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AS Science for Public Understanding
Student Sheets
Human Karyotypes
There are many forms of genetic disease, such as Cystic Fibrosis and Down’s syndrome.
Genetic diseases are passed on from one generation to another, unlike contagious diseases, which can be caughtfrom one another just in the air or by contact.
Genetic diseases are caused by changes in the genetic material or DNA within our cells. These changes or
mutations are spontaneous (i.e. they can occur at any time without any warning) or they can be induced by
contact with certain chemicals or harmful radiation (mutagens).
Chromosome mutations can also occur, where the distribution of the chromosomes is affected. One of thecommonest chromosome mutations, that you have probably heard about, or even had personal experience of, iscalled non-disjunction, and causes what it known as Down’s syndrome.
Down’s syndrome occurs when, at a crucial stage in cell division, the chromosomes do not all separate equallyinto the resulting new cells. Some of the chromosomes lag behind, resulting in an extra one in one cell, and oneless in the other. The cell that has the extra chromosome can still act as a sex cell and if it fuses with a ‘normal’egg or sperm cell, the resulting baby will have not 46 but 47 chromosomes in every cell.
A few types of major chromosomal abnormalities, including missing or extra copies or gross breaks and rejoining(translocations), can be detected by microscopic examination; Down’s syndrome, in which an individual’s cellscontain a third copy of chromosome 21, is diagnosed by karyotype analysis.
How can we detect Down’s syndrome in a baby?
This exercise will enable you to identify the typical Down’s syndrome karyotype.
A karyotype is the name given to a picture of the chromosomes from one cell.
The cells which were photographed above have been treated with colchicine, which allows the chromosomes to staytogether. There are two photographs here, showing Karyotypes stained in different ways. The second one showsbands along the chromosomes.
Copies may be made for use in schools and colleges Student Sheets
AS Science for Public Understanding
If a baby is suspected of having inherited the Down’s mutation, cells can be obtained from the amniotic fluid orumbilical cord, and then tested as shown below.
The chromosomes are paired upand numbered. There will be 23pairs of chromosomes in a humanbody cell.
The individual chromosomes arecut out from the photograph.
S9.5/Genetic Diseases/chapter 6/page 2 Malcolm Ingram The Nuffield Foundation, 2004 Copies may be made for use in schools and colleges AS Science for Public Understanding
Student Sheets
Karyotyping a sample
In this exercise you will act as the medical technician responsible for identifying whether the baby has Down’s ornot. You can imagine the sense of responsibility that this has, when you consider the effect that the result will haveon the prospective parents. So it is very important that you make no mistakes in your analysis.
1. Study the picture on page 4, of chromosomes taken from a baby. Cut out each chromosome, and place them on a sheet of white paper.
2. Study each chromosome very carefully, noting the size and banding patterns. (Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands. Differences in size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype.) 3. Carefully match the pairs of chromosomes until you have accounted for every one. Try to number the pairs using the pattern template below.
4. Is this baby a male or female? How do you know? 5. Do you have any other comments about this karyotype? Copies may be made for use in schools and colleges Student Sheets
AS Science for Public Understanding
Pre-natal screening - issues arising
• Use the weblinks section to research the risks and issues involved in antenatal screening for Down’s syndrome.
Who are the individuals that should decide whether screening for a genetic disease such as Down’s syndrome takesplace? What considerations should be involved in the decision e.g. medical, social, financial? Do you think prenatal screening should be compulsory for everyone, e.g. mothers with increased risk of a geneticcondition? Who should make the final decision about whether a termination is to take place? What information is needed by parents before screening takes place? This document has been downloaded from the Science for Public Understanding website S9.5/Genetic Diseases/chapter 6/page 4 Malcolm Ingram The Nuffield Foundation, 2004 Copies may be made for use in schools and colleges


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