Prenatal diagnosis and trisomies — Grade 12 Life Sciences

Why test before birth?

Some genetic disorders, especially trisomies (three copies of one chromosome instead of two), can be detected before birth. Results help parents and doctors plan care, prepare for the birth, and make informed decisions.

Prenatal diagnosis checks for chromosome number and structure. Many single-gene disorders (such as cystic fibrosis) can also be tested for, if a family has a known risk.

Prenatal tests

Amniocentesis — a fine needle is inserted through the abdominal wall into the amniotic sac to draw out a small sample of amniotic fluid. The fluid contains fetal cells, which are grown in the laboratory and their chromosomes examined. It is usually done from about week 15 of pregnancy.
Chorionic villus sampling (CVS) — a small sample of the chorionic villi (placental tissue) is taken, usually at about week 10 to 13. Results come sooner than from amniocentesis.

Risk: both tests carry a small risk of miscarriage. They are offered when the benefit of knowing the result is judged to outweigh that risk, and the parents give informed consent.

Reading a karyotype

A karyotype is an ordered picture of a person’s chromosomes, arranged in homologous pairs by size, centromere position and banding pattern. A normal human karyotype has 46 chromosomes (23 pairs): 22 autosomal pairs and one pair of sex chromosomes (XX in a female, XY in a male).

✏️ Worked example

1Count the chromosomes. The karyotype shows 47 chromosomes, with three copies of chromosome 21.

2Normal count = 46. An extra chromosome gives 47, so this is a numerical abnormality: trisomy.

3Because the extra chromosome is number 21, the diagnosis is trisomy 21 (Down syndrome).

Trisomies and non-disjunction

Trisomy	Extra chromosome	Condition
Trisomy 21	Chromosome 21	Down syndrome
Trisomy 18	Chromosome 18	Edwards syndrome
Trisomy 13	Chromosome 13	Patau syndrome

Trisomies usually arise from non-disjunction: a pair of homologous chromosomes (or sister chromatids) fails to separate during meiosis. One gamete then has an extra copy, and after fertilisation the zygote has three copies. The risk of non-disjunction in the mother increases with her age, which is why age is a key risk factor.

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (3 marks)

Describe amniocentesis, including when it is performed and what cells are examined.

Show memo

A fine needle is inserted through the abdominal wall into the amniotic sac, guided by ultrasound ✓. A small sample of amniotic fluid is drawn off from about week 15 ✓. Fetal cells in the fluid are grown in a laboratory and their chromosomes examined (karyotype) ✓.

Question 2 · (4 marks)

Compare amniocentesis and chorionic villus sampling. Give ONE advantage of CVS and ONE reason why the parents must give informed consent before either test.

Show memo

Amniocentesis samples amniotic fluid from about week 15; CVS samples placental tissue (chorionic villi) at about week 10–13 ✓✓.
Advantage of CVS: it can be done earlier, so results come sooner ✓.
Consent is needed because both tests carry a small risk of miscarriage and the results may lead to difficult decisions ✓.

Question 3 · (5 marks)

A karyotype of a fetus shows 47 chromosomes including three copies of chromosome 18. (a) State the diagnosis. (b) Explain how the extra chromosome could have arisen during meiosis. (c) Name the condition.

Show memo

(a) Trisomy 18 ✓
(b) Non-disjunction ✓: the chromosome 18 pair (or its sister chromatids) failed to separate during meiosis ✓, so one gamete received two copies of chromosome 18; fertilisation by a normal gamete gave three copies ✓.
(c) Edwards syndrome ✓

Question 4 · (4 marks)

Explain why the risk of trisomy increases with maternal age, and state what the term non-disjunction means.

Show memo

Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate during meiosis ✓✓.
A woman's oocytes begin meiosis before she is born and stay paused for many years ✓; the longer they wait, the more likely chromosomes are to separate incorrectly ✓.