GENETICS AND EUGENICS 831 



chicken is crossed with another blue chicken, one-fourth the progeny- 

 are black, one-fourth are white and the remaining two-fourths are 

 blue. The blacks and the whites are homozygous for their respective 

 colors while the blues are heterozygous. The inheritance of color in 

 short-horn cattle, and of the color in the four-o'clock flower are also 

 examples of this. In still other cases in which there is not neutrality 

 of dominance, the dominant effect may not be complete. Very often 

 the heterozygous individuals can be picked out from the homozygous 

 dominant ones by casual inspection. 



Inheritance of Sex 



In certain lower phyla of animals and in the early embryos of 

 higher forms, including mammals, both male and female repro- 

 ductive systems are present in each individual. Typically in 



Fig-. 433. — Half of all possible fertilization combinations will be (XY) and half 

 will be (XX). The XY combination will be males and the XX will be females. 



higher forms, one of these systems is repressed, and we speak 

 of the individual as being either male or female. The deter- 

 mination of whether the male or the female system will develop is an 

 inherited trait. The chromosome that carries the gene of sex deter- 

 mination has come to be designated as the X-chromosome. In typical 

 cases, the female has in each of her germ cells, two of these X-chromo- 

 somes and the male has only one. Therefore, when, in the process of 

 spermatogenesis, the chromosomes match with their homologous chro- 

 mosomes and separate again to give each of the resultant cells one 

 complete set of chromosomes, the X-chromosome cannot pair with an- 

 other like itself, and when the chromosomes are distributed in sets, 

 half the sets will lack an X-chromosome. The sperm cells that do not 

 have an X-chromosome will produce males while those that have such 

 a chromosome will produce females. 



