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Mutations
13
...
The change can be as large as an entire chromosome or as
small as a single nucleotide in one gene (k Box 13
...
Box 13
...
Box 13
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Symptoms may include restricted
growth and short stature, lack of
secondary sexual development,
learning difficulties, challenging
behaviour, a constant desire to
eat and obesity
...
7
...
Mutations usually arise
due to an error in cell division and are often harmful to the individual, e
...
Prader–Willi syndrome (k Box 13
...
13
...
2 Changes to the chromosome structure
Different types of chromosome mutations most often arise in germinal cells
(ovary or testes) during meiosis or by mutagens such as radiation
...
Chromosome mutations affect protein production by changing the genes on the
affected chromosome; some are not harmful but others may contribute to
developmental difficulties or death
...
19)
...
19 Turner syndrome
is characterised by short
stature, non-functioning
ovaries, puffiness
(lymphoedema) of the hands
and feet, skeletal
abnormalities, heart defects,
high blood pressure and kidney
problems
...
7
...
They arise when
a change in a single gene has occurred following the alteration of one base pair in the
DNA sequence
...
13
...
To function correctly, each cell depends on thousands of proteins to do their jobs in
the right places at the right times
...
When a mutation occurs to a protein that plays a critical role in the
body, it can cause disease
...
8
...
If the mutation occurs in the embryo it may disrupt development and result in
miscarriage
...
13
...
4 Sex-linked inherited conditions
Sex-linked inherited conditions result from a mutant allele located on either the X or
the Y chromosome, which are different shapes and do not share the same genes (k
Box 13
...
Haemophilia is an example of an inherited sex-linked disease passed on
the X chromosome from mothers to their sons
...
21)
...
• Females have two X chromosomes
...
The genotype of mothers can therefore be:
• XX – no haemophilia, or
• XXd – no haemophilia, but will be a carrier with a 50% chance of passing the d
allele to her offspring in every pregnancy
...
Box 13
...
Box 13
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It also
follows dental and general
surgery unless clotting factors
are given beforehand
...
22 Acquired (rather than
inherited) forms of haemophilia
are rare forms of the disease that
are not present at birth but can
develop suddenly at any stage in
the lifespan of a person who does
not have a family history of
haemophilia
...
Sometimes
acquired haemophilia is
associated with bone marrow
problems, tumours and cancers,
some medications or vitamin K
deficiency
...
13
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However, there is a 50% probability that they are carriers
...
It is also possible for all the children in the
family to inherit the normal allele or all to inherit the haemophilia allele
(k Box 13
...
GAMETES
ovum
MOTHER
FATHER
XXd
XY
X
Xd ovum
sperm
XX
XY
XXd
XdY
unaffected
girl
unaffected
boy
carrier
girl
affected
boy
X
Y
sperm
Fig
...
16 Sex-linked inheritance for haemophilia from a carrier mother
...
However,
daughters would not necessarily have haemophilia; this is because the
normal blood clotting allele from the mother is dominant so code from
the haemophilia allele will not be expressed
...