Q.During a medical investigation, an infant was found to possess an extra copy of chromosome no. 21. Identify the disorder the child is suffering from. Describe the symptoms the child is likely to develop later in life.

The disorder is Down’s syndrome, caused by trisomy of chromosome 21 — an extra copy of this chromosome in the cells.

Children with Down’s syndrome typically show a distinct set of physical and developmental features. The most common symptoms include a flat facial profile, a protruding tongue, and a single deep crease across the palm of the hand (simian crease). They also have a broad, short skull, a short neck, and often a small, open mouth. …

The scene is set in the fundamentals of Mendelian genetics, but this particular disorder steps beyond simple dominant-recessive inheritance. It involves a change in chromosome number — a condition called aneuploidy. Normally, humans have 46 chromosomes (23 pairs). When an extra copy of a specific chromosome appears, the genetic balance is disturbed, and a syndrome with multiple effects follows.

In this case, the extra copy of chromosome 21 means the child has three copies instead of the usual two. This condition is known as Down syndrome (also called trisomy 21). It is one of the most common chromosomal disorders, and its incidence is known to increase with maternal age, though the NCERT textbook does not specify exact figures.

The symptoms a child with Down syndrome is likely to develop later in life are well-documented in the NCERT syllabus. They fall into three broad categories: physical features, intellectual disability, and associated health problems.

Physical features are often noticeable at birth or early childhood. The child typically has a flat face with a broad, flat nose. The eyes have an upward slant (similar to the epicanthic fold seen in some Asian populations), and there is often a protruding tongue because the oral cavity is smaller than usual. The hands are short and broad, and there is a characteristic single palmar crease (a single deep line across the palm, instead of the usual two). The neck is often short, and the overall stature is short.

Intellectual disability is a hallmark of Down syndrome. The child will have moderate to severe intellectual disability, meaning learning, reasoning, and problem-solving skills are significantly below average. Speech development is delayed, and memory may be impaired. However, with early intervention and special education, many children can achieve a good degree of independence. …