Genotyping uses SNP microarrays, it only looks at certain positions (bases or single nucleotide polymorphisms – SNP) in the genome, for example 23andme looks at about half a million positions but the human genome has 3 billion positions. Gene expression is something completely different. It describes what the genes do. Genes get expressed and then transcribed into proteins. To measure Gene Expression, one measures mRNA molecules through transcriptomics