The first person to show how traits are passed from one generation to the next was the Austrian monk Gregor Mendel, also regarded as the Father of Genetics. Scientists at the time had no idea how inheritance worked. They believed that traits were inherited and blended together.
In the mid-19th century, Mendel carried out breeding experiments on pea plants, studying the inheritance of different characteristics. His research discoveries have greatly influenced our understanding of genetics.
Mendel used the common garden pea for his experiments, which is a self-pollinating plant. He realised that characteristics are not blended during inheritance. For example, the shape of a pea pod has no effect at all on the colour of the flowers. He also looked at other characteristics such as:
When Mendel crossed white-flowered plants with red-flowered plants, all the offspring had red flowers. Afterwards, he crossed two of the red-flowered offspring. This time, around three-quarters of the offspring had red flowers and one-quarter had white flowers.
The diagrams below show the outcomes of each cross in Mendel’s experiment. Let’s use these representations:
Mendel deduced that specific inherited units determine characteristics, and these units remain do not change when passed on to the next generation.
Mendel demonstrated that some traits could be masked and then reappear in future generations. These are now known as recessive alleles. Recessive alleles are only expressed if the plant inherits one copy from each parent. This is why there were no white-flowered offspring in the first cross.
All of the red-flowered offspring from the first cross had the hereditary unit for white flowers. However, they were not expressed because the unit is recessive. The plants had red flowers because the hereditary unit for red flowers is dominant. The white-flowered plants in the second cross have two copies of the recessive hereditary unit, so it was expressed.
Despite being published in a reputable scientific journal, Mendel’s findings were initially overlooked. Scientists had not yet discovered DNA, genes and chromosomes, so they didn’t realise Mendel had made a significant discovery. As a result, his work was quickly forgotten.
Between the late 19th and early 20th centuries, scientists began studying the behaviour of chromosomes during cell division. Around the same time, scientists rediscovered Mendel’s work on genetics. They observed that chromosomes behave similarly to Mendel’s hereditary units, which we now call genes. This led to the belief that genes are located on chromosomes.
In the mid-20th century, James Watson and Francis Crick determined the structure of DNA and how genes work.