Why does % recombination max out at 50% for the linked genes?

this questions is in relation to linked genes? why can not the % recombination exceed 50%

Why does % recombination max out at 50% for the linked genes?
Imagine a case where there is NO linkage at all. Two genes are located on different chromosomes, so that during meiosis the segregation of alleles of one gene is completely independent of alleles of the other gene (this is stated in Mendel's Second Law and is known as the law of independent assortment).





Start by crossing pure-bred homozygote parents AABB x aabb. The parental gametes will be AB and ab. F1 generation offspring will ALL have the genotype AaBb. With independent assortment the F1 offspring will produce the following gametes:


AB (25%) - present in parental generation


Ab (25%) - NOT present in parental generation


aB (25%) - NOT present in parental generation


ab (25%) - present in parental generation





50% of the F1 gametes were not present in the parental generation, so are recombinant gametes (recombinant gametes are those gametes that differ from both of the haploid gametes that made up the diploid cell). The recombination frequency is 50%.





If the genes ARE linked then when the F1 gametes are formed the alleles of the two genes will not assort independently. The A and B alleles will have been located close to each other on the same chromosome from the AABB parent (the same is true for a and b). The closer together the two genes are, the less likely it is that cross-overs will occur between them, so when gametes are formed there will be more AB and ab gametes, and fewer Ab and aB. Thus linked genes have a recombination frequency that is less than 50%.





The recombination frequency CANNOT be greater than 50%. For it to be greater than 50% then the Ab and aB gametes would have to be MORE frequent in the F1 generation than the parental types (AB and ab). This would mean that crossovers between the linked genes would have to be MORE likely to occur than not, and only in one direction. This CANNOT happen. Any (hypothetical) mechanism that could promote crossing over to occur between the AB and ab chromosomes from the parents (resulting in Ab and aB), would also cause crossovers to happen in the other direction (resulting in AB and ab chromosomes again). No matter how very high the rate of crossing over this can only ever result in random assortment (giving a recombination frequency of 50% as above).