Question

(i) Differentiate between Mendelian and Chromosomal disorders.

(ii) Which disorder is known as "Inborn Error of Metabolism" ? Mention its cause and any one symptom. Name the harmful compounds formed.

Hint:

PKU is the inborn error of metabolism: phenylalanine hydroxylase is missing, so phenylpyruvic acid forms and causes mental retardation.

Answer 1

Step 1: Part (i): Mendelian disorders:
A Mendelian disorder is caused by a change (mutation) in a single gene.
It follows Mendel's principles of inheritance and can be traced in a family pedigree. It may be dominant or recessive, autosomal or sex-linked.
Examples are haemophilia, colour blindness, sickle-cell anaemia, cystic fibrosis and phenylketonuria.

Step 2: Part (i): Chromosomal disorders:
A chromosomal disorder is caused by absence, excess or abnormal arrangement of whole chromosomes or chromosome parts.
It does not follow Mendelian ratios. It often arises from non-disjunction of chromosomes during meiosis.
Examples are Down syndrome (trisomy 21), Turner syndrome (XO) and Klinefelter syndrome (XXY). Karyotyping detects it.

Step 3: Part (i): Summary of three differences:
Cause: single gene mutation versus change in chromosome number or structure.
Pattern: follows Mendelian inheritance versus does not follow it.
Detection: pedigree and gene tests versus karyotype study.

Step 4: Part (ii): The disorder and its cause:
Phenylketonuria (PKU) is called the inborn error of metabolism. It is an autosomal recessive disorder.
The gene for the enzyme phenylalanine hydroxylase is mutated. So the enzyme is missing or non-functional.
Phenylalanine therefore cannot be changed into tyrosine.

Step 5: Part (ii): Symptom and harmful compounds:
Phenylalanine builds up in the body. It is converted into phenylpyruvic acid and other phenyl derivatives (phenylketones).
These compounds are harmful to the brain and are passed out in urine. A symptom is mental retardation (slow brain development).

Final Answer:
(i) Mendelian: single gene change, follows Mendel's laws. Chromosomal: chromosome number or structure change, no Mendelian pattern. (ii) PKU; phenylalanine hydroxylase gene mutation; mental retardation; phenylpyruvic acid and other phenyl derivatives.