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.