Genetics for the clinician / C.A. Clarke.

  • Clarke, Cyril A. (Cyril Astley)
Date:
1964
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Genetics for the clinician / C.A. Clarke. Source: Wellcome Collection.

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    THE CHROMOSOMES OF MAN 23 result of an inversion. Sometimes genes appear twice in the same chromo¬ some (duplication) again because of aberrant crossing over. (c) 'Translocation' can occur between non-homologous chromosomes. This happens when two pieces of chromosome (not usually of the same size) are broken off from two non-homologous chromosomes and their positions exchanged. Two chromosomes must be broken since normally the terminal portion of any chromosome repels a fragment which tries to attach itself to it. The usual result of a translocation is that one chromo¬ some will have too little chromatin and the other too much. The indi¬ vidual inheriting the former will probably be inviable while some of those inheriting the latter may be abnormal (see mongolism, p. 27). (d) Homologous chromosomes may fail to separate and pass one into each gamete; instead both members of a pair pass into the same gamete, and the other is left without a chromosome of that pair. This is known as non-disjunction, and can happen either with the sex chromosomes or with the autosomes and the abnormaHty can arise either at the first or second meiotic division. In the case of the autosomes the individual who receives the extra chromosome is known as 'trisomie' for that chromosome pair. Point Mutations These are sudden changes at single gene loci and can occur at any stage of the nuclear cycle—either in the resting stage or at any period of mitosis or meiosis. When they happen at oogenesis or spermatogenesis, any zygote with the gamete bearing the mutation will either show or carry this, the rest of the siblings usually being normal. Mutations occur¬ ring in somatic cells are not inherited. Here only some of the cells of the animal or plant are affected (depending on how early after the formation of the zygote the mutation has occurred) and the result is a mosaic. If the mutation occurs in the spermatocyte or oocyte and is transmitted to the next generation, it wiU, if it be an autosomal dominant, appear in the first generation in either sex. If it be a sex-linked recessive it will show m the first heterogametic individual who carries it (that is, in Man, the male). If it be an autosomal recessive it may be carried for many genera¬ tions without manifesting itself. Because normally the genes are finely adjusted to their gene complex mutations are generally unfavourable and tend to be eliminated. This aspect is dealt with in later chapters.