Y is still possible. Because the male specific region is very small and contains no essential genes, it is even possible to artificially induce XX males and YY females to no ill effect. Multiple XY pairs Monotremes possess four or five (platypus) pairs of XY sex chromosomes, each pair consisting of sex chromosomes with homologous regions. The chromosomes of neighboring pairs are partially homologous, such that a chain is formed during mitosis. The first X chromosome in the chain is also partially homologous with the last Y chromosome, indicating that profound rearrangements, some adding new pieces from autosomes, have occurred in history. Platypus sex chromosomes have strong sequence similarity with the avian Z chromosome, (indicating close homology), and the SRY gene so central to sex-determination in most other mammals is apparently not involved in platypus sex-determination. Human Y chromosome In humans, the Y chromosome spans about 58 million base pairs (the building blocks of DNA) and represents almost 2% 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 Y-linked traits, or holandric traits (from Ancient Greek ὅλος hólos, "whole" + ἀνδρός andrós, "male"). Loss of Y chromosome Men can lose the Y chromosome in a subset of cells, which is called the mosaic loss of chromosome Y (mLOY). This post-zygotic mutation is strongly associated with age, affecting about 15% of men 70 years of age. Smoking is another important risk factor for LOY. 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 LOY (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. However, it was thought that the loss of Y could also be an effect rather than a cause and/or a "neutral karyotype related to normal aging". In 2022, a study showed that blood cells' loss of the Y chromosome in a subset of cells (mLOY), reportedly affecting at least 40%