Genetic screening: ethical issues - Nuffield Council on Bioethics

108
Congenital hypothyroidism
Mode of inheritance : usually not inherited - about 5-10% of cases due to a
known genetic defect
Abnormal development or function of the thyroid gland resulting in lack of production
of thyroid hormone (thyroxine) occurs in about 1 in 4,000 babies in the UK. The baby
is usually normal at birth because the mother’s thyroxine has been able to pass to the
baby. Unless treatment with thyroxine is started within the first few weeks of life,
growth and mental development will be delayed. Screening is carried out by measuring
specific hormones in the blood taken from the baby at around the end of the first week
(the same blood sample as used for PKU - see later).
Cystic fibrosis
Mode of inheritance : recessive
Cystic fibrosis (CF) is a serious inherited disease affecting the lungs and digestive
system of babies, children and young adults. People with CF have sticky mucus in
their lungs and are particularly prone to chest infections. They also have difficulty in
digesting foods, especially fatty foods, and may later develop liver problems.
Treatment (antibiotics, physiotherapy, digestive enzymes) can greatly help but does not
cure the condition. The average life expectancy for a person with CF is about 20-30
years. The disorder is inherited and the change in the gene responsible for about 85%
of the cases can now be detected. For the disease to develop, a defective gene must
be inherited from each parent. Parents who have only one of a pair of defective genes
are known as carriers and are themselves completely healthy. About 1 in 20 of the
white population in the UK are carriers of the gene; the disease occurs in about 1 in
2,000 babies born. If both parents are carriers, the risk of any baby having the disease
is 1 in 4.
Down’s syndrome
Mode of inheritance : usually not inherited
The disorder, which is genetic but usually not inherited, affects about 1 in 600 babies
born overall, although the risk of having a child with Down’s syndrome rises sharply
when the mother is over 35 years of age.
The vast majority of individuals with Down’s syndrome have an extra copy of
chromosome 21, are born with specific physical characteristics and have severe
learning disabilities. A very small percentage are inherited due to a translocation.
Duchenne muscular dystrophy (DMD)
Mode of inheritance : X-linked
Duchenne muscular dystrophy is a serious progressive disease of muscles affecting
about one in 3,500 newborn boys. There are no signs of disease at birth, and affected
boys develop and grow normally until around 18 months of age. From the ages of 7
to 12, affected boys become wheelchair bound. Death from chest infection or heart
failure usually occurs by the early 20s or before.