Day 19 has 209 protein-coding genes, more than any other day of Chromosome 1. On this very gene-dense day is the gene ACKR1 (atypical chemokine receptor 1) which encodes the famous Duffy chemokine receptor.
Like A/B/O blood types and the Rhesus antigen, the Duffy receptor is the basis of a blood type, where different people have different versions of the protein on blood cells. The Duffy receptor is the entry point for malaria into blood cells.
ACKR1 is one of the few places in the genome where there are strong differences between ethnic groups. An inactivating mutation is found in almost all people of West African descent and almost no people of European descent. Strong geographic differences like these are almost always due to natural selection, and the ACKR1 inactivation is thought to confer protection against malaria where it is endemic.
Click here to see the Fyb- mutation, which will be flashing. Note that it is before the protein-coding part of the gene. It disrupts the binding site for a transcription factor needed to turn the gene on in blood cells.