By now it may be obvious that the author emphasizes the historical importance of the discoveries that made cytogenetics the science it is today. That is why the first chapter was written in such detail and the student is often referred back to it in order to freshen his recollection. The first studies of chromosomes that determine sex were undertaken at the end of the last century. As mentioned in Chapter 1, Henking in 1891 for the first time described what is now known as the X chromosome. Half of the sperm of the insect Pyrrhocoris apterus received this chromosome and half did not. This system is now known as the X-O system. A much more common system is the X-Y system, which will be discussed first.
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