Ans. is 'a' i.e., X-linked Recessive o The clues in this question are ? Only males are manifesting disease. Females are acting as carriers -3 sisters sons had suffered from the disease. o Both these are features of X-linked recessive disorders. X-linked disorders o Except for a few conditions, all X-linked disorders are X-linked recessive. o As male has only one X-chromosome, the male with affected gene on X-chromosome will always manifest the disease. o On the other hand, female has 2 X-chromosomes, heterozyogous female will be carrier because of expression of normal allel on the other X-chromosome. o Note ---> Female with Turner syndrome (only one 'X' chromosome) can manifest X-linked recessive disorders. o 50% boys of the carrier mother will be affected. o All daughters of affected father will become carrier, i.e. affected father will transmitt the defective X chromosome to all the daughters. o Father will not transmitt the disease to son as boys do not inherit X-chromosome from father. About other options o In X-linked dominant disorder the females will also manifest the disease. o In autosomal dominant and autosomal recessive disorders, both males and females are affected.