Genetics / D.J. Cove.

  • Cove, D. J.
Date:
1971
Catalogue details

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

Credit: Genetics / D.J. Cove. Source: Wellcome Collection.

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    haploid organisms 7 (3) Since the progeny do not produce yellow-green conidiospores these two alternative types of genetic information contained in the zygote cannot get mixed in any way. (4) Since the ascospores produced from the zygote give rise to mycelium bearing only yellow or green conidiospores, the information in the zygote must be sorted out precisely during the formation of the ascospores, so that each ascospore receives only one of the two alter¬ native types of information. These conclusions can be represented symbolically. Suppose we call the two alternative pieces of genetic information a and A. The breeding scheme can now be represented as in figure 3. parents a A y gametes a A I J A zygotes — ascospores a A (in equal numbers) progeny a A Figure 3. (For explanation, see above.) Cells which have only one copy of a particular type of genetic information are said to be haploid. Cells having two copies are called diploid. In the scheme in figure 3 therefore, only the zygote is diploid. Notice also that the scheme proposes that there are two different types of cell division. The more common type, corresponding to asexual cell reproduction, results in the daughter cell receiving the same information content as that of the mother cell from which it came. This is the process by which a haploid cell gives rise to haploid cells. However during the process whereby the diploid zygote gives rise to haploid ascospores an orderly reduction of information content must occur. This type of cell division whereby a diploid cell can give rise to haploid cells is called a reduction division.