Genetic technology : a new frontier / Office of Technology Assessment.

Date:
[1982]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic technology : a new frontier / Office of Technology Assessment. Source: Wellcome Collection.

    54/360 (page 30)
    30 passed from parents to offspring, to a study of the underlying agent by which this transmission is accomplished. That shift began in the garden of Gregor Mendel, an obscure monk in mid-19th century Austria. By analyzing generations of controlled crosses between sweet pea plants, Mendel was able to identify the rudimentary characteristics of what was later termed the gene. Mendel reasoned that genes were the vehicle and repository of the hereditary mechanism, and that each inherited trait or function of an organism had a specific gene directing its devel¬ opment and appearance. An organism's observ¬ able characteristics, functions, and measurable properties taken together had to be based some¬ how on the total assemblage of its genes. Mendel's analysis showed that the genes of his pea plants remained constant from one gen¬ eration to the next, but more importantly, he found that genes and observable traits were not simply matched one-for-one. There were, in fact, two genes involved in each trait, with a single gene contributed by each parent. When the genes controlling a particular trait are iden¬ tical, the organism is homozygous for that trait; if they are not, it is heterozygous. In the Mendelian crosses, homozygous plants always retained the expected characteristics. But heterozygous plants did not simply display a mixture of their different genes; one of the two tended to predominate. Thus, when homozy¬ gous yellow-seed peas were crossed with homo¬ zygous green-seed plants, all the offspring were now heterozygous for seed color, possessing a green gene from one parent and a yellow from the other. Yet all of them turned out to be indistinguishable from the yellow-seed parent: Yellow-seed color in peas was dominant to green. But even though the offspring resembled their dominant parent, they could be shown to contain a genetic difference. For when the het¬ erozygotes were now crossed with each other, a certain number of recessive green-seed plants again appeared among the offspring. This oc¬ curred whenever an offspring was endowed with a pair of genes that was homozygous for the green-seed trait—and it occurred at a rate consistent with the random selection of one of two genes from each parent for passage to the new generation. (See figure 5.) Genes were real—Mendel's work made that clear. But where were they located, and what were they? The answer, lay within the nucleus of the cell. Unfortunately, most of the contents of the nucleus were unobtainable by biologists in Mendel's time, so his published findings were ignored. Only during the last decades of the 19th century did improved microscopes and new dyes permit cells to be observed with an acuity never before possible. And only by the Figure 5.—The Inheritance Pattern of Pea Color Y = yellow gene Homozygotís^fltow-seed ition:YY. g = green gene have the genetie < tion: OS- Each parent contributes only one seed-color gene to the off¬ spring. When the two YY and gg homozygotes are crossed, the genetic composition of all offspring is Yg: Parents Genes V \). '(S Offspring All Yg offspring are heterozygous, and all have yellow seeds, indicating that the Y yellow gene is dominant over the g green gene. When these Yg heterozygotes are crossed with each other: Female genes (i) 1Щ 0, Male genes (9) ^9) ^ il4 oTIKe'tofâTarèTiomozygous yy, navlh^ yeiiow seeds Î V« Of Hie total ar« Ьогтгу9(Ш9 mvmg Уг of fKe total are heterozygous Yg, having yellow swds* Thus, У4 of these offspring will have yellow seeds, but their individual genetic composition, YY of Yg, may be different. SOURCE: Office of Technology Assessment.