3.2 Applications and skills

3.2.3 Non-disjunction and aneuploidy

Figure 3.2.3a – Non-disjunction during meiosis results in an abnormal chromosome numberFigure 3.2.3a – Non-disjunction during meiosis results in an abnormal chromosome number

Figure 3.2.3b – Karyogram: abnormal male, Down’s syndromeFigure 3.2.3b – Karyogram: abnormal male, Down’s syndrome

  • People with Down’s syndrome have lower life expectancy, as well as many other mental and physical difficulties.

Skill: Deducing sex and diagnosing aneuploidy

  • A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
  • For each of the karyograms below, determine the sex and identify the chromosomal abnormality (if applicable).

Source: Figures 3.2.3c–e from wellcomeimages.org


Age and incidence of non-disjunction

  • Studies show that the chance of non-disjunction is influenced by the age of parents.

Figure 3.2.3f – Risk of Down’s syndrome as a function of maternal ageFigure 3.2.3f – Risk of Down syndrome as a function of maternal age

  • The correlation between maternal age and the incidence of Down’s syndrome has been clearly established.
  • There is evidence that paternal age may be correlated to increased non-disjunction in sperm. However, the link between paternal age and risk of Down’s syndrome is not clear.

Pre-natal screening and associated risks




Performed at

10–12 weeks

14–16 weeks


  • Sampling tool inserted through vagina and cervix using ultrasound guide
  • Fetal cells taken from chorion (part of the placenta)
  • Needle inserted through abdominal wall using ultrasound guide
  • Fetal cells taken from amniotic fluid

Figure 3.2.3g – CVSFigure 3.2.3g

Figure 3.2.3h – AmniocentesisFigure 3.2.3h

Source of images: urmc.rochester.edu

  • Both of these procedures are invasive and increase the risk of spontaneous abortion by 0.5–2%, so they are not recommended for low-risk pregnancies.
  • Prenatal screening is recommended for parents who are known carriers of a genetic disease, mothers older than 35 years, and parents with a family history of genetic disease.

Figure 3.2.3i – Down’s syndrome featuresFigure 3.2.3i – Trisomy 21

People with Down syndrome have characteristic physical features.

Figure 3.2.3j – Patau syndromeFigure 3.2.3j – Patau syndrome
Patau syndrome has a high mortality rate and is very rare. Most children do not survive their first month.

Figure 3.2.3k –Turner syndromeFigure 3.2.3k –Turner syndrome
The symptoms of Turner syndrome can be treated with hormone therapy. XO females have normal intellectual development.

Science and social responsibility

Pre-natal screening raises ethical issues over selective abortion. In India, for example, it is illegal for practitioners to disclose the sex of a fetus, while in some US states it is illegal to abort a fetus showing markers for Down syndrome.

Figure 3.2.3l – SpermFigure 3.2.3l – Sperm
Aneuploidy (abnormal chromosome number) in sperm cells.

Food for thought

There are many types of aneuploidy, but Turner syndrome is the only monosomy that produces live births. Down syndrome is the only trisomy that results in a near normal lifespan. Suggest reasons.

Course link

HL students will learn in 11.1.4 how meiosis produces egg and sperm.


How could you investigate the link between environmental mutagens (e.g. nuclear radiation) and incidence of chromosomal abnormalities?