The human X chromosome has a unique biology that was shaped by its evolution as the sex chromosome shared by males and females. We have determined 99.3% of the euchromatic sequence of the X chromosome. Our analysis illustrates the autosomal origin of the mammalian sex chromosomes, the stepwise process that led to the progressive loss of recombination between X and Y, and the extent of subsequent degradation of the Y chromosome. LINE1 repeat elements cover one-third of the X chromosome, with a distribution that is consistent with their proposed role as way stations in the process of X-chromosome inactivation. We found 1,098 genes in the sequence, of which 99 encode proteins expressed in testis and in various tumour types. A disproportionately high number of mendelian diseases are documented for the X chromosome. Of this number, 168 have been explained by mutations in 113 X-linked genes, which in many cases were characterized with the aid of the DNA sequence.

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A monosome was identified as chromosome 7 of the A genome of the tetraploid cottons by its linkage with the Lc1yg2R2 markers. This makes eight chromosomes of the tetraploids now identified by monosomes. For over a decade the N locus has been considered to be linked with the Lc1yg2R2 loci. Monosomic analysis, however, reveals that N is on a separate chromosome. Studies with a telocentric of chromosome 7 show that the Lc1yg2R2 loci are located in the long arm. It is suggested that the R2 locus rather than the Lc1 locus is in the proximal position. These findings are discussed in relation to data, particularly that involving the linkage group, which supposedly illustrate the compensatory recombination phenomenon in cotton.

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