Animal breeding / by A.L. Hagedoorn.

  • Hagedoorn, Arend L. (Arend Lourens), 1880-1953.
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.

    73/410 (page 45)
    Chapter Seven Inheritance of Sex I n all our domestic animals and birds, the sex ratio is approximately equality. This observation, coupled with facts of sex-linked inheritance, led to the hypothesis that in every species one of the sexes would be heterozygotic for something for which the other sex was thought to be homozygotic. When the cytology of the chromosomes developed, it was found that this something is not one gene, but a complete chromosome. In some animals, the male sex is heterogametic, and the female sex homogametic. This means that in the cells of the males, one chromosome is single, while all the others are double, whereas in the females all the cells have a complete double set of chromosomes. In other animals, however, the reverse is true, the female sex being heterogametic. This is so in butterflies and in birds, whereas in mammals the males are heterogametic. This means that, in respect to all the genes carried on the un¬ paired sex chromosome, the female in birds or the male in mammals must necessarily be heterozygous. The other sex, how¬ ever, may be either homozygous or heterozygous. Of course it may so happen that animals of either sex lack the gene in question altogether. This situation leads to some remarkable consequences. We must look upon all the specially male or female qualities as caused by a difference between one and two doses of genes on this chromosome. When we mate a female bird to a male that lacks one of the genes the female carries on the sex chromosome, half of the off¬ spring will have this gene, and the others will lack it. As, how¬ ever, the birds into which the sex chromosomes enter will