Ans. is 'b' i.e., 36 weeks Fetal Hb - (a2, g2)Adults Hb - (a2, b2,)During the initial fetal life, the major hemoglobin synthesized in the body is fetal hemoglobin, because in fetus, the chain synthesized predominantly is the g chain.b chain is present in only trace amount in early embryosThe rate of synthesis of g and b chains and the amount of HbA and HbF are inversely related during intrauterine stageAs the age of the fetus increases, the synthesis of b chains and the amount of adult hemoglobin in the blood increasesAt 36 weeks of gestational age the rate of synthesis of b chain exceeds that of g chain. This is called switch overWhat exactly is switch over?Switch over is the point at which the rate of synthesis of b chain exceeds the rate of synthesis of g chain.Don't confuse it with the concentration of hemoglobin remember that despite having higher rate of synthesis of bchains it is not HbA, but HbF. which is present in treated concentration when switch over occurs.When does it begin and end?* Switch over occurs at 36th gestational week, so that may be taken as the beginning of switch over and after that b chains remains the predominant chain synthesized.