Mendelian Inheritance
The patterns of trait transmission discovered by Gregor Mendel: segregation and independent assortment of alleles.
Mendelian inheritance is the pattern of trait transmission established by Gregor Mendel through his pea-plant experiments of 1856–1863, published in 1866. By crossing pure-breeding lines that differed in single, clearly defined traits — round versus wrinkled seeds, tall versus short stems — and counting the offspring of each generation, Mendel discovered the two laws that became the foundation of genetics.
The law of segregation states that each individual carries two copies (alleles) of each gene and that each gamete receives one of them at random. In a monohybrid cross of two heterozygotes (Aa × Aa), the offspring genotypes appear in the ratio 1:2:1, and because the dominant allele masks the recessive in heterozygotes, the dominant phenotype appears in the ratio 3:1 — the ratio Mendel observed for all seven of his traits.
The law of independent assortment states that alleles of different genes are distributed into gametes independently, producing the classic 9:3:3:1 dihybrid ratio. The law has an important exception: genes on the same chromosome are linked and travel together, and the frequency with which linked genes recombine became the basis of genetic mapping after Thomas Hunt Morgan's Drosophila experiments in the 1910s.
Mendel's work was ignored until its rediscovery in 1900 by de Vries, Correns, and Tschermak, after which it rapidly became classical genetics. Molecular biology later explained the laws: genes are DNA sequences carried on chromosomes, alleles are sequence variants, chromosomes segregate during meiosis, and dominance reflects the biochemical function of the gene products. Some human conditions, such as sickle-cell disease and cystic fibrosis, follow simple Mendelian patterns, while most traits are polygenic and vary continuously. The same genes whose transmission Mendel characterized are today's targets of genome-editing tools such as CRISPR-Cas9.
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biology genetics gregor mendel heredity
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