Each atDNA testing company shows a slightly different table of cMs for each chromosome. You can see a report of cM for various companies at http://www.isogg.org/wiki/CentiMorgan. Don’t get hung up on the exact numbers – it’s the overall concept that counts. The average number of crossovers is the cMs in that chromosome divided by 100. Since the number of crossovers per chromosome must be a whole number, I’ve shown several alternatives above. Although the average may be 1 or 2 or 3, possibilities may include 0 or 4 (or more sometimes). The point is that there are only a few crossovers expected for each chromosome, in one generation, and if more occur on some chromosomes, there tends to be fewer on some other chromosome. This process occurs in each generation – for instance when a parent recombines the grandparent’s chromosomes and passes a single chromosome to you. It also happens when a grandparent recombines the Great grandparent’s chromosomes and passes a single chromosome to your parent. So there are about 35 crossover points at each and every generation, and they add up, depending on which generation is under consideration. This will be described for each generation in more detail below.