Which of the following are chromosomal disorders?
I. Colour blindness
II. Down’s syndrome
III. Phenylketonuria
IV. Turner’s syndrome
V. Thalassaemia
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → II and IV
Genetic disorders fall into two groups. Mendelian disorders come from a change (mutation) in a single gene. Chromosomal disorders come from the absence, excess or abnormal arrangement of one or more whole chromosomes.
I. Colour blindness — Not a chromosomal disorder. It is a sex-linked recessive Mendelian disorder. A mutation in a gene on the X chromosome affects the red or green cone cells of the eye.
II. Down’s syndrome — A chromosomal disorder. The person has an extra copy of chromosome 21 (trisomy of 21). It arises when chromatids fail to separate during cell division.
III. Phenylketonuria — Not a chromosomal disorder. It is an autosomal recessive Mendelian disorder. A gene mutation leaves the person without the enzyme that converts phenylalanine into tyrosine.
IV. Turner’s syndrome — A chromosomal disorder. It is caused by the loss of an X chromosome in human females, giving 45 chromosomes with XO.
V. Thalassaemia — Not a chromosomal disorder. It is an autosomal recessive Mendelian blood disease. A mutation or deletion of a globin gene reduces the making of one of the globin chains of haemoglobin.
NCERT Extract:
Mendelian Disorders – “Broadly, genetic disorders may be grouped into two categories – Mendelian disorders and Chromosomal disorders. Mendelian disorders are mainly determined by alteration or mutation in the single gene. … Most common and prevalent Mendelian disorders are Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalassemia, etc.”
Chromosomal Disorders – “The chromosomal disorders on the other hand are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. For example, Down’s syndrome results in the gain of extra copy of chromosome 21. Similarly, Turner’s syndrome results due to loss of an X chromosome in human females.”