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Chap. IV. SEX DIFFERENCES

It was mentioned on p. 12, Chapter II, that when chromo­ some pairs are examined it is found that there is one pair which is not identical in males and females: all the others are. In mammals the female has two large chromosomes, called “ X ” , while the male has one large X and a small “ Y ” chromosome. Thus all female mammals have two X chromosomes in every body cell and one X in every egg cell, and in this respect the egg cells are alike. The males o f mammals, on the other hand, have an X and a Y chromosome in every body cell, and consequently produce two kinds of sperm, one half of which carry an X chromosome and one half a Y .2