Genetics 47 



There are other means of producing chromosomal maps, liowever, 

 and these give a check on the method. For example, it is possible to 

 map chromosomes by studying the effects of induced deletions of 

 small portions of the chromosome and of chromosomal changes such 

 as inversions and translocations, as well as other techniques. Tliese 

 have confirmed the linear order of the genes mapped by crossoNcr 

 studies, but these maps vary from genetic maps, often strikinglv, iu 

 spacing and other details. Regions of the chromosome that are 

 heterochromatic and that seem to lack genes with major effect are 

 not easily studied by the recombination analysis; in the main these 

 are responsible for the differences. Progress has been made in local- 

 izing specific genetic effects at visible regions of the chromosomes in 

 Drosophila (with its giant polytene salivary-gland chromosomes) 

 and in Zca (where the chromosomes have characteristic chromo- 

 meres visible under the microscope). 



Sex Chromosomes 



A specialized sort of linkage occurs in animals and plants with differ- 

 entiation of sexes. In these organisms where there are special sex 

 chromosomes, as opposed to the other chromosomes (known as 

 autosomes), one sex usually has two homologous sex chromosomes. 

 The other sex has only one chromosome homologous with these and 

 either lacks the second or has another only partiallv homologous 

 chromosome. In Drosophila and man the female is the homogametic 

 sex with two X chromosomes (everv gamete contains an X), while 

 the male is heterogametic with an X chromosome and its partial 

 homologue Y (there are two kinds of gametes). It is clear that the 

 transmission of genes that are located on the sex chromosomes will 

 be different from those on the autosomes. Furthermore, the charac- 

 teristics affected by these genes will show genetic linkage with sex. 

 Sex chromosomes in these organisms differ from the autosomes in 

 that they are specialized into two different regions. A portion of the 

 two different sex chromosomes in the heterogametic sex will synapse 

 and crossing-over may occur. In contrast to these pairing segments, 

 there are the differential segments of the X and Y that do not pair. 

 The differential segment of the Y usually contains few if any genes 

 with detectable effect, and it is heterochromatic and smaller than 

 the differential segment of the X. When it does carry genes, they 

 are passed from father to son and females never show the traits in- 

 volved. The heterochromatic portion of the Y is necessary for fer- 

 tility; therefore it cannot be completely without effect. 



In some groups the female is the heterogametic sex, while the male 



