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•  Non-disjunction can cause Down syndrome and other chromosomal abnormalities

Non-disjunction refers to the chromosomes failing to separate correctly, resulting in gametes with one extra, or one missing, chromosome (aneuploidy)

The failure of chromosomes to separate may occur via:

  • Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
  • Failure of sister chromatids to separate in Anaphase II (resulting in only two daughter cells being affected) 



Chromosomal Abnormalities

If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body 

Conditions that arise from non-disjunction events include:

  • Patau’s Syndrome (trisomy 13)
  • Edwards Syndrome (trisomy 18)
  • Down Syndrome (trisomy 21)
  • Klinefelter Syndrome (XXY)
  • Turner’s Syndrome / Fragile X (monosomy X)

Down Syndrome

Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)

  • One of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
  • The other parental gamete was normal and had a single copy of chromosome 21
  • When the two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21

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•  Studies showing age of parents influences chances of non-disjunction

Studies show that the chances of non-disjunction increase as the age of the parents increase

  • There is a particularly strong correlation between maternal age and the occurrence of non-disjunction events
  • This may be due to developing oocytes being arrested in prophase I until ovulation as part of the process of oogenesis 

Other studies also suggest that:

  • The risk of chromosomal abnormalities in offspring increase significantly after a maternal age of 30  (Table 1 / Figure 1)
  • There is a higher incidence of chromosomal errors in offspring as a result of non-disjunction in meiosis I  (Figure 2)
  • Mean maternal age is increasing, leading to an increase in the number of Down syndrome offspring  (Figure 3)

Studies into the Development of Chromosomal Abnormalities

Down syndrome statistics