Even when both the parents are Rh positive, children born to them can be Rh negative. How does this happen?
Rh typing is one among the 30 different blood grouping system currently in use. In this grouping human beings are differentiated as Rh positive and Rh negative based on the presence or absence of an antigen (a type of protein). Here Rh refers to the fact that it was first found in Rhesus monkeys.
This type of blood grouping was discovered by Karl Landsteiner and Alexander Wiener after 40 years of discovery of ABO blood grouping. Rh typing is genetically most complex blood typing system than others.
Like other traits, Rh factors are under the control of genes and it follows a common pattern of genetic inheritance.
Offspring receive a copy of gene from their parents, so that they are having a pair of genes for Rh factors. Since Rh positive (Rh+) gene is dominant over Rh negative (Rh-), a single Rh positive gene (i.e. Rh+Rh-) is enough to express its trait.
In the case of Rh negative which is recessive, to express the character both genes must be negative (Rh-Rh-).
The genotype of Rh positive person may be have Rh+Rh+ (homozygous) or Rh+Rh- (heterozygous) genotype and it is Rh-Rh- (Homozygous) in negative persons.
Homozygous Rh positive (Rh+Rh+) parents produce only one type of gametes with Rh+ gene.
Likewise Rh negative parents (Rh-Rh-) produce games with only Rh- genotype. Union of these gametes Rh+ and Rh- produces an offspring with Rh+Rh- genotype which is positive.
But Rh positive heterozygous parent (Rh+Rh-) produces two different types of gametes i.e. Rh+ and Rh-. Fusion of these gametes (Rh+ or Rh-) with Rh- negative parent genotype gives two possible combinations: Rh+Rh- (Rh positive) and Rh-Rh- (Rh negative).
While both parents are heterozygous (Rh+Rh-) they produce Rh+ and Rh- gametes. The union of Rh- gamete from father and Rh- gamete from mother results in Rh negative child. The other possible combination are homozygous and heterozygous Rh positive children.
Dr. P. MARIAPPAN
Assistant Professor of Zoology
Rajah Serfoji Govt College
Thanjavur, Tamil Nadu