3.2 Applications and skills
3.2.3 Non-disjunction and aneuploidy
- Non-disjunction is the failure of homologous chromosomes to separate during anaphase of meiosis.
- When non-disjunction occurs, meiosis results in cells with abnormal chromosome numbers (aneuploidy).
- In most cases, the gametes with extra or missing chromosomes do not survive.
- Down’s syndrome, or Trisomy 21, is a human condition that results from the fusion of a normal gamete and a gamete having an extra chromosome number 21 (from non-disjunction). The resulting embryo has 47 chromosomes instead of 46.
- People with Down’s syndrome have lower life expectancy, as well as many other mental and physical difficulties.
Skill: Deducing sex and diagnosing
- 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
- Studies show that the chance of non-disjunction is influenced by the age of parents.
- 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
- Chorionic villus sampling (CVS) and amniocentesis are two procedures used for sampling fetal cells to check for chromosomal abnormalities.
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.
People with Down syndrome have characteristic physical features.
Figure 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.
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.
How could you investigate the link between environmental mutagens (e.g. nuclear radiation) and incidence of chromosomal abnormalities?