Recombination

This was one of the confusing things about early genetics but when you have a pair of chromosomes the DNA from one, can swap with the other. Sometimes it is a small bit and sometimes a larger piece. Not only that but some can switch back again. The long strand of DNA,about 2 metres long when stretched out, is often broken and repaired. Sometimes the DNA is just attached back to the other chromosome of the pair.

So this action now confuses any inheritance pattern that we might expect because it is a random event. Some generations it might happen and some it will not. In the 22 pairs the recombination rate is fairly even across all the chromosomes but where the breaks occur is not even along its length. This makes sense as a break might occur inside a working gene and the risk factor of it being non-functional is too high. The X and Y chromosomes can technically swap DNA but it is extremely rare because the two chromosomes have a lot of differences.For genealogists this is not an issue to worry about

It is worth mentioning here that cM values in genealogy relate to the chance of DNA swapping positions and being lost - "lost" meaning not inherited because that chromosome is not passed on. A value of 100cM means the segment had more chance to be lost than a 10cM match. That means the relationship between two people with a 100cM match is closer than a 10cM match. - there were less generations and less chance of recombination.

This diagram shows one chromosome pair for each parent. Each parent donates one of the pair to the new child.This produces four, different, possible outcomes for each child (only one chromosome from 23)

This last diagram shows the four possible combinations of chromosomes that have re-combined. It shows how, the mixing of the DNA by swapping and the chance of each chromosome being inherited, provides a lot of variation?

That is why the amount of DNA transferred to each generation is not a straightforward mathematical equation. In the chart below there are ranges of inheritance because the variation of how much DNA is swapping between chromosomes is variable each time it happens and it may be a single event or a double event

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