DNA (well, nuclear DNA, the stuff which, generally speaking, people are talking about when they talk about genetics) is arranged on chromosomes. Imagine you have a paired set of pieces of string. Say, eight pieces of string, two of them are 4cm long, two are 5cm long, two are 6.5cm long and two are 12cm long (all arbitrary). The two sets are essentially identical, you got one from mum and one from dad. These pieces of string don't really do anything, they are just a framework you can stick important stuff on to. Imagine now that the pieces of string have knots along them. The pairs have the knots in the same place. Okay, now imaging that at each knot there is a note which says something like "green eyes", "tall", "short", "smart", "freckles" etc etc. If you line the pairs of string up, each knot will be next to a knot in it's homologous partner. Now look at the notes, you might find a pair which says "Short" "Short", in which case you'd be a short person. Another pair might say "idiot" "genius", in which case you'd either be an idiot (if idiot was dominant to genius), a genius (if genius was dominant to idiot), somewhere in between (if they were codominant) or, perhaps, you might be an insane genius, if the affect was additive (this is a bit advanced and certainly way beyond the layman, I won't get into it further).
So, the strings are chromosomes, the knots are loci and the notes are genes. It doesn't matter what the note says, if it's at the same place as another note, they're the same gene. An allele is a different type of note/gene. (A height locus ('knot') might have four alleles - pygmy, short, tall, giant).
Within a species, every individual has the same number of pieces of string with the same number of knots in the same places. Every knot always has a note.
Generally, we hideously oversimplify and what I've described is only a tiny amount of what is going on. We generally only discuss Mendelian genetics, which only represent a small percentage of the traits we're interested in (height, intelligence, how yellow a jungle carpet is, whether or not a Black headed python will eat mice, etc etc are not Mendelian and the situation is far more complex).
Before anyone points it out, yes, I deliberately left out sex chromosomes, there are gene duplications, I know I oversimplified, there are haploids and polyploids, etc etc etc.
2.40am is probably not the best time to be explaining this
I'll probably read it in the morning and think "string, knots and notes" what on earth? :shock:
:lol: