Animal breeding / by A.L. Hagedoorn.

  • Arend Lourens Hagedoorn
Date:
[1939]
Catalogue details

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

Credit: Animal breeding / by A.L. Hagedoorn. Source: Wellcome Collection.

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    Heredity 19 ferent kinds of cells differ so much in composition, if we did not know that the genes within the cells are localized in a separate nucleus. Inside this nucleus the genes are arranged in strings, the chromosomes; outside the nucleus in the cell cytoplasm other cell-ingredients are present in varying proportions in different tissues. The chromosomes in the nucleus are paired, and in every pair one chromosome is originally derived from the mother and one from the father. When a certain gene is inherited from both parents, the chromosomes are so arranged in the nucleus that those two genes are placed side by side, and this is possible for all the genes, because they are arranged in a definite order on the chromosomes. In the higher organisms every cell contains a great many genes, and it is evident that the number of different genes runs into the tens of thousands. In the beginning it was thought that those genes would probably be protoplasmatic, living corpuscles, but it becomes more and more probable that we are dealing with chemical substances of a special kind (autocatalisers), and that only one molecule of each gene is present in each cell. The genes multiply in the cell, and when every gene present has doubled its volume, the whole chromosome splits into two identical halves. Before every cell division the chromosome pairs go to opposite pairs of the nucleus, which divides before the cell does, and in this way one complete cell containing a pair of chromosomes of each kind for a short while contains two pairs, and then divides into two equivalent cells, each having one pair of each chromosome. In Genetics it has become usual to denote genes by means of letters of the alphabet. If both the father and the mother have contributed a certain gene A to the fertilized egg, the egg nucleus, as far as gene A is concerned, becomes AA. By duplica¬ tion these two A's become four, so that for a short while the nucleus contains AAAA. At the next nuclear division and cell division, we get again two complete cells, each containing AA. Now any cell can only be in one of three possible conditions in respect to any given gene. The case in which both parents contributed A, we have just discussed. It is possible that the parents did not both contribute the same gene. If at the moment of fertilization, only the egg or only the sperm contains A, in the chromosomes at one spot there will be a gene A in one
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