Question

Mendelian Disorders
Read the following passage carefully and answer the given questions.
Genetic disorders may be grouped into two categories - Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in a single gene. These disorders are transmitted to the offspring on the same lines as in the principle of inheritance. The pattern of inheritance of such Mendelian disorders can be traced in a family by pedigree analysis. Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc. The Mendelian disorders may be dominant or recessive. By pedigree analysis, one can easily understand whether the trait in question is dominant or recessive. Similarly, the trait may also be linked to the sex chromosome or an autosome.

Which one of the following diseases is an autosomal dominant disorder?

• A. Sickle-cell anaemia
• B. Myotonic dystrophy
• C. Phenylketonuria
• D. Thalassemia

Hint:

Dominant means one faulty copy is enough. Three of the four diseases need two faulty copies.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
An autosomal disorder is caused by a gene on a non-sex chromosome. A dominant disorder shows up even when only one copy of the faulty allele is present. A recessive disorder shows up only when both copies are faulty.

Step 2: Check option 1, Sickle-cell anaemia:
It is caused by a change in the beta globin gene on chromosome 11. A person needs two faulty copies to have the disease. So it is autosomal recessive and option 1 is wrong.

Step 3: Check option 2, Myotonic dystrophy:
This muscle disease is caused by a repeat expansion in a gene on an autosome. One faulty copy is enough to cause it. So it is autosomal dominant and option 2 is right.

Step 4: Check option 3, Phenylketonuria:
The enzyme phenylalanine hydroxylase is missing when both copies of the gene are faulty. So it is autosomal recessive and option 3 is wrong.

Step 5: Check option 4, Thalassemia:
Thalassemia is caused by faulty globin genes and shows up in the person who gets two faulty copies. It is autosomal recessive, so option 4 is wrong.

Final Answer:
Myotonic dystrophy is the autosomal dominant disorder, which is option 2.