Genetics - Introduction
Reading the instructions
Copying genes - Mitosis
Copying genes - Meiosis
Meiosis and Fertilisation
The sex chromosomes
Is there a gene for it?
Genes and personality
The Maternal Effect
We have seen that not all genes work in such a simple way. Many of the characteristics that breeders look for are the product of a number of genes, and many do not exhibit such clear patterns of dominance and recessiveness. One only has to look at a close relative of the pea - the lupin.
There was also the question of in-breeding. One could mate individuals for a number of generations, but if they were closely related, sooner or later they would deteriorate. It was known too, that if one then mated non-related individuals, the offspring might be more successful than either of the parents, so-called 'hybrid vigour'.
Another reason, then, that no one said "Why didn't I think of that!" - as Thomas Henry Huxley did when he heard of Darwin's theory - might be that, though his ideas worked with peas, their relevance to the complexities of everyday breeding practice was difficult to see. In other words it was an answer looking for a problem.
Moreover there were other effects that were passed from generation. The fact that syphilis could be caught was well known to Victorian society, as was the fact that it appeared in later generations. It was a factor in the idea of 'hereditary taint' and only much later was the parasite responsible for the infection discovered. Another example was the passion at that time for tulips. Most favoured were the striped ones, which defied the usual breeding methods, since the effect was caused by a viral infection in the bulbs.
However, there are two modes of non-Mendelian transmission that have recently become important in genetic studies, and we look at them next.
Bibliography and good reading.
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Derby TV/TS Group. Text copyright Jed Bland.
08.04.98 Last amended 04.07.05