IB Biology SL

Revision Notes

3.2.4 Non-disjunction


  • Sometimes children can be born with chromosomal abnormalities or an abnormal number of chromosomes, this can be due to non-disjunction occurring during meiosis
  • Non-disjunction occurs when chromosomes fail to separate during meiosis
  • This can occur during either anaphase I or anaphase II
  • The gametes may end up with one extra copy of a particular chromosome or no copies of a particular chromosome
  • These gametes will have a different number of chromosomes compared to the normal haploid number
  • If the abnormal gametes are fertilized, then a chromosome abnormality occurs as the diploid cell (zygote) will have the incorrect number of chromosomes

Nondisjunction, downloadable AS & A Level Biology revision notes

Image showing how chromosomes failing to separate properly during meiosis can result in gametes with the incorrect number of chromosomes

Down Syndrome

  • An example of a chromosome abnormality is Down syndrome, technically called Trisomy 21
  •  Individuals with this syndrome have a total of 47 chromosomes in their cells as they have three copies of chromosome 21
  • Non-disjunction occurs during anaphase I (in this case) and the 21st pair of homologous chromosomes fail to separate

Age & Non-disjunction

  • Many studies have shown that there is a correlation between age and the incidence of non-disjunction
  • It is believed that as the age of the parents increases the incidence of non-disjunction increases
  • The age of the mother has been found to increase the chance of having a child with Down Syndrome
  • The table below shows how the age of the mother can increase the chance of having a baby with Down Syndrome

Insert 3.2.4 Non-disjunction table outlining link between age and risk of down syndrome

Table showing how age of mother can increase the chance of having a child with Down Syndrome

Methods used for Karyotype Analysis

  • A karyotype is a technique used to produce an image of all of the chromosomes of an individual from a single cell. It can can be used to check for any chromosomal abnormalities
  • Two methods can be used to obtain cells from an unborn child for chromosome testing:
    • Amniocentesis
    • Chorionic villus sampling


  • A needle is inserted through the mother’s abdomen wall and a small sample of amniotic fluid is taken. The sample will contain some fetal cells for analysis
  • This procedure usually takes place around 16 weeks of pregnancy
  • There is a small risk of miscarriage associated with the procedure (approx 1%)
  • The procedure also poses a small risk of infection

Chorionic Villus Sampling (CVS)

  • A long tube is inserted through the vagina and then cervix in order to take a small sample of the developing chorion (a membrane surrounding the embryo which forms part of the placenta)
  • This procedure can be carried out earlier in the pregnancy; around 10 – 12 weeks
  • CVS has a slightly increased risk of miscarriage associated with the procedure (approx 2%)
  • There is also a small risk of infection

Insert image 3.2.4 Non-disjunction Normal Karyotype and 3.2.4 Non-disjunction Down syndrome Karyotype side by side

Image comparing a normal Karyotype to that of someone with Down Syndrome


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