560 GENETICS AND EVOLUTION 



haired cats imisi be homozygous. We can deduce, then, that cat B and 

 the kittens produced by cats B and C have the genotype II. All the short- 

 haired cats have at least one L gene. The fact that any of the offspring 

 of the male cat has long hair proves that he is heterozygous, with the 

 genotype LI, The kitten produced by cat B received one I gene from 

 its mother but must have received the other from its father. The fact 

 that cat C gave birth to a long-haired kitten proves that she, too, is 

 heterozygous, and has the genotype LI. It is impossible to decide, from 

 the data at hand, whether the short-haired cat A is homozygous LL or 

 heterozygous LI. A test cross with a long-haired male would be helpful 

 in deciding this. Further mating of the short-haired male with cat B 

 would give half long-haired ancf half short-haired kittens, whereas fur- 

 ther mating of the short-haired male with cat C would give three times 

 as many short-haired kittens as long-haired ones. 



276. The Genetic Determination of Sex 



The sex of an organism is a genetically determined trait. There is 

 an exception to the general rule that all homologous pairs of chromo- 

 somes are identical ni size and shape: the so-called sex chromosomes. 

 In one sex of each species of animals there is either an unpaired chromo- 

 some or an odd pair of chromosomes, the two members of which differ 

 in size and shape. In most species the females have two identical chromo- 

 somes, called X chromosomes, and males have either a single X chro- 

 mosome or one X plus a generally somewhat smaller one called the Y 

 chromosome. The existence of these unpaired chromosomes was discov- 

 ered by C. E. McClung in 1902, when he was studying the process of 

 meiosis in the testes of grasshoppers. He made the shrewd guess that 

 these might play some role in sex determination. In a few animals, the 

 butterflies and birds, the system is reversed and the male has two X 

 chromosomes and the female one X and one Y. The Y chromosome 

 usually contains few or no genes and in most species the X and Y 

 chromosomes are distinguished by their different size and shape. Yet in 

 meiosis the X and Y chromosomes act like homologous chromosomes; 

 they undergo synapsis, separate, pass to opposite poles, and become in- 

 corporated into different gametes (Fig. 32.4). Human beings have 23 

 pairs of chromosomes; males have 22 pairs of ordinary chromosomes, 

 called autosomes, one X and one Y chromosome, whereas females have 

 22 pairs of autosomes and two X chromosomes. 



It is not the presence of the Y chromosome, however, which deter- 

 mines maleness, for in a number of species the male has no Y chromo- 

 some at all, just a single X chromosome. Whether an individual is male 

 or female is determined by the presence of one or two X chromosomes. 



The experiments of C. B. Bridges revealed that the sex of fruit 

 flies, Drosophila, is determined by the ratio of the number of X chromo- 

 somes to the number of haploid sets of autosomes. Males have one X 

 and two haploid sets of autosomes, a ratio of 1 : 2, or 0.5. Females have 

 two X and two haploid sets of autosomes, a ratio of 2 : 2, or 1.0. By 

 genetic techniques possible in fruit flies, Bridges established abnormal 



