became useful because chromosomes also behaved like separate structures that could be passed on.

In the 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.