Genetics / D.J. Cove.

Cove, D. J.

Date:: 1971

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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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2 Patterns of inheritance I: and no confusion can occur. It is with multicellular organisms, that confusion can occur. At the organism level humans only reproduce sexually; two parents are involved, each of which supplies one gamete to form a zygote which develops into another individual. However, from a cellular point of view by far the most common method of cell reproduction in humans is the asexual one. In some, multicellular organisms asexual reproduction occurs not only at the cellular level, but also at the organism level. Some examples of this will probably be familiar. The strawberry plant, Fragaria grandiflora, for example sends out long shoots at the ends of which a new plant can develop, which will become independent of the parent plant when the shoots wither. No gametes are involved in this process, nor therefore is a zygote, only asexual cell reproduction has occurred and yet two individuals have arisen where before there was only one. The fungus, Aspergillus nidulans, is another example of a eukaryotic organism which can reproduce both sexually and asexually. As it is relatively simple, it will be used in this book to illustrate some of the basic facts about the mechanism of inheritance and so it is first necessary to consider its life cycle. Aspergillus is a fairly common mould often found growing on food, and is related to Pénicillium, the mould used to make penicilHn, and also to the moulds used in the making of certain cheeses. Aspergillus consists of long tubular cells, which are typically eukaryotic in structure. Each cell is however very large and contains many nuclei. It is possible that during the course of evolution Aspergillus lost the cell walls that divided each cell with its single nucleus from its neighbours, and so the multinucleate cells found today were formed. These cells are called hyphae, and branch as they grow. Collectively the branching cells are called a mycelium. The mycelium grows by the process of asexual cell reproduction, but Aspergillus can also reproduce asexually at the organism level. The cells of the mycelium produce, also by asexual cell reproduction, enormous numbers of spores, called conidiospores (or sometimes conidia). It is these spores which give Aspergillus and other moulds their characteristic green colour. Each square centimetre of Asper¬ gillus mycelium produces about 2 x 10^ spores, and so a mycehum the size of a petri-dish (i.e. 9 cm in diameter) produces about 10^ spores. Any one of these spores can germinate and grow to a similar size producing another 10^ spores, in about five days. The mycelium which develops from a conidiospore is almost always identical to that from which the spore came. This then is the asexual form of reproduction shown by Aspergillus.