jaumzaum

Hello. I'm studying genetic linkage and the recombination frequency calculation (that is assumed to be directly proportional to allele distance) considers that only one cross over will take place. I know this is an estimation, but I want to know what the biological mechanism is that justify the probability that EXACTLY ONE crossover will take place.In other words, what is the probability of no cross overs taking place, or a double cross over, or a triple cross over, and why are these probabilities lower than for a single crossover? And also, it is considered that all regions in the chromosome have the same chance to suffer a cross over, is this correct? If not, what are the most probable and the least probable sites and why?

I was looking for a type of organism in which the was no differentiation between the gametes, both are 100% identical (of course, there could be mutations, but no allele differentiation in matching-type regions, like the positive + and negative - mathing types in some algaes), and with a sexual reproduction. In isogamous organisms we have a small genetic differentiation between the gametes, as the + and - one I said. Is there an organism like this in nature? Also, is there any isogamous organism in which the gametes are produced in the same organ or region?