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The second big idea: The gene

Why do offspring resemble their parents? The discovery of the molecule that is passed from a parent to their offspring was a key development in biology.

Discovering heredity

The nature of heredity was discovered by a monk called Gregor Mendel who carried out thousands of breeding experiments with pea plants. From his results, he surmised that a 'particle' from each parent is passed on to their offspring and that these two 'particles' interact to create similar traits in the offspring.

Inheritance of traits

Even though Mendel had clearly shown inheritance involved a particle being passed from parent to offspring, he had not been able to identify the particle, so his work was treated with scepticism. Almost half a century later, in the early 1900s, these views were changed.

Could the secrets to heredity lie inside cells? Chromosomes had been observed and behaved exactly as Mendel’s particles should, but how they could carry hereditary information was still a mystery.

The central dogma of biology

Mendel’s work was finally accepted when DNA was identified as the molecule responsible for inheritance. DNA has functional lengths called genes that equate to Mendel’s particles. These lengths of DNA control a trait by way of producing a protein. This is the central dogma of biology.

DNA is normally found inside cells in a structure called the nucleus. In the nucleus, the DNA is unravelled. However, when DNA is being replicated, it becomes tightly wound into fibres and forms structures called chromosomes. 

Genes are inherited

We now know that genes are the unit of inheritance. We inherit two copies of each gene, one from each of our parents.

All the cells we have contain exactly the same genes, but not all of these genes are expressed. Some genes are switched on to produce proteins, whereas others are switched off. The type of cell, tissue or organ depends on which genes are expressed.

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