In my opinion, the Post Mendelian Era starts at the beginning of the 20th century. The first important discovery made of the 20th century was Karl Landsteiner’s ABO blood group system. Karl was observing samples of blood from various people and how they interacted, when he discovered that in certain mixtures the red cells massed together, and in others they did not. His explanation for this was that the people’s red blood cells had different antigens on their surface, which he labeled as A and B, and that the serum of blood contained antibodies that fought unknown blood group antigens. Another blood type was O. O could be accepted by A or B types. The last of the four blood types was AB which was discovered later, it contained both antigens and accepted any of the other three types. The significance of this was that people started getting their blood types labeled, and matching them for transfusion. It is important to the community of Genetics because it became a very important inherited characteristic in genetic research.
In 1902 Walter Sutton was studying the chromosomes of a grasshopper. The conclusion he came to was. that chromosomes were of separate types, that they also occurred in pairs, and with one from each pair given by each parent, and that those paired chromosomes separated as germ cells formed, and must rejoin at fertilization. Coincidentally a man in Germany, Theodor Boveri had a similar conclusion. His conclusion was that male sperm nuclei and female egg nuclei contribute equal amounts of genetics information to embryos, and that it’s essential to the normal development of the organism.
William Bateson was writing to a zoologist name Adam Sedgwick in 1905, and in his letter he used the word “genetics” for the first time ever to reference the study of inheritance and its physical basis. Soon after, in 1906 he also used the word again in a speech he gave at the Third International Conference on Hybridization and Plant breeding. The title of the conference was actually changed to “Report of the Third 1906 International Conference on Genetics”.
The next advance in genetics was made by G.H. Hardy and Wilhelm Weinberg. They came up with the Hardy-Weinberg equilibrium. The idea behind it is that the frequency of alleles and genotypes remains constant in a population from one generation to the next unless some influences it. Things such as selection, non-random mating or mutation. Although not a particularly large discovery, it is still important.
The first large scale research project of the Post Mendelian Era is the Treasury of Human Inheritance. Started in 1909 by Karl Pearson, the Treasury of Human Inheritance is a project that is dedicated to documenting conditions inherited by humans. It continued for about 50 years, the bulk of the sections being written by Julia Bell.
The next coining of a term was in 1909 by Wilhelm Johannsen> He showed that certain traits like seed size are subject to change even in genetically identical plants. It was in his book ‘The Elements of an Exact Theory of Heredity’ that he rejected the other existing term for physical units of heredity, ‘pangene’, and suggest simply using ‘gene’ instead.
The last discover was made in 1910 when Thomas Hunt Morgan was curious about what the physical basis for hereditary factors could be. So being a man of science, he set out to find the answer. The organism of study was the fruit fly Drosophila because it was cheap, bred quickly, and chromosomes of exceptional size. A white-eyed variant randomly was discovered in the mix of the red-eyed ‘wild-type’ flies. This is where Thomas makes his discovery. The discovery made was that the inheritance of the white eyes was linked to the sex of the flies, and thus was linked to the X chromosome. This is important because it’s the first definite link between a specific chromosome and an inherited characteristic.