The diseases of infants and children.

  • Cautley, Edmund, 1864-1944
Date:
1910
Catalogue details

Licence: In copyright

Credit: The diseases of infants and children. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    species. The number in an ovum or sperm cell is half that of a body cell, and union is necessary to raise the number to that of a body cell. He regarded the chromosomes as made up of an immense number of small parts or determinants, and the characters of the offspring as dependent on the relative combinations and arrangements of these male and female determinants. Hence arise variations. The most interesting theory of heredity is that advocated by Mendel, and it has the great advantage of being supported by many experimental data. His conclusions are based on the results of experiments on the edible pea, in order to discover the law of inheritance in hybrid varieties. Of two contrasting characters, e.g., size, colour, shape of seed, there is always one in each cross which appears exclusively in the offspring, and this he called the dominant, and one which does not appear, named by him the recessive. A dominant parent only produces dominant gametes, and a recessive produces recessives. Therefore, the offspring are dominants. Thus.—DxR=D (R), the recessive being latent. On allowing the hybrids to become self-fertilised the characters of both parents appear in the offspring in the form of one pure dominant, one pure recessive, and two impure dominants ; that is, apparently dominants but containing a latent and unrecognisable recessive factor. This proves that the recessive character is present, though latent, in the hybrid pro- duced by the union of a dominant and a recessive. From this it follows that two kinds of germ cells are produced by the hybrids, the parental characters being segregated into dominant and recessive gametes in equal numbers. The union of a dominant sperm cell (male gamete) with a dominant ovule (female gamete) produces a pure dominant. A pure recessive is due to the union of a recessive with a recessive. The impure dominants are formed by the union of a dominant male or female gamete with a recessive female or male gamete. The pure dominants and recessives are called homozygotes, and the impure dominants heterozy- gotes. The dominant factor is present in three out of four zygotes. The descent may be tabulated thus :— D(E). MALE. GAMETES. FEMALE. D unites with D to form DD Homozygote. j> M R DR Heterozygote. ^ }, D DR Heterozygote. :> J. R RR Homozygote. Each gamete contains only the unsplittable inherited factor, and only one of a contrasting pair, that is, is pure for that factor. The zygotes contam two, being formed by the union of two gametes, and these two characters are splittable, that is, are segregate from each other in the zygote, similar gametes unite into a zygote, segregation is into like gametes.