Explanation
The genetic code of apple snails is, like in all life forms (except retro-viruses) stored in DNA. Each cell of the snail has 2n DNA strands in its nucleus, and each strand is available in two, nearly identical copies. So there are n pairs of DNA strands, and fom each pair, one strands is received from the father, the other from the mother.
In a normal cell situation, the DNA strands are invisible with a microscope, but when a cells is about to divide into two new cells, the DNA is duplicated and each old strand and its new duplicate is compacted into a chromosome. The chromosomes are, in constrast to the non-compacted DNA strands, visible under a microsope (that's why the chromosomes got so much attention in the past before DNA was well understood).
Now about the colour genes: somewhere in the DNA, there are sequences that encode the way the pigments are to be build (such sequence is called a 'gene'). These genes can consist of a single piece of DNA, or several pieces that depends on each other.

'Shell colour genes' section.

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