DOWN’S SYNDROME
Down’s syndrome
On completion of this activity you should be able to:
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describe the causes of Down’s syndrome
describe the typical features of Down’s syndrome
explain the difference between a screening test and a diagnostic test
describe the relationship between the age of a mother and her risk of having
a child affected by Down’s syndrome
 explain in simple terms the reason for the above relationship
 analyse and interpret a pedigree chart
 draw a pedigree chart spanning three generations, using the correct
symbols.
Down’s syndrome is a genetic condition that causes impairments in physical
and intellectual development. It occurs in approximately one in every 800 live
births. Individuals with Down’s syndrome have 47 chromosomes instead of the
usual 46. Individuals have an extra copy (three instead of two) of chromosome
21. Down’s syndrome affects all population groups and is distinguished by a
number of features occurring together, including low muscle tone, a face that
appears flatter with eyes slanting upward, small ears and an unusually wide
neck and a deep crease across the palm of the hand. Some individuals may
have heart problems or visual problems or may develop Alzheimer ’s disease.
Although people with Down’s syndrome have learning difficulties, these vary
in severity.
Children with Down’s syndrome have a higher incidence of infection,
respiratory, vision and hearing problems as well as thyroid and other medical
conditions. However, with appropriate medical care most children and adults
with Down’s syndrome can lead healthy lives.
Prenatal diagnosis
Two types of procedures are available to pregnant women: screening tests and
diagnostic tests.
Screening test: A test or inquiry used on people who do not have or have not
recognised the signs or symptoms of the condition being tested for. It divides
people into low and higher risk groups.
Diagnostic test: Refers to the analytical process involved in obtaining a
result/diagnosis. For example, the diagnostic test on an amniocentesis sample
(invasive procedure) is the karyotype. Newer test uses the polymerase chain
reaction to amplify a marker for chromosome 21. A positive PCR result will
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DOWN’S SYNDROME
show 1.5 times the expected amount of DNA from chromosome 21. PCR
results can be ready in 48 hours, karyotyping can t ake a week for results.
There are three chromosomal patterns that result in Down’s syndrome.
1.
Non-disjunction
The risk of carrying a child with Down’s syndrome increases as the
mother’s age increases.
Trisomies appear to be associated with an increase in maternal age. The
major reason for this is thought to be that the eggs in a woman ’s ovaries
have been held at the crossing-over stage in meiosis from when she was
in the womb at approximately 6 months’ gestation. It is the ‘wear and
tear’ in the spindle fibres (the machinery for cell division) that is
thought to be a major cause of non-disjunction, leading to trisomy 21.
Figure 1 Increasing risk of Down’s syndrome with maternal age.
2.
Translocation
Translocation accounts for only 3–4% of all cases. In translocation a part
of chromosome 21 breaks off during cell division and attaches to another
chromosome. The presence of an extra piece of the 21st chromosome
causes the characteristics of Down’s syndrome. Unlike trisomy 21,
translocation may indicate that one of the parents is carrying
chromosomal material that is arranged in an unusual manner.
Use the following websites to gain a deeper understanding of the
process.
http://www.ccs.k12.in.us/chsBS/kons/kons/chromosome mutations web
quest/translocation.htm
http://www.embryology.ch/anglais/kchromaber/abweichende03.html
3.
Mosaicism
Mosaicism occurs when non-disjunction of chromosome 21 takes place
in one of the initial cell divisions after fertili sation. When this happens,
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DOWN’S SYNDROME
there is a mixture of two types of cells, some containing 46
chromosomes and some with 47. The cells with 47 chromosomes contain
an extra 21st chromosome. Because of the ‘mosaic’ pattern of the cells
the term ‘mosaicism’ is used. This type of Down’s syndrome occurs in
only 1–2% of all cases of Down’s syndrome and individuals normally
have much milder symptoms because of the mixture of cells with 46 and
47 chromosomes.
Questions
Study the following family trees showing the inheritance of a Robertsonian
translocation of chromosome 21. Squares represent males and circles represent
females.
1.
Which couple had a surviving affected daughter?
2.
Couples 14 and 15 decided not to undergo genetic testing. Give a
possible reason for this choice.
3.
What relation is the person indicated by the arrow to the female in
couple 15?
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DOWN’S SYNDROME
4.
Researchers of a study of couples with a child who inherited Down ’s
syndrome from a Robertsonian translocation made the following
statement.
‘Having a healthy child before the birth of an affected child strongly
influenced the parental decision-making regarding a future pregnancy.’
Couple X has an unaffected first child and a second child with Down ’s
syndrome.
Couple Y has one child and it inherited Down ’s syndrome.
(a)
Which couple do you think would be more likely to have a strong
willingness for a future pregnancy? Give reasons for your answer.
(b)
Alison is married to Ken. Alison inherited the carrier status for the
Robertsonian translocation of chromosome 21 from her father. Alison
has a paternal uncle who has Down’s syndrome. Alison has had three
spontaneous abortions and tests indicate that all three children were
suffering from Down’s syndrome.
Draw the family tree to show the described inheritance of this
characteristic.
Answers
1.
Couple 15.
2.
The couples may have decided to not have any more children. Find ing
out who was the carrier may cause problems regarding feelings of guilt
etc.
3.
Brother.
4.
(a)
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Research suggests couple Y. Couple Y may feel the need for a
‘normal’ child, while couple X already have a ‘normal’ child.
© Learning and Teaching Scotland 2011