Kinky hair disease is a disorder where an affected child has peculiar white stubby hair, does not grow, brain degeneration is seen and dies by age of two years. Mrs A is hesitant about having children because her two sisters had sons who had died form kinky hair disease. Her mother's brother also died of the same condition. Which of the following is the possible mode of inheritance in her family -

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