Quote Originally Posted by paulh View Post
I've seen a number of web sites that say there is a gene on each end of a "rung". That is wrong.

Two genes are alleles because they are at the same locus (location in the DNA in the chromosomes). It takes hundreds or thousands of "rungs" in a DNA ladder to make a gene, and each DNA "ladder" contains millions of "rungs". The sequence of "rungs" determines the gene's makeup. And this is a pretty simplistic description.

Chromosomes come in pairs -- 23 pairs in humans, 18 in boa constrictors, and (I think) 18 in ball pythons. There is one DNA "ladder" in each chromosome of a chromosome pair. The corresponding locations in the two chromosomes of a chromosome pair are the same locus. We say there is one locus with two alleles, but it must be understood that the two alleles are in separate chromosomes.

The genes for two different traits are in the DNA "ladders" in different chromosome pairs. Or they are different series of "rungs" in different locations in the same chromosome pair.

A sperm or egg gets one chromosome from each parental chromosome pair. When a sperm and egg fuse, the chromosome pairs are reestablished, which also reestablishes all the gene pairs.

A pair of chromosomes:
-----a----B---------- = the DNA ladder in one chromosome.
-----a----b---------- = the DNA ladder in the other chromosome. Each - stands for a gene. The a and b genes are different genes in different loci. The B and b genes are alleles.

By the way, if you see an X-shaped structure in a cell's picture, it is not a chromosome pair. It is a single chromosome that is dividing to become two rod-shaped chromosomes.

Hope this helps. A genetics text will have a more detailed explanation.
Ok, so my genetics vocab was off, correct? As in, If you picture it as a ladder (Not saying the DNA ladder) a ladder as in a visual reference for comparing two Chromosomes. That two chromosomes "match up" if they are allelic. There are at the same locus/rung (Location on the dna strand) and therefore create the "acts like super/ allelic mutation" that we call say, a vanilla cream/superstripe etc.

In other words, aside from the actual breakdown of the DNA strand = ladder, and the use of the word "gene", I got the concept right? That the two chromosomes are allelic because they are on the same locus in the DNA strand, and therefore would not be able to have all four of those allelic chromosomes (super specter ivory) in a single snakes genetic make up?

I'm just trying to wrap my brain around all the vocabulary, while still keeping it simple enough that people will read it without their eyes glazing over lol. Thanks a bunch for the help Paul, it is much appreciated!