HEREDITY AND VARIATION 



whose development has occurred under identical conditions. In man two 

 kinds of twins are known. Fraternal twins may be of the same or opposite 

 sex and bear no more resemblance to one another than other brothers and 

 sisters, but identical twins are always of the same sex. A comparable situa- 

 tion is known in the nine-banded armadillo, which gives birth to four young, 

 all of the same sex, and in certain insects, which produce by the method of 

 polyembryony large numbers of young, all of the same sex. These situations 

 were clarified bv studv which revealed that identical twins in man and other 

 mammals and the quadruplets of the armadillo, as well as the polyembryos of 

 insects developed from single zygotes, had identical hereditary constitutions. 



Furthermore, the discovery and study of the so-called sex chromosomes 

 led to the interpretation proposed by E. B. Wilson in 1905 that sex was 

 determined at the time of fertilization by the chance combinations of the 

 gametes formed. In the insect, Protenor, there are 14 chromosomes in the 

 female but only 13 in the male (see Fig. 2.12/1, /?, and C, p. 42); and 7 

 pairs of chromosomes in the female, but only 6 pairs and an extra or odd 

 chromosome in the male. The extra chromosome of the male can be clearly 

 seen to be similar to one of the pairs of the female, the largest pair in this 

 instance. This unpaired chromosome of the male and the comparable pair of 

 the female are called X-chromosomes or sex chromosomes. All the other 

 chromosomes, which occur in pairs in both males and females, are known as 

 autosomes. If a haploid set of autosomes is designated as A, then a formula 

 for the chromosome number of any somatic cell or primordial germ cell of a 

 female Protenor would be 2.-1 + 2A', where A' stands for a sex chromosome, and 

 the formula for every mature ovum would be A + A'. The chromosome con- 

 tent of somatic cells and undifferentiated germ cells of a male Protenor would 

 be 2A + A'. One-half of the spermatozoa could be represented by A + A' and 

 the other half hy A -\- 0. Wilson pointed out that, when an ovum (.4 + X) 

 was fertilized by one kind of spermatozoon (,4 + A"), the zygote would have 

 the number of chromosomes characteristic of the female (2^-1 + 2X). If the 

 ovum {A + X) was fertilized by the other kind of spermatozoon {A + 0), then 

 the zygote would have the number of chromosomes characteristic of the male 

 (2,4 + A"). Since by the process of mitosis during development each cell 

 receives the same number of chromosomes that the zygote has, it can be seen 

 how the concept arose that sex was determined by the number of X-chromo- 

 somes present in the zygote. 



It was soon discovered that the male did not always diflfer from the female 

 in number of chromosomes and that, although the male had only one X- 

 chromosome, it sometimes had a Y-chromosome that segregated from the X- 

 chromosome at the disjunctional division (see Fig. 5.7, p. 135). The 

 Y-chromosome, like the X, is known as a sex chromosome. Drosophila males 

 have such a Y-chromosome (see Fig. 2.12£', p. 42) and produce two classes 

 of spermatozoa, which can be represented as .4 + X and .4 + T. Union with 

 A + X ova yields female-producing zygotes (2/1 + 2X) and male-producing 

 zygotes (2/1 + XT). In Protenor, which is an example of forms with XO 



209 



