Q: Both my parents' blood group is O +ve. When my brother got his blood test done, it showed O -ve. Is this possible? What precautions should he take for his day-to-day life and marriage?

A:The Rh system is the second most important blood group system after the ABO blood group system. Two closely linked but separate genes on chromosome 1 encode Rh proteins - the RHD gene that encodes the D antigen and the RHCE gene encoding the Cc and Ee antigens. Rh-negative blood group means the absence of D protein on the red blood cells and this results from deletion of the RHD gene on both chromosomes. Rh antigens exist in 3 loci: Cc, Dd, and Ee. Of individuals who are Rh positive, 45% are homozygous (gene present on both chromosomes) and 55% are heterozygous (gene present on only one chromosome). Frequency of Rh negativity is rare in Asians. The paternal heterozygosity determines the likelihood of an Rh-positive child being born to an Rh-negative mother. The Rh system is much more complicated than the ABO system because there are more than 30 combinations possible when inherited, however for general usage, the Rh proteins are grouped into two families - either positive or negative. If the parents' Rh type is Both +, then possible allele combinations are ++ and ++ and the possible Rh in the children are ++ positive. If the parents' Rh type is Both +, then possible allele combinations are ++ and +- and the possible Rh in the children are ++ or +- positive. If the parents' Rh type is Both +, then possible allele combinations are +- and +- and the possible Rh in the children are ++, +- or -- positive or negative. If the parents' Rh type is Both -, then possible allele combinations are -- and -- and the possible Rh in the children are -- negative. If the parents' Rh type is one + and one -, then possible allele combinations are ++ and -- and the possible Rh in the children are +- positive. If the parents' Rh type is one + and one -, then possible allele combinations are +- and -- and the possible Rh in the children are ++ and +-positive negative. Rh negative children can have parents that are either Rh positive or Rh negative. If both parents are Rh negative then the children will be Rh negative. No precautions need be taken.