Genetics Picture

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The word gene, first introduced in 1910, was used as an abstract hereditary unit controlling a specific hereditary characteristic within a specific species. The existence of the genes was predicted in studying hereditary characteristic, like for instance the color of flowers, from known "parents" for many generations. The most famous studies were done by Mendel, an Austrian monk, on various characteristics of pea plants. The factor found deciding the color of flowers was present in different versions. In one case it gave white flowers, in another red. The same thing applied to the structure of the seed surface: some are wrinkled, others are smooth. These different varieties of one specific gene are called alleles. Each organism has two alleles for each characteristic, one from each parent. In every generation they are split up when the gametes are created during the reduction division (meiosis): each haploid germ cell has only one allele of the original pair. At conception a new combination is created. Both the alleles in the pair can be identical, and the individual is then said to be homozygotic (Greek: homos=same, zygon=pair) for that pair of alleles. If we call the alleles deciding the color of the pea flower for fr (red color) and fw (white color), an individual can have the following combinations: frfr = homozygote, fwfw = homozygote, frfw = heterozygote. The germ cell will have either the allele fr or fw.

The alleles for different hereditary characteristics is often transmitted to the gametes independently of each other, since the different genes often are placed on different chromosomes or far from each other on the same chromosome. A pea plant can for instance have the alleles frfw color of the flower and slsk for long respectively short stem. The gametes can then contain either of the following combinations: frsl, frss, fwsl or fwss. When the gametes are formed the alleles are transmitted for color, fr and fw, independently of the alleles for the length of the stem, sl and ss.