Q.What is Down's syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down's syndrome increases if the age of the mother exceeds forty years?

Down's syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21 — a condition called trisomy 21. Instead of the normal 46 chromosomes, affected individuals have 47.

The main symptoms include:

• Mental retardation (varying degrees)
• A characteristic flat facial profile, with a small nose and upward-slanting eyes
• A short neck and broad hands with a single palmar crease
• Delayed physical and motor development
• Often associated with heart defects and other organ abnormalities

The cause is non-disjunction during meiosis — the failure of chromosome 21 to separate properly, so one gamete ends up with two copies instead of one. When that gamete fuses with a normal one, the zygote has three copies. …

Down’s syndrome is one of the most common chromosomal disorders you will study in genetics. It is named after John Langdon Down, who first described the condition in 1866, but its cause — an extra chromosome 21 — was not understood until 1959. The NCERT textbook presents it as a classic example of aneuploidy, where the total number of chromosomes in a cell deviates from the normal 46.

In a healthy individual, each cell contains 23 pairs of chromosomes. In Down’s syndrome, there is an extra copy of chromosome 21, making the total 47 instead of 46. This condition is therefore also called trisomy 21. The extra genetic material disrupts the normal course of development, leading to a set of characteristic symptoms.

Symptoms and features — The NCERT lists the following key symptoms:

• Physical appearance: A flat facial profile, a protruding tongue, a short neck, and a single palmar crease (often called a simian crease). The eyes have an upward slant, and the ears are small and low-set.
• Growth and development: Short stature, broad hands with short fingers, and poor muscle tone (hypotonia) in infancy.
• Intellectual disability: Moderate to severe intellectual impairment is a consistent feature. Children with Down’s syndrome learn more slowly and may have delayed speech and motor skills.
• Associated health problems: Congenital heart defects, increased risk of respiratory infections, and a higher likelihood of developing leukaemia or Alzheimer’s disease later in life.

Cause — The root cause is nondisjunction during meiosis. Normally, during the formation of eggs or sperm, the two copies of each chromosome separate so that each gamete gets one copy. In nondisjunction, the pair of chromosome 21 fails to separate. As a result, one gamete ends up with two copies of chromosome 21 instead of one. When this gamete fuses with a normal gamete (which has one copy), the resulting zygote has three copies — trisomy 21.

Approach it through the mechanics of meiosis itself: trace a single egg cell arrested in prophase I for decades, and reason how that prolonged arrest raises the chance the spindle apparatus mis-handles chromosome 21 at ovulati …