Revision summary
Down’s syndrome is usually trisomy 21 from meiotic non-disjunction. Lejeune linked the phenotype to the extra chromosome. Translocation and mosaic forms exist. Maternal age raises risk of the common form. Counselling uses karyotype; racial labels are obsolete.
Model answer
Introduction
Down’s syndrome is a chromosomal condition, usually trisomy 21. Anthropology meets it in human genetics, growth, and counselling.
Body
Cause
- John Langdon Down described the phenotype. Lejeune showed an extra chromosome 21.
- Most cases are non-disjunction in meiosis, risk rising with maternal age.
- Translocation Down’s can be inherited from a balanced carrier. Mosaics are milder in some tissues.
Features and care
- Hypotonia, a flat facial profile, single palmar crease, and a risk of heart defects are common.
- It is not a ‘race type’. Older racial labels were a scientific error.
- Genetic counselling uses karyotype, and now NIPT, without treating the child as a social rank.
Flow diagram
flowchart TD DS[Down syndrome] --> T21[Trisomy 21] DS --> TR[Translocation] DS --> MO[Mosaic] T21 --> ND[Meiotic nondisjunction]
Conclusion
Down’s syndrome is trisomy 21 in most cases. The anthropological task is accurate cytogenetics and humane counselling, not a racial cartoon.
Quick related
Students also ask
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Is every case inherited from a parent?
No. Most are new non-disjunction. Translocation cases need parental karyotypes.
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Does paternal age matter as much?
The strong, replicated risk is maternal age for free trisomy 21.
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