'Crossing Over' ...

iansmith

Most of the genes do have a amino acid sequence that are identical or similar (meaning that a a.a from a certain group [+ charge, - charege, polar, non-polar and aromatic] is change by another a.a from the same group.

Some genes do have different a.a sequence. For example, a +ve charge a.a is change by a -ve charge a.a. This change migth not affect the function of the protein. In the other hand, the protein could become non-functional or functional under certain conditional (refer to as conditional mutant). Other mutation will introduce a stop codon and the protein becomes non-functional.

Most genes from individuals are not that different but some genes show a lot of different phenotype, eye and skin color for example. Crossing over can be beneficial for individuals that carries dangerous mutation. Crossing over also allow the exchange of gene and a mixture of genes in the population.

Monique

Basically you have got two genes, paternal and maternal. These two genes can be completely homologous, than both parents will have blue eyes (hypothetically). Or, the genes have a small difference in the first exon, two alleles. Now the one gene codes for blue eyes and the other for green eyes (hypothetically). Imagine crossing over occurs in the middle of the gene. Since only the first exon is different, and the rest is the same.. only switching of the genotype occurs from paternal to maternal chromosome.

You must understand that genes are highly conserved sequences, there really isn't that much variation in them to cause massive structural changes when they are interchanged.

I have sequenced genes of unrelated individuals, they are completely the same!

Crossing-over DOES have an evolutionary advantage though, a genome is reshuffled, creating more genomic diversity and less inbreeding, simply stated :)

Monique

It is also important to keep in mind that only about 2% of the genome is coding, less than 1% exons! (heard that in a lecture, not sure where to get the data from)

iansmith

A long explantion is that the population of a given specie has a mix of individuals traits and is heterogenous. Therefore when a massive selection pressure (i.e. disease) appears not every individuals are wipe out and the few that are left can rebuilt the population and the specie.

Also recombiantion can also have a bad effect. Non-functional and disfunction (I don't know if it's the rigth term) can be pooled together and pass on.