When writing up a punnett square, you need a box with however many possible combinations of genes. Whichever letters you choose is completely arbitrary.

your example, spinner x super pastel:
S = spider, s=normal at spider loci, P=pastel, p= normal, N=pinstripe, n=normal

Now since there are 3 genes in this cross (spider, pin, pastel) you need to take into account all 3 of them.

There are 4 combinations of genes in each gamete possible for the spinner: SNp, Snp, sNp, snp. So there will be four columns along the top with each column headed by one of those gene combinations.

The super pastel only has one gene combination possible: snP.

So for this breeding, you would have a 4 x 1 box.
You end up with 1/4 SsNnPp (spinner blast), 1/4 SnnPp (bumblebee), 1/4 ssNnPp (lemon blast), 1/4 ssnnPp (pastel).

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