Brent Cornell

    
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) 

Non-Disjunction

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 (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

    
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