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Down syndrome in humans is due to?

A) Three copies of chromosome 21
B) Three X chromosomes
C) Three Y chromosomes
D) Two Y chromosomes
E) Monosomy

Ans: Three copies of chromosome 21

Down syndrome 

This was the first autosomal aneuploidy, described by John Longdon Down (1866) under the name of mongoloid idiocy or mongolism.
  • 21-Trisomy
  • 47 chromosomes instead of 46 with three copies of chromosome 21 instead of  usual two.
Down syndrome Karyotype

Down syndrome Karyotype

  • Karyotype: XXY (reason: Non-disjunction of chromosome of pair during meiosis)  
Reason for Down Syndrome
Reason for Down Syndrome : Non-disjunction of chromosome of pair during meiosis
  • The frequency of non-disjunction  of chromosome pair 21 increases with mother age.
  • The frequency of abnormality is 1in 600 live births.
  • Clinical symptoms : Male with slowly degenerating testes, enlarged breasts.
































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