Two siblings with osteogenesis imperfect, but their parents are normal. Mechanism of inheritance is -

Ans. is 'c' i.e., Germ line mosaicism o In some autosomal dominant disorders (e.g. osteogenesis imperfecta) phenotypically normal parents have more than one affected child. This clearly violates the law of inheritance. Studies indicate that gonadal (germ line) mosaicism may be responsible for such unusual inheritance. Germline mosaicism o Germline mosacism results from a mutation that occurs postzygotically during early embryonic development. In these a poion of the egg or sperm cells of a parent carries the mutation. o Because the mutation affects only cells destined to form the gonads the gametes carry the mutation but somatic cells of the individual are completely normal. o A phenotypically normal parent who has germ line mosaicism can transmit the disease causing mutation to the offspring through the mutant gamete. o Because the progenitor cells of the gamete carry the mutation there is a definite possibility that more than one child of such a parent would be affected. o Obviously, the likelihood of such an occurrence depends on the propoion of germ cells carrying the mutation.