"The P values of Bonferroni corrected thresholds for suggestive, 5 and
1% genome-wide significant levels were 1, 0.05 and 0.01, respectively,
divided by the number of SNPs used in the GWAS. The suggestive level
was first proposed by Lander and Kruglyak [17] and represents the
threshold where, under the null hypothesis, one false positive is
expected per genome scan."
So the suggestive line is calculated as the -log10( 1 / number of variants)
Reference: Guo, Y., Huang, Y., Hou, L., Ma, J., Chen, C., Ai, H., ... & Ren, J. (2017). Genome-wide detection of genetic markers associated with growth and fatness in four pig populations using four approaches. Genetics Selection Evolution, 49(1), 21.