안녕하세요 ^^

In the question it says, 

"if two gray progeny of cross IV mate with each other"

so we have to determine the genotype of the "gray progeny of cross IV mate"

As you see in ichart,  cross IV is this.  GG * gg --> 100% Gg  

Gray is dominant, and white is recessive . For recessive allele to be express as recessive phenotype, it has to be homozygous recessive.  

All gray progeny (=offspring)  have genotype Gg.  

If two gray progeny  Gg mate with each other, we can draw a punnett square and get the 3:1 offspring ratio of Gray to white.

이해가 되었나요?