Down’s syndrome is caused by:
(c) Trisomy of 21st chromosome
Understanding Down’s Syndrome and Chromosomal Abnormalities
Down's syndrome is a genetic disorder that affects a person's physical features, intellectual development, and health. It is one of the most common chromosomal abnormalities in humans.
What Causes Down’s Syndrome?
Down’s syndrome is primarily caused by an error in cell division that results in an extra full or partial copy of chromosome 21. This condition of having three copies of a particular chromosome instead of the usual two is known as trisomy.
Trisomy of Chromosome 21
Normally, a person has 23 pairs of chromosomes, totaling 46 chromosomes. One set of 23 comes from the mother, and the other set of 23 comes from the father. In the case of Down's syndrome, a person has three copies of chromosome 21 instead of the usual two. This is specifically called Trisomy 21.
This extra genetic material from the extra chromosome 21 leads to the characteristic features and health issues associated with Down's syndrome. The extra chromosome copy affects the expression of many genes located on chromosome 21, disrupting normal development.
Analyzing the Options
Let's look at why the other options are not the cause of Down's syndrome:
- (a) Trisomy of 14th chromosome: Trisomy 14 is a rare and severe chromosomal abnormality, often leading to miscarriage or significant developmental issues, but it is not the cause of Down's syndrome.
- (b) Trisomy of 22nd chromosome: Like Trisomy 14, Trisomy 22 is also a rare and severe condition, often incompatible with life or causing significant problems, but it is not Down's syndrome.
- (c) Trisomy of 21st chromosome: This is the specific genetic abnormality known to cause Down's syndrome.
- (d) Short arm of 23rd chromosome: Chromosome 23 refers to the sex chromosomes (X and Y). Abnormalities involving the short arm of the 23rd chromosome (like deletions on the Y chromosome's short arm) are related to sex determination and other conditions, but not Down's syndrome. Down's syndrome involves an autosomal chromosome (chromosome 21), not a sex chromosome (chromosome 23).
Therefore, the presence of an extra copy of chromosome 21, resulting in Trisomy 21, is the genetic basis for Down's syndrome.
Key Concept: Trisomy
Trisomy is a type of aneuploidy (an abnormal number of chromosomes) where there are three copies of a specific chromosome in a cell's nucleus, instead of the normal two copies. In Down's syndrome, this trisomy occurs with chromosome 21.
| Condition | Chromosomal Anomaly |
|---|---|
| Down's Syndrome | Trisomy of 21st chromosome |
| Typical Person | Two copies of each chromosome (including 21) |
Revision Table: Causes of Genetic Conditions
| Condition | Chromosomal Cause |
|---|---|
| Down's Syndrome | Trisomy 21 |
| Edwards Syndrome | Trisomy 18 |
| Patau Syndrome | Trisomy 13 |
| Turner Syndrome | Monosomy X (absence of one X chromosome in females) |
| Klinefelter Syndrome | Extra X chromosome in males (e.g., XXY) |
Additional Information on Down’s Syndrome Genetics
While Trisomy 21 is the most common cause (accounting for about 95% of cases), Down's syndrome can also be caused by other chromosomal changes involving chromosome 21:
- Translocation Down's Syndrome: In about 3-4% of cases, a portion of chromosome 21 breaks off during cell division and attaches to another chromosome (often chromosome 14). The person still has the usual two copies of chromosome 21, but they also have the extra piece attached elsewhere. This can be inherited.
- Mosaic Down's Syndrome: This is a rare form where some cells in the body have the usual 46 chromosomes with two copies of chromosome 21, while other cells have 47 chromosomes with three copies of chromosome 21. This occurs due to an error in cell division after fertilization.
However, the vast majority of Down's syndrome cases result from simple Trisomy 21, where every cell in the body has three complete copies of chromosome 21.
Important Questions from Heredity and Variation
The probability of having a haemophilia carrier daughter by a haemophilia carrier mother and a normal father is:
Which of the following statements are correct about Thalassemia?
A. In β-Thalassemia, production of β-globin chain is affected, and in α-Thalassemia, production of α-globin chain is affected.
B. α-Thalassemia is controlled by two closely linked genes HBA1 and HBA2.
C. The genes HBA1 and HBA2 are located on chromosome 11 of each parent.
D. β-Thalassemia is controlled by a single gene HBB.
Choose the correct answer from the options given below:
Match List-I with List-II:
List-I List-II (A) Phenylketonuria (I) Incomplete dominance (B) Haemophilia (II) 9:3:3:1 (C) Snapdragon (III) Pleiotropy (D) Dihybrid cross (IV) Sex-linked Choose the correct answer from the options given below:
Chromosomes in meiocytes of butterflies are:
The karyotype in Klinefelter’s syndrome is:
