186 FUNDAMENTALS OF CYTOLOGY 



When the chromosome map constructed on the basis of such data is 

 compared mth the Unkage map for the same chromosome, it is found 

 that the two agree with respect to the serial order of the genes (Fig. 131). 

 This is gratifying proof of the value of the crossover method long used 

 for the determination of such order. The spacing of the genes, however, 

 differs in some regions, showing that the occurrence of crossing over 

 is not uniform in frequenc.y throughout the length of the chromosome 

 as was originally assumed in constructing the linkage map. A lower 

 frequency in certain chromosomal regions, notably near the kinetochore, 

 yields recombination percentages lower than the average; hence the genes 

 in such regions appear closer together in the linkage map than they do 

 in the chromosome map showing the true positions. The successful 

 construction of such chromosome maps accurately summarizing large 

 bodies of observational data is surely one of biology's major achievements. 

 Their usefulness and appearance suggested an earlier remark (page 97) 

 that the salivary-gland chromosome is a sort of ''biological spectrum" 

 indicating the genetical composition of the organism. 



The Special Case of Sex. — Sex, like other characters of the organism, 

 has a genie basis. Its inheritance in dioecious organisms, however, 

 differs from that of other characters because of a special type of chromo- 

 somal mechanism. This specialization is manifested in the differentiation 

 of one chromosome pair from the others in its influence upon sex and 

 often in its visible morphology. The members of this pair are therefore 

 known as sex chromosomes, although the other chromosomes, called 

 autosomes, may also have some share in the determination of sex. In 

 addition, there is a further differentiation of the sex chromosomes into 

 two kinds, usually designated as X and Y (Figs. 132-136). The indi- 

 viduals of one sex, nearly always the female, have two similar chromo- 

 somes {XX), while those of the other sex have two unlike ones (XY). 

 In a few organisms, notably birds and lepidoptera, the females have the 

 XY pair. In some species there is no Y, one sex therefore having only 

 one sex chromosome. Again, the X or the Y may comprise two or more 

 elements. In certain monoploid organisms, e.g., bryophyte gametophytes 

 and some algae, the single genome includes an X in the female and a Y 

 in the male, both X and Y being present in the zygote and the asexual 

 sporophyte. 



In all the above cases the behavior of the sex chromosomes in meiosis 

 and syngamy normally results in an approximate numerical equality of 

 the two sexes. This is shown in the following examples. 



The XY type referred to above is the one of most frequent occurrence, 

 being found in plants and animals of many natural groups. In Fig. 132 

 it is seen that the disjunction of the two X-chromosomes in the female 

 animal yields gametes of but one class: every egg carries an X. Dis- 



