Why does crossing over promote genetic variation?

Question

why does crossing over generally occur in long homologous chromosomes?

Answer 1

Crossing over is the exchange of corresponding alleles at the same locus between non- sister chromatids in homologous chromosome pairs at metaphase 1 in meiosis. This shuffles the alleles and produces new combinations of alleles in the gametes produced.

The furthe apart the loci are on the chromosomes the greater is the chance of a chiasma forming (point of attachment between non-sister chromatids) and therefore of crossing over taking place - this will therefore happen more on long chromosomes.

Answer 2

Some additional information about crossing over.  Genes that are carried on the same chromosome are said to be linked and usually are inherited together.  Linkage decreases the likelihood of new combinations of the linked genes.  So a chromosome with the gene sequence of ABC would most likely be inherited as ABC.  If the organism is heterozygous with abc on the other chromosome, the gametes would get either ABC or abc.  With crossing over, there new gene combinations possible such as ABc, AbC, Abc, aBc, aBC, etc.  The new combinations may produce offspring that have an advantage in survival.

Geneticists also used the crossing-over frequency to determine the sequence of genes and distance of one gene away from another gene long before modern techniques were available.