Heredity by John Waller

Author:John Waller [Waller, John]
Language: eng
Format: epub
ISBN: 9780192507693
Publisher: OUP Oxford
Published: 2017-06-05T00:00:00+00:00

The triumph of Mendelism

In the year 1900 Mendel’s laws of segregation, independent assortment, and dominance were recovered from historical obscurity. The British experimental biologist William Bateson emerged as a key exponent of Mendelism having read of Mendel’s experiments with pea plants while on a train from Cambridge to London. He introduced a now familiar syntax: ‘allele’ to refer to one of two or more different forms of the same hereditary unit; ‘heterozygote’ to denote a situation in which an organism carries two different alleles for a particular trait; and ‘homozygote’, when only one of the allelic forms is present. At about the same time, the Danish scientist Wilhelm Johannsen contributed the term ‘gene’ and introduced the conceptual division between the ‘genotype’, that is, what an organism inherits, and its ‘phenotype’, that is, how the hereditary endowment is expressed. But demonstrating the validity of Mendel’s theory would take time.

A key step involved finding evidence that the behaviour of chromosomes during the formation of sex cells, or ‘meiosis’, accords with the patterns of inheritance as described by Mendelian theory. In 1902 and 1903, the American Walter Sutton, building on the work of Theodor Boveri, published the results of his studies of meiosis in grasshoppers. Sutton described how, in cells containing the full complement of chromosomes, identical-looking chromosomes duplicated, causing a doubling of their number. The cell then divided into two daughter cells that divided again to produce four sex cells, each containing just one copy of each chromosome (Figure 13). The pieces were falling into place: at conception two sex cells, each with half the normal complement of chromosomes, fuse in order to create a ‘zygote’ with the full number of chromosomes necessary to form a new being. As Sutton realized, if genes are located on the chromosomes, then Mendel’s laws would naturally follow from what he had seen happen during meiosis. And yet, these observations still left room for scepticism: even Bateson doubted that Mendelian alleles had anything to do with chromosomes.

13. Simplified schematic of meiosis. In a nucleus containing two pairs of chromosomes each parent having supplied one of each type, the chromosomes duplicate before undergoing crossing over and two divisions in order to produce four gametes.

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