<P> In humans, the Y chromosome spans about 58 million base pairs (the building blocks of DNA) and represents approximately 1% of the total DNA in a male cell . The human Y chromosome contains over 200 genes, at least 72 of which code for proteins . Traits that are inherited via the Y chromosome are called holandric traits (although biologists will usually just say "Y - linked"). </P> <P> Some cells, especially in older men and smokers, lack a Y chromosome . It has been found that men with a higher percentage of hematopoietic stem cells in blood lacking the Y chromosome (and perhaps a higher percentage of other cells lacking it) have a higher risk of certain cancers and have a shorter life expectancy . Men with "loss of Y" (which was defined as no Y in at least 18% of their hematopoietic cells) have been found to die 5.5 years earlier on average than others . This has been interpreted as a sign that the Y chromosome plays a role going beyond sex determination and reproduction (although the loss of Y may be an effect rather than a cause). And yet women, who have no Y chromosome, have lower rates of cancer . Male smokers have between 1.5 and 2 times the risk of non-respiratory cancers as female smokers . </P> <P> The human Y chromosome is normally unable to recombine with the X chromosome, except for small pieces of pseudoautosomal regions at the telomeres (which comprise about 5% of the chromosome's length). These regions are relics of ancient homology between the X and Y chromosomes . The bulk of the Y chromosome, which does not recombine, is called the "NRY", or non-recombining region of the Y chromosome . The single - nucleotide polymorphisms (SNPs) in this region are used to trace direct paternal ancestral lines . For details, see human Y - chromosome DNA haplogroup . </P> <P> The following are some of the gene count estimates of human Y chromosome . Because researchers use different approaches to genome annotation their predictions of the number of genes on each chromosome varies (for technical details, see gene prediction). Among various projects, the collaborative consensus coding sequence project (CCDS) takes an extremely conservative strategy . So CCDS's gene number prediction represents a lower bound on the total number of human protein - coding genes . </P>

In humans the y chromosome contains genes that cause a human being