586 



COLLEGE ZOOLOGY 



Other half of the males are color-blind, hav- 

 ing the gene in the X chromosome and no 

 normal gene in the Y chromosome. 



When a normal father is mated to a 

 color-blind mother, all sons, but none of the 

 daughters, are color-blind; and half of the 

 grandsons and half of the granddaughters 

 are likewise affected. Traits such as color 

 blindness are said to be sex-linked, since 

 their genes are borne by the X chromosome 

 that also bears the genes influencing sex. 

 Hemophilia (bleeder's disease) is another 

 well-known sex-linked trait. There are many 

 sex-linked traits; for example, the genes for 

 over 30 sex-linked traits are known for man, 

 and about 150 sex-linked genes have been 

 found in the X chromosome of the fruit fly. 



Linkage 



It has been found by breeding experi- 

 ments that the genes borne by the ordinary 

 chromosomes or autosomes are also linked 

 together in heredity, as one would expect, 

 since at the reduction division, during 

 maturation, entire chromosomes are sepa- 

 rated. The number of linkage groups in an 



organism has always been found to be equal 

 to the reduced number of chromosomes. 

 Thus in the fruit fly, 4 groups of linked 

 traits have been discovered, the genes for 

 which have been located in the 4 chromo- 

 somes. Special technics have been developed 

 in recent years for study of autosomal link- 

 age in man. As a result, two well-established 

 cases of autosomal linkage are known. 



Crossing over 



The clue to the localization of genes in 

 the chromosome was furnished by the phe- 

 nomenon known as crossing over. Certain 

 crosses between fruit flies did not give the 

 expected Mendelian ratios. The explanation 

 suggested was that during the union of the 

 chromosomes (synapsis), at the time of the 

 reduction division and the subsequent sepa- 

 ration of homologous chromosomes, an ex- 

 change of parts occurred between these 

 homologous chromosomes. In other words, 

 parts of a chromosome crossed over from one 

 chromosome to its mate (homologue). A 

 diagram of how this might occur is presented 

 in Fig. 414. 



CI 



ID 



12 3 4 



Figure 414. Diagram of crossing over during maturation (meiosis) with exchange of genes 

 between homologous chromosomes. A, a; and B, b represent allehc pairs of genes in homologous 

 chromosomes, which pair in synapsis: (1) pair of homologous chromosomes; (2) each chromo- 

 some becomes duplicated to form four chromatids (a tetrad); (3) crossing over takes place 

 when one chromatid of each chromosome breaks and becomes reattached to the chromatid of 

 the opposite (homologous) chromosome; (4) in the anaphase as shown by the arrows, the two 

 groups of chromosomes go to opposite ends of the cell. 



Location of genes 



During the process of crossing over, those 

 genes that are closest together in the chro- 

 mosome would tend to remain together 

 when the homologous chromosomes ex- 



change parts. With this assumption as a 

 basis, and with the results of thousands of 

 breeding experiments as material to work 

 with, the locations of hundreds of genes 

 have been determined in the chromosomes 

 of the fruit fly {Drosophila) . 



