Question

Alkaptonuria is a ______________ disorder
A. Sex linked dominant
B. Sex linked recessive
C. Autosomal recessive
D. Autosomal dominant

Answer 1

Hint:-The main cause of Alkaptonuria is a mutation of the homogentisate 1,2-dioxygenase (HGD)
gene of humans. Hence it is not linked with the X chromosome of our body.

Complete step-by-step solution:-Alkaptonuria is one of the rare genetic metabolic disorder which is mainly characterized by
the accumulation of homogentisic acid in our body.
- Affected individuals are not able to synthesize the enzyme which is required for the
breakdown of homogentisic acid.
- The affected individuals usually have dark urine which eventually turns black when exposed
to air.
- The affected individuals may also develop discoloration of the skin covering the cartilages within the body such, even in some cases the sclera layer of the eye may become decolorized.
- The HGD gene codes for the enzyme which is required for the breakdown of homogentisic acid but mutations in the HGD gene cause alkaptonuria.
- Alkaptonuria is inherited as an autosomal recessive disease, this recessive genetic disorder will occur only when an individual inherits the same abnormal gene for the same trait from each of his parents. But if an individual receives one normal gene and one gene for the disease the individual will only be a carrier for the disease and will not be affected by the disease. Hence any kind of symptoms will also be not shown by the individual.
- If both of the parents are the carrier of the disease then there are 25% chances that the child will be affected with the disease, whereas 50% chances are there the child will only be the carrier and 25% chances are there that the child will neither be the carrier nor be affected. As it is not an X-linked genetic disorder the chances are the same for both females and males.
- ‘Alkaptonuria is an Autosomal recessive disorder’, so the correct option is C.

Note:-
The HGD gene contains certain instructions which help in the encoding for an enzyme known as homogentisate 1,2-dioxygenase, this enzyme is very much essential for the breakdown of homogentisic acid. Therefore mutations in the HGD gene result in a deficiency of homogentisate 1,2-dioxygenase enzyme, which in turn will lead to excess levels of homogentisic acid. Even though homogentisic acid is rapidly cleared from the body by the action of kidneys but it also slowly accumulates in the various tissues of the body, specifically in the connective tissue such as cartilages.Eventually, the color of affected tissue to a slate blue or black, even long-term, and chronic accumulation of homogentisic acid will ultimately lead to the damage of the affected tissue.