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More complex genetics: 6. Genetics of Down syndrome - genetic screening for abnormalities - ethical issues

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INDEX of biology notes on more complex genetics - inherited sex/non-sex linked examples

(6) Down's Syndrome (NOT a sex-linked inherited genetic disorder, but there is a gender link)

Causes and effects

Down's syndrome is  genetic disorder that causes reduced muscle tone and reduced cognitive development.

 It is caused by the presence of an extra chromosome from a random error in the cell division of sex cells (gametes) in meiosis.

A sperm cell with the normal 23 chromosomes fertilises a female egg cell with 24 chromosomes.

This results in an individual with 47 chromosomes rather than the ‘normal’ 46.

An individual with the Down syndrome has an extra copy of the 21st chromosome rather than the normal compliment of two for the human genome.

In 95% of cases, the extra copy of chromosome 21 comes from the mother in the egg, and only 5% from the male sperm.


What increases the chances of a child having Down syndrome?

Down syndrome occurs in around 1 of 800-1000 live births and is the most common genetic cause of developmental delay.

There are many factors the chance of a child having Down Syndrome.

These risk factors include geographic region, maternal education, marital status, and  ethnicity.

However, because Down Syndrome is due to extra copy of chromosome 21, the incidence increases significantly as a woman gets older i.e. older women are much more likely than younger women to give birth to an infant with Down syndrome.


A note on genetic screening for genetic abnormalities - ethical issues

Genetic screening involves a test for the presence of a particular allele or other genetic abnormality in the foetus.

An amniocentesis procedure is one way of performing a genetic screening test.

A needle is inserted into the amniotic fluid that surrounds the foetus in the womb and a fluid is sample is withdrawn.

Cells in the fluid from the foetus are then examined for the presence of genetic abnormalities such as Down syndrome and cystic fibrosis.

Amniocentesis does carry a 1% risk of miscarriage, and therefore usually only offered to pregnant women who are at a higher risk of having a child with a genetic abnormality.

For most mothers risk of genetic abnormalities in the bay are low, but the increases for mothers who have previously carried a foetus with a genetic abnormality, those with a family history of a genetic condition and older mothers.

Pregnant mothers can be offered a blood test between 10–14 weeks of pregnancy as alternative initial screening for Down's Syndrome, but if the blood test suggests a possibility of having a Down syndrome child, amniocentesis is normally then offered to those mothers who appear to have a higher risk.

Although not as accurate as amniocentesis, the blood test does not pose any risk to the foetus or the mother; it simply identifies those women who may wish to take the riskier amniocentesis procedure.

Whatever the method used, detection of an abnormality, poses not only potential medical problems, but ethical and moral issues including a decision to terminate a pregnancy.

Genetic screening is very controversial, for a wider discussion of its implications see ...

Genetic screening - using data from the human genome project - potential medical treatments - issues

Summary of learning objectives and key words or phrases about Down Syndrome

Be able to understand the genetics of Down's syndrome from due to an extra chromosome abnormality.

Be able to discuss the ethical issues of using controversial genetic screening for abnormalities.

Know some of the risk factors increasing the chances of a child having Down Syndrome.



INDEX of biology notes on more complex genetics - inherited sex/non-sex linked examples


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