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CHAPTER 39 



matic cells. Since the normal male has no 

 Barr body, it is the condensed X which is 

 inactivated. Human sex chromatin is not 

 present at fertilization; it first appears at 

 about the twelfth day of development. 



From Barr body cytology and the inac- 

 tivation of a half dozen or so different loci, 

 we cannot determine how much of an X 

 chromosome is inactivated in a normal hu- 

 man female. Still other X chromosome loci 

 whose gene action can be studied in sepa- 

 rate individual cells, need to be discovered. 

 A gene whose action gives rise to a product 

 diffusible between cells will be of little or 

 no use in determining the length of the in- 

 activated segment. 



Sex chromatin occurs in many mammals H 

 besides human beings. In the mouse, al- 

 though sex chromatin is absent one of a 

 female's X's is heteropycnotic during mito- 

 sis. One locus has been found which fails 

 to show the inactivation expected, 7 suggest- 

 ing the presence of an X chromosome re- 

 gion not heteropycnotic in the female and, 

 therefore, not routinely inactivated. Since 

 reciprocal X-autosome translocations occur 

 in mice, we may ask whether such rearrange- 

 ment has any effect upon the functioning of 

 the rearranged autosomal genes. This ques- 

 tion can be studied in females heterozygous 

 for such a translocation when the nontrans- 

 located autosomal homolog carries suitable 

 recessive alleles of genes whose loci span 

 a large portion of the linkage group. In 

 some cases the phenotype is that of the nor- 

 mally dominant allele present in the struc- 

 turally rearranged autosome; in others, a 

 mottled or variegated phenotype results. 

 Moreover, according to studies of different 

 rearrangements between a given autosome 

 and the X, in the latter cases, the portion 

 of the body showing the recessive pheno- 

 type decreases as the distance from the au- 



'■See M. L. Barr (1959). 

 •See L. B. Russell (1963). 



tosomal locus to the point of union with 

 the X increases. Consequently, autosomal 

 loci can be inactivated by translocation to 

 the X, the greater the distance from the 

 breaking point, the less inactivation. Since 

 breakage in an autosome occurs at several 

 positions, the decreasing strength of inac- 

 tivation has been found to proceed in either 

 direction. Thus, inactivation of autosomal 

 loci in X-autosome rearrangements can ex- 

 plain cases of variegated-type (V-type) po- 

 sition effects. 



In two instances, the autosomal break 

 points involved in rearrangement with the 

 X were located in slightly different positions; 

 both, however, were close to a given gene. 

 In one case, the normal autosomal allele 

 was inactivated and produced variegation; 

 in the other no variegation occurred. Re- 

 sults from the investigation of the latter 

 case strongly suggest the nonsuppressed 

 wild-type gene was in an autosomal frag- 

 ment which had joined an X region inca- 

 pable of causing inactivation. These studies 

 also suggest — as does the already-mentioned 

 finding of an apparently unsuppressible X 

 locus — that, normally, in the segment re- 

 placed by the autosomal fragment some X 

 chromosome genes near the point of break- 

 age are always functional. There are, then, 

 two possible reasons for nonvariegation of 

 an autosomal gene attached to the X chro- 

 mosome: attachment to a portion of X in- 

 capable of causing inactivation and excessive 

 distance from a portion of X capable of 

 causing inactivation. 



In light of the preceding discussion, per- 

 haps the same genetic material can be het- 

 erochromatic or euchromatic, the primary 

 determinant being the degree of coiling. 

 Chromosome hypercoiling during interphase 

 seems to prevent the functioning of the gene 

 contents. In mammals, this system appar- 

 ently compensates for the difference in gene 

 dosage existing between male and female. 



