Linked genes and Crossing over

[Grumpy]

Im my biology text it reads

"....[i]with so many genes, there must be more than one pair of genes on each chromosome. If genes for different characteristics are found on the same chromosome, they are called linked genes"

and

"......[i]sometimes when crosses are done with linked genes, the characteristics produced in the offspring seem to indicate that the linkage has com 'unstuck' and recombination has occured; this is referred to as crossing over. Crossing over is a swapping of chromoid parts of homologous chromosomes early in meisois"

Can anybody explain to me in basic terms what this means. I sure it is quite simple, perhaps i just need it expained a different way

[James]

During meiosis, in order to create genetic variation for the gametes, the 2 chromosomes within a homologous pair 'swap' certain genes. If, whislt crossing over, each chromosome happened to have the same genes, it would appear that the genes had not crossed over.

Heres a diagram: http://www.accessexcellence.org/RC/VL/GG/comeiosis.html