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GRADUAL OR DISCONTINUOUS. "Was recombination suppression in mammals a discontinuous process (due to successive inversions or translocations), or did it evolve gradually?" http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn

NO CROSSOVER. "There is now evidence from several taxa (mammal, birds and plants) for more than a single event of cessation of recombination between X and Y chromosomes. The extent of DNA sequence divergence between homologous X/Y gene pairs of a single species has been found to vary greatly among genes, suggesting that different regions of the sex chromosomes have been nonrecombining for very different amounts of time, given that sequence divergence begins once crossing over between X and Y stops." http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn

SINGLE LOCUS. "By this type of process, a single locus takes control of sex determination, that is, derived states with a single sex-determining gene can evolve. Single locus sex determination, such as that of mammals, with a single Y-linked gene, SRY, controlling sex, might also be able to evolve directly, but no detailed model has yet been worked out to show how this could occur." http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn

NEW SEX GENES. "Another possible type of change is the emergence of a new sex-determining gene, epistatic to the previous sex determiner, so that the original genes no longer control sex development, as in the fly Musca domestica, and probably also in the medaka fish, Oryzias latipes. The sex-determining region of the fly, Megaselia scalaris, appears to transpose between chromosomes, creating new potential sex chromosomes." http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn

"TRANSPOSITIONS of autosomal genes have occurred onto established Y chromosomes in mammals. All genes known on the D. melanogaster Y appear to be male function genes without X chromosome counterparts, which were probably acquired from the autosomes." https://lnkd.in/gCh5RAX View in LinkedIn

RECOMBINATION. "Finally, sex chromosome systems, once established, have been modified in various ways that have no direct consequences for recombination frequencies." http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn

CLEAR MORPHOLOGY. "Likewise, X-autosome fusions generate XY1Y2 systems, Z-autosome fusions generate ZW1W2 systems, and W-autosome fusions generate Z1Z2W systems. These neo-sex chromosome systems can often be identified by light microscopy, without molecular cloning or linkage mapping." https://lnkd.in/g2CPWtY View in LinkedIn

ODD NUMBER. "A fusion between a sex chromosome and an autosome is usually detected because it creates an odd number of chromosomes in one sex. With XY sex determination, a Y-autosome fusion creates an X1X2Y system, with the unfused homologue segregating as a neo-X chromosome." https://lnkd.in/g2CPWtY View in LinkedIn

ACCUMULATED VIROIDS. "The large chromosome size does not imply absence of genetic degeneration, nor a high expressed gene content. On the other hand, accumulation of repetitive sequences such as transposable elements on the Y does not necessarily imply degeneration. It may merely be a sign that recombination is rare." http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn

"NEO-SEX chromosome systems are known in many plants and animals, including XY1Y2 in the plant Rumex acetosa, and multiple sex chromosomes due to translocations in African misteltoes, termites, African pygmy mice and monotremes." http://www.nature.com/hdy/journal/v95/n2/full/6800697a.html View in LinkedIn