Thread: Question about Genetic Units

Dawkins writes in TSG that the smaller a genetic unit is, the less likely it is to be broken down during the meiotic process. This makes sense, but then he goes on to say that the smaller genetic units will be usually be more easily passed down from generation to generation. I'm pretty sure I'm misunderstanding this but I have a question: Doesn't meiosis randomly pick genes from each corresponding chromosomes to be fused as one and therefore even if smaller genes don't get broken up, their chances of being passed down in a single sperm cell decreases- instead of increases; a larger unit might be broken up but it would be passed down.

Dawkins states that a genetic unit with a size of 1/100 of a chromosome would have a 99% chance of getting passed down. How?

Where am I going wrong?

All he's really saying there is that alleles that lie close together have a lower recombination frequency than those that lie farther apart; that is, they exhibit varying degrees of linkage. The larger a block of alleles is, the greater the chance that it will experience a cross-over event.


More specifically he is saying that the longer a DNA sequence (however you decide to define it) is, then the more likely it is to be broken up by a cross-over event. A genetic unit (in Dawkins speak) could be an entire chromosome; in this case then it almost certain that this large unit will be split up.


In the example you used I think it assumes - I'm purely guessing here - at the very least one cross-over per chromosome per meiotic division (100% chance of a crossover); 50% chance for a unit the size of half that chromosome etc. Continuing the arithmetic, a unit of size 1/100 of the chromosome would have a 99% chance of NOT being split. Or, to put it another way: a bigger target is easier to hit than a smaller one.




Nice image showing recombination:




image022.jpg