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Gregor Mendel (Jul. 20, 1822-Jan. 6, 1884) was a priest, teacher, and pioneer in the field of genetics. He presented his paper on his experiments in 1865; shortly thereafter, his scientific career was cut short as he was appointed abbot of his monastery and became increasingly involved in administrative duties. The scientific community did not take a general interest in his discoveries until well after his death.
 Mendelian genetics
Through experimentation on peas Mendel demonstrated several properties of inherited traits. He postulated that traits are determined by gene pairs. An organism's outward appearance, or phenotype, is dictated by the alternative forms, or alleles, of genes in the pair. Given a dominant allele D and a recessive allele r, the possible gene pairs are DD, Dr, rD, and rr. The recessive allele then is present in all but the first case, although the recessive trait will be expressed only in the last case.
Mendel further postulated that an organism's genetic makeup, or genotype, is inherited. He noted that for each trait the gene pair is formed by the receipt of one allele from each parent. This observation is identified as the Law of Segregation, or Mendel's First Law. He also noted that the combination of alleles inherited for a given trait is independent of that for other traits. This observation is identified as the Law of Independent Assortment, or Mendel's Second Law.
- Mendel's Paper in English, MendelWeb