For an explanation, see the main splats page
The principles of genetics
- 1828 Karl Ernst von Baer published his The Embryology of Animals which strongly opposed preformationism, showing that mammals also have 'eggs', or ova.
- In 1866, Gregor Mendel published his experiments on the crossbreeding of pea plants. These revealed that inheritance was not blending, as had been assumed.
- The basic principles of genetics were first set out in Mendel's laws, although they have since been found to be more complicated in some cases.
- Genes may occur in multiple forms called alleles. Some alleles may mask (be dominant over) other alleles which are called recessive because they 'recede'.
- Homozygous individuals generally breed true when crossed with another similar-appearing homozygous individual, unless there are two mutations at the same time.
- Heterozygous individuals may not 'breed true' if crossed with another heterozygote, as heterozygotes produce gametes with different genes for a single trait.
- In some cases, alleles may show incomplete dominance, with intermediate heterozygotes, rather than the classic Mendelian dominant/recessive pattern.
- Gregor Mendel showed that the genes do not blend to an average, but that the original character, in its original form, can return in a later generation.
- While Darwin was still puzzling about blending inheritance, Mendel had provided the answer and published it, but sadly, nobody noticed, for almost forty years.
- Independent assortment of genes, as described by Mendel, may be restricted by linkage effects where two genes are nearby on the same chromosome.
- In 1902, William Bateson coined the terms F1, F2, allelomorphism, homozygote, and heterozygote for use in the discussion of genetics experiments.
- In 1905, William Bateson and Reginald Crundall Punnett reported the discovery of two new genetic principles of interest: linkage and gene interaction.
- In 1905, Edmund Wilson and Nellie Stevens independently described the behaviour of sex chromosomes, showing that XX determines female, XY determines male.
- In 1905, Lucien Claude Cuénot discovered the first lethal allele, the yellow coat colour allele in mice, showing that genes could play a role in selection.
- In 1906, C. W. Woodworth and William Ernest Castle introduced the fruitfly Drosophila melanogaster as new experimental material for genetic studies.
- In 1908, G. H. Hardy and Wilhelm Weinberg independently proposed the Hardy-Weinberg Law, that gene frequencies remain constant in the absence of selection.
- In 1909, F. A. Janssens suggested that the chiasmata between chromosomes could be taken as evidence for the phenomenon of crossing over among linked genes.
- In 1909, Castle and Phillips showed that an ovary from a black guinea pig, transplanted into a white one, still gave black offspring if mated to a black male.
- In 1909, Wilhelm Johannsen showed that natural selection demands a source of genetic variability and introduced the terms 'genotype' and 'phenotype'.
- In 1910, Thomas Hunt Morgan proposed a theory of sex-linked inheritance for the first mutation discovered in the fruit fly, Drosophila, white eye.
- The identification of white-eye in Drosophila melanogaster as a sex-linked gene was followed by the gene theory, including the principle of linkage.
- In 1911, Thomas Hunt Morgan first proposed that the Mendelian factors, otherwise the genes, were in fact arranged in a line on chromosomes in some way.
- In 1913, Alfred Sturtevant used crossing-over frequencies to get relative distances for a map of genes on the chromosome, using sex-linked genes in Drosophila.
- Plants often hybridize outside their species: plant hybrids can be created that cross species 'barriers', and even different genera may be hybridized.
- Plants are often polyploid, especially cultivated varieties, although it also happens naturally in some groups. Tetraploids can often reproduce successfully.
- In animals, triploid individuals rarely survive. Triploid plants can survive, though some of them do not breed very well, due to problems at meiosis.
- In animals, trisomic individuals, with one extra chromosome, can sometimes survive, and this may be related to the number of genes on the extra chromosome.
- In 1927, Karpchenko got a tetraploid cabbage-radish hybrid, thus creating the new genus, Raphanobrassica, with a full gene complement from each parent.
- In 1937, Albert Francis Blakeslee and Oswald Avery used colchicine to produce artificial polyploidy in plant cells, making a new tool for experimental genetics.
- In 1941, George Beadle and Edward Tatum irradiated fungus, Neurospora crassa, and based on the results, they then proposed the one gene one enzyme hypothesis.
- Beadle and Tatum's irradiated Neurospora allowed them to establish conclusively that the gene produces its effect by regulating particular enzymes.
- In 1946, Joshua Lederberg and Edward Lawrie Tatum studied the process of conjugation in Escherichia coli, where bacteria interchanged genetic material.
- In 1947, Barbara McClintock published her hypothesis of transposable elements (her 'jumping genes') to explain curious colour variations in corn.
- In 1952, William Hayes demonstrated a variety of forms of conjugation in bacteria, a method by which bacteria can exchange genetic information.
- Genes differ in their effect according to the parent they come from in some cases. This is called 'imprinting', and is not yet fully understood and explained.
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