Iris Cohen schreef
Perhaps what he meant is that if you have two genes from different loci on
the chromosome, they are not alleles. Alleles are two different genes found
in the same location on different chromosomes, having a different effect on
the same trait, such as a gene for a normal pigment enzyme and a defective
gene which does not create that enzyme. When you breed a plant with the
defective gene on both sets of chromosomes, you get white flowers.
Iris,

+ + +
I doubt he meant anything really.
Genes tend to have fixed positions (ie occur on a certain spot on a certain
chromosome), but this is no requirement. Also the fact that many different
genes can have an effect on a certain property does not make the discussion
easier. Alleles are the same gene, in another edition. Usually alleles are
found on the same chromosome (ie on a particular chromosome, not both on the
same copy of that particular chromosome), and as chromosomes come in pairs
there are usually two alleles present. But this is all very general:
pictures do help (a lot).
PvR