Basanti a 7 year old girl, presents with bowing of legs and difficulty in walking. Her height was below average for her age group. Her one of the sibling also faces the same problems. On X-ray examination there is metaphyseal widening and funnel-like beaking of the metaphyses. Investigations showed a serum calcium of 9 mg/dl, serum phosphorus of 2.5mg/dl. The possible cause is: