In genetics,
locus refers to the physical location on a chromosome where a gene is found.
http://ib.bioninja.com.au/standard-l...-and-loci.html
When he says "gene frequency data for some loci," I don't think he's talking about geographic loci where Jews congregate, but the comparative allele frequency between Jewish and Gentile populations at chromosomal loci.
From wikipedia:
Allele frequency, or gene frequency, is the relative frequency of an allele (variant of a gene) at a particular locus in a population, expressed as a fraction or percentage.[1] Specifically, it is the fraction of all chromosomes in the population that carry that allele.