citations

http://www.cdc.gov/ncbddd/sicklecell/index.html

http://sickle.bwh.harvard.edu/scd_inheritance.html

http://www.mayoclinic.com/health/sickle-cell-anemia/DS00324

http://www.mamashealth.com/sickle_cell.asp

http://www.google.com/imghp?hl=en&tab=wi