308 



ANIMAL BIOLOGY 



through his daughters, who are normal, to half of his grandsons; 

 and from a color-blind woman to all of her sons and none of her 

 daughters. When both parents are color-blind, all the children 

 show the defect. This behavior is readily accounted for if we as- 

 sume that the gene for color-blindness when present is associated 

 with the factors for sex on the X chromosome, and that color- 

 blindness develops in males 

 when it is received from one 

 parent, and develops in fe- 

 males when it is received from 

 both parents. Thus a color- 

 blind man is always hetero- 

 zygous for the character, while 

 a color-blind woman is ho- 

 mozygous. A woman who is 

 heterozygous has normal vi- 

 I Ra lib He sion, but is a 'carrier,' that is, 



Fig. 195. - Diagram showing a pos- produces game tes half of which 

 sible mechanism ot crossing-over during 



synapsis of homologous paternal and carry the gene lor color-bhnd- 

 maternal chromosomes. The segments ness. It is obvious why color- 

 indicate the assumed linear arrange- blindness is ver y much less fre- 

 ment ol the genes with allelomorphic 



genes opposite each other. I, pair of quent in women than m men. 



chromosomes which have entered and (Fig. 194.) 



emerged from the synaptic state with- Color-blindness and hemo- 

 out any crossing-over; 11a, chromo- 

 somes winding about each other at phiha thus serve to illustrate 

 synapsis; 116, separation of these chro- the association of genes of 

 mosomes involving breaking at the different characters in the 

 pomts ol crossing; lie, their emergence 



from synapsis with the members of the same chromosome and the 

 pairs of allelomorphic genes inter- association later of their re- 

 changed. (From Wilson.) spective characters in the 



adult — independent assortment does not occur. Incidentally, 

 it also clearly shows that whatever characters are borne by the 

 X chromosome are not transmitted by a father to his sons, and so 

 perhaps minimizes, from the standpoint of heredity, the importance 

 usually ascribed to descent in the direct male line. 



3. Crossing-over 



However, the presence of different genes in the same chromo- 

 some — we know that chromosomes are really linkage groups of 

 genes - - by no means indicates that such genes must always be 



