Exactly, it's insidious because you can't really prove something by its absence. While it's easy to prove your spider isn't homozygous by it producing 1 non spider offspring it's hard to prove it is homozygous by it not producing any normal offspring. If your spider produces only a large number of spider offspring maybe you where just very lucky with a heterozygous spider. We can however put numbers on how lucky you would have to be to help you decide if it’s proven to your satisfaction or not. The following shows the odds of producing x for x number of spiders using a heterozygous spider and just being lucky. The first column is the number of spiders produced in a row and the 2nd column out of how many heterozygous spiders you world expect to see one start out with this many in a row:
1 2
2 4
3 8
4 16
5 32
6 64
7 128
8 256
9 512
10 1024
11 2048
12 4096
13 8192
14 16384
15 32768
So, for example, if your spider produces 10 for 10 spiders out of his first babies with normals is he a proven homozygous spider or are you just very lucky? This year I produced 5 for 5 normal males from a pastel bred to a normal. If you take into account both not getting any pastels and not getting any females that is 10 for 10 on bad luck coin flips for 1 in 1024 bad luck. Jared Horenstein produced 5 for 5 cinnamon females from a cinnamon to a normal this year so there is a 1 in 1024 good luck example when taking both morph and gender into consideration. I’d certainly be interested in hearing about any spiders that had gone 10 for 10 producing spiders but maybe somewhere around 15 for 15 I would start to consider it a proven homozygous spider. But even then there is still the chance it’s just the lucky 1 in 32,768.