568 GENERAL CONSIDERATIONS 



x-chromosome or both an x-chromosome and a ^/-chromosome, while the 

 males have the two a;-chromosomes. In this case it is the type of egg cell 

 which is fertilized that determines sex. 



In man, and perhaps in all mammals, there are an a:-chromosome and 

 a ^/-chromosome in the male and two rc-chromosomes in the female. Man 

 also has 46 autosomes. The formulae for the two sexes may be written: 



cf = 46 -h a; + 2/; 9 = 46 + 2x. The sex cells are 23 + a; or 23 + y. 



611. Twins. — The production of twins may be mentioned in connec- 

 tion with sex. There are two types, known respectively as identical and 

 fraternal twins. Identical twins agree precisely in their characteristics 

 and are always of the same sex. For this reason it is assumed that they 

 are developed from a single zygote, each of the two sections of the early 

 embryo developing into a complete individual. In the case of the 

 nine-banded armadillo it has been found that the four young usually 

 produced are identical quadruplets resulting from the development of a 

 single zygote and possessing a common placenta. On the other hand 



A b C D e F q H i ) K L m n 



i^ Vl < y i ] i' fH«J >' * m m, , m LW i ' J J ^ 



0##0#0#00##00 P*t«rn4l 



• 00##«00#0###0 Maternal 



AbcDEFghljKLMn 



Fig. 391. — Diagram of two homologous chromosomes, paternal and maternal, showing 

 that similar genes are placed at the same level and at definite locations in the length of the 

 chromosomes. Both genes of a pair may be dominant (black discs with capital letters) 

 or may be recessive (white discs with small letters), or one may be a dominant and the other 

 recessive. 



fraternal twins, being produced from two zygotes, may differ in sex and in 

 other characteristics to the same extent that two offspring produced 

 by the same parents at different times may differ. 



612. Sex-linked Characters. — The sex chromosomes may not only 

 determine the sex but also carry genes for many other characters. It thus 

 follows that these characters will be determined when sex is determined 

 and will be associated with one sex or the other. In the case of color 

 blindness in man the defect is carried in some of the x-chromosomes 

 and is recessive, while normal vision, which is dominant, is carried in the 

 other rr-chromosomes. The sex chromosome carrying the defect may 

 be indicated by x. A color-blind father (xy) and a normal mother 

 {xx) will have no color-blind children, since the zygotes will be either xy 

 or XX. In the F2 generation, however, one-half the grandsons and one- 

 half the granddaughters are free from this defect and the other half of 

 the granddaughters carry the gene for color blindness as a recessive and 

 the other half of the grandsons are color-bHnd. A normal father with a 

 color-blind mother will have only color-blind sons while the daughters 

 will be normal. Also one-half the grandsons and one-half the grand- 



