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Back in school, you probably studied Gregor Mendel’s classic experiments in pea plants, in which he examined the inheritance of certain appearance traits in these plants. This was, perhaps, the first truly revolutionary experiment in genetics, because it proved that heredity was not acquired—it was handed down from generation to generation, in some kind of physical manner. Mendel’s work also showed that there were rules which governed how these traits were passed on, and that these rules tended to follow some simple mathematical ratios.
Although Mendel published his work in 1865, it wasn’t until after his death that the importance of the work was understood. In the 1890s, scientists working on similar experiments rediscovered Mendel’s earlier work on peas, and in 1905, William Bateson—a firm believer in the important of Mendel’s experiments—coined the term genetics, derived from the Greek word genesis, meaning origin.
Bateson, along with co-researcher Reginald Punnett, also first proposed the idea of genetic linkage after noting that some traits were likely to be passed on in groups. Essentially, linkage refers to the fact that genes which lie in close proximity on a chromosome are likely to be inherited together, because they are less likely to be separated during recombination.
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Genes and Chromosomes
Following the rediscovery of Mendel’s research, scientists began investigating the way in which genetic information was passed on. In 1910, after his experiments studying the inheritance of mutations in fruit flies, researcher Thomas Hunt Morgan made a case for the existence of chromosomes.
Three years later, Alfred Sturtevant, a student of Morgan, showed that genes were arranged on chromosomes in a linear fashion. After Morgan observed the phenomenon of linked genes, Sturtevant proposed the idea that the physical distance between genes on a chromosome affected the likelihood that the genes would be inherited together, essentially showing the link between the phenomenon of inherited characteristics, and the physical properties of chromosomes and the physical distance between genes.
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Watson and Crick and the Double Helix
Chromosomes are made up of both protein and DNA, and for some time, scientists did not know which of these macromolecules was responsible for inheritance.
In 1928, Frederick Griffith discovered the process of transformation—the process by which cells can take in, incorporate, and express foreign DNA—after noting that genetic information dead bacteria could be transferred to living bacteria. It wasn’t until 1944, however, that a group of researchers discovered that DNA was the molecule responsible for this transfer of genetic information. Finally, the Hershey-Chase experiment in 1952 also confirmed that it was DNA and not protein that was responsible for inheritance.
In 1953, what is perhaps the most well-known advance in genetics was made: James D. Watson and Francis Crick determined the structure of DNA. Crucial in this discovery was the research of a third scientist, Rosalind Franklin, who supplied x-ray crystallography work which allowed the duo to determine that DNA has a helical structure composed of two strands of DNA, each with a carbon-phosphate backbone, and pairs of nucleotides arranged like the rungs on a ladder.
Unfortunately, Rosalind Franklin has never received the credit she should have been entitled to for her role in Watson and Crick's discovery, and in fact it was not until well after her death that the importance of her work became public knowledge. It is entirely possible that had she not died at just 37, she would have been nominated for a Nobel Prize along with the famous duo (Nobel Prize rules do not allow posthumous nominations).
Determining the structure of DNA was particularly important because it suggested two crucial possibilities about genetic inheritance: first, that genetic information was carried by the sequence of nucleotides on the DNA strands, and second, that duplication of DNA could be achieved if the strands were unwound, and each single strand was used as a template for a new strand. Both of these possibilities turned out to be the case.