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8 9 the UnsUnG monk peas and their peculiar traits While Darwin was pondering over his mechanism, a Moravian monk, Gregor Mendel 182284, had already been experimenting with heredity for years. Starting in 1856, Mendel had begun breeding pea plants on a hunch that inheritance was mathematically predictable. By 1865 he had tested over 29,000 plants and amassed enough evidence to show that it was possible to accurately forecast the ratios between paired traits such as smooth vs wrinkled peas, or tall vs dwarf plants after controlled crossbreeding. For example, crossing purebred tall and dwarf pea plants produced only tall specimens. However, crossing these with each other again, dwarfness reappeared in the next generation, with talldwarf plants in a 31 ratio. Mendel concluded that pairs of particles now known as alleles, one dominant and one recessive, were at work see example, opposite top. Mendel was right. We also now know that other species, such as snapdragon flowers, can also display incomplete dominance when red and white varieties are crossed lower example opposite. And there is codominance, where neither allele is recessive. An example is the ABO bloodgroup system, which is controlled by three alleles, A, B and o. o is recessive to both A and B, and causes Otype blood, while A and B are codominant. You inherit two alleles, one from each parent, and can therefore either end up with AAA, Ao , BBB, Bo , ABAB , or Ooo blood groups. We also now know that just one letter change on chromosome 9 makes the difference between O and A, but more of that to come. Darwin didnt hear of Mendels work and it was not fully recognised until 1900 by William Bateson 18611926. Above Mendels original experiment on peas. If the original purebred plants pairs of particles were TT tall, dominant and dd dwarf, recessive, then the second generation would all be Td, all tall, while in the third generation equal numbers of TT, Td, dT and dd would, because of the dominance of the T, produce the observed 31 ratio of tall to dwarf peas. Above An example of incomplete dominance, here in snapdragons. The original plants are RR red, partially dominant and ww white, recessive. The second generation are all Rw, pink, while the third generation show red, pink and white children in the ratio 121.
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