Describe a chromosome mutation as a change in the chromosome number or structure, using Down’s syndrome as an example (47 chromosomes instead of 46)

Sourav PanNovember 4, 2024

Answered

A chromosome mutation refers to a change in the structure or number of chromosomes within a cell. This can involve alterations in chromosome number (such as aneuploidy) or structural changes (like deletions, duplications, inversions, or translocations). One well-known example of a chromosome mutation is Down syndrome, which is characterized by the presence of an extra copy of chromosome 21.

Chromosome Mutation: Down Syndrome

1. Normal Chromosome Count:
   • Typically, humans have 46 chromosomes, arranged in 23 pairs, with one chromosome of each pair inherited from each parent.
2. Aneuploidy:
   • Down syndrome is primarily caused by a type of chromosomal mutation known as aneuploidy, specifically trisomy 21. This condition occurs when there is an extra chromosome 21, resulting in a total of 47 chromosomes instead of the usual 46.
3. Mechanism of Trisomy 21:
   • Trisomy 21 usually results from a failure in the normal separation of chromosomes during cell division, a process known as nondisjunction. This can occur during meiosis (the formation of egg and sperm cells) or mitosis (cell division in somatic cells). When nondisjunction happens, one gamete receives two copies of chromosome 21 while another receives none. If this gamete with the extra chromosome participates in fertilization, the resulting zygote will have three copies of chromosome 21.
4. Phenotypic Effects:
   • The additional genetic material from the extra chromosome 21 disrupts normal development and leads to the characteristic features associated with Down syndrome, which may include:
     • Distinctive facial features (e.g., flat facial profile, slanted eyes)
     • Short stature
     • Developmental delays and intellectual disabilities
     • Increased risk for certain health issues (e.g., heart defects, gastrointestinal problems)
5. Other Forms:
   • While most cases of Down syndrome are due to complete trisomy 21, some individuals may have a form known as translocation Down syndrome, where part of chromosome 21 is attached to another chromosome. There is also mosaic Down syndrome, where only some cells have the extra chromosome.