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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    1. Patterns of inheritance I: haploid organisms To understand the mechanism of inheritance, it is first necessary to consider the ways in which cells and organisms reproduce. Two different processes of reproduction are found at the cellular level, and for many species at the organism level too. By far the most common process of cell reproduction found in eukaryotic* and prokaryotic* organisms is the process whereby a cell results from the division of a parent cell, and in turn divides to give rise to daughter cells. There is very good evidence that the information inherited by these daughter cells is almost always identical to that possessed by the mother cell. Much of the evidence for this will be found in later chapters of this book. This process is usually called asexual reproduction, and it is the process of asexual cell reproduction which accounts for the growth of bacterial cultures and also for the development of a human from a single cell to the final adult organism with a cell content of about lO'^-lO^'* cells. In eukaryotic organisms, a cell can sometimes be formed, not by asexual reproduction, but instead by the fusion of two special cells. This process involving cell fusion is the central one of the second process of reproduction, sexual reproduction. Prokaryotic organisms also show sexual reproduction, but the mechanisms involved are more diverse and more complex. These will be described in this book but only after the mechanism of inheritance in eukaryotes has been dealt with. Returning to sexual reproduction in eukaryotic organisms, the specialised cells which fuse are called gametes, and the cell which is formed by their fusion is called a zygote. Confusion often arises when these two processes of reproduction are considered at the cellular and organism level. Where the whole organism consists of a single cell then these two levels are the same * These terms are defined brieñy in the Index of Definitions and Glossary on page 207. 1