Page:Popular Science Monthly Volume 85.djvu/438

434 all probability associated with the distribution of the maternal and paternal sex chromosomes.

One of the most striking cases of sex-linked inheritance is that form of color-blindness known as Daltonism, in which the affected person is unable to distinguish between red and green. It is known that males are more frequently affected than females, and that color-blindness is in some way associated with sex. It requires two determiners for colorblindness, one from the father, the other from the mother, to produce a color-blind female, whereas only a single determiner is necessary to

 Parents, white-eyed &#x2640; and red-eyed &#x2642;, F1, red eyed &#x2640; and white-eyed &#x2642; ("Criss-cross inheritance"), F2, equal numbers of red-eyed &#x2640; and &#x2642; and white-eyed &#x2640; and &#x2642;. The distribution of sex chromosomes is shown on the right, as in Fig. 61.

produce a color-blind male, just as is true of sex. The accompanying diagrams illustrate the method of inheritance of color-blindness. As in the previous diagrams X represents the sex determiner, O its absence, and X the sex determiner which carries the factor for color-blindness. (Diagrams from Morgan.) It will be seen that a color-blind father and a normal mother have only normal children, but the father transmits to his daughters and not to his sons the sex determiner which carries the factor for color-blindness. But since color-blindness does not develop in females unless it is duplex (i. e., comes from both father and mother), whereas it develops in males if it is simplex (i. e., comes from either parent) all the daughters will appear normal although carrying one determiner for color-blindness, while all the sons will be normal because they carry no determiner for color-blindness. But these daughters transmit to one half of their children the single determiner for color-blindness, and if any of those receiving this determiner are males they will