Chapter 16.7 – Screening for clinically important genes
Genetic techniques can determine whether an unborn child might be affected by a genetic disorder,
this is called genetic screening.
Genetic screening
Genetic disorders, such as sickle cell anaemia are the result of gene mutations. Gene mutations can
arise if one or more nucleotides in DNA are either deleted or replaced by other nucleotides. If the
mutation results in a dominant allele, all individuals will have the genetic disorder. If the allele is
recessive, the genetic disorder will only occur in those that are homozygous recessive (2 copies of
the recessive allele). Heterozygous individuals will not have symptoms of the disease but carry one
copy of the mutant allele, thus have the capacity to pass the disease on to their offspring if the other
parent is also heterozygous or homozygous recessive.
It is important to screen individuals who may carry a mutant allele. Such individuals often have a
family history of disease. Screening can work out the probabilities of a couple having offspring with a
genetic disorder. As a result, potential parents who are at risk can obtain advice from a genetic
councillor about the implications of having children, based on their family history and the results of
genetic screening.
It is possible to fix hundreds of different DNA probes in an array on a glass slide. By adding a sample
of DNA to the array, any complementary DNA sequences in the donor DNA with bind to one or more
probes. In this way it is possible to test simultaneously for a number of genetic disorders.
Another area where genetic screening can be valuable is in the detection of oncogenes, which are
responsible for cancer. Cancers may develop as a result of mutations that prevent the tumour
suppressant genes inhibiting cell division. Mutations of both alleles must be present to inactivate
the tumour suppressant genes and to initiate the development of a tumour. Some people inherit
one mutated tumour suppressant gene and are therefore at greater risk of developing cancer.
If a mutated gene is detected by genetic screening, people who are at greater risk of developing
cancer can make informed choices about their lifestyle and any future treatment. They can choose
to give up smoking, lose weight, eat healthier and avoid mutagens (anything that can induce a
mutation) as far as possible. They can also check themselves regularly for signs of cancer, which can
lead to earlier diagnosis and a higher chance of treatment being successful. They may choose to
undergo gene therapy. This treatment is in the developmental stage but trials are taking place on
gene therapy for bladder, brain, liver, ovarian and prostate cancers.
Genetic screening is used in conjunction with genetic counselling. The expert advice provided by a
counsellor helps people to understand the results and implications of the screening and so make
appropriate decisions.
Genetic counselling
Genetic counselling is a special form of social work, where advice and information are given that
enable people to make personal decisions about themselves or their children. One important aspect
of genetic counselling is to research the family history of an inherited disease and to advise parents
on the likelihood of their children inheriting the disease.
Consider a mother who has a family history of sickle cell anaemia. If the mother herself is unaffected
but carries the gene that causes sickle-cell anaemia, she must be heterozygous. Suppose she wanted
to have children with a man who has no history of the condition. In this case it can be assumed that
Genetic techniques can determine whether an unborn child might be affected by a genetic disorder,
this is called genetic screening.
Genetic screening
Genetic disorders, such as sickle cell anaemia are the result of gene mutations. Gene mutations can
arise if one or more nucleotides in DNA are either deleted or replaced by other nucleotides. If the
mutation results in a dominant allele, all individuals will have the genetic disorder. If the allele is
recessive, the genetic disorder will only occur in those that are homozygous recessive (2 copies of
the recessive allele). Heterozygous individuals will not have symptoms of the disease but carry one
copy of the mutant allele, thus have the capacity to pass the disease on to their offspring if the other
parent is also heterozygous or homozygous recessive.
It is important to screen individuals who may carry a mutant allele. Such individuals often have a
family history of disease. Screening can work out the probabilities of a couple having offspring with a
genetic disorder. As a result, potential parents who are at risk can obtain advice from a genetic
councillor about the implications of having children, based on their family history and the results of
genetic screening.
It is possible to fix hundreds of different DNA probes in an array on a glass slide. By adding a sample
of DNA to the array, any complementary DNA sequences in the donor DNA with bind to one or more
probes. In this way it is possible to test simultaneously for a number of genetic disorders.
Another area where genetic screening can be valuable is in the detection of oncogenes, which are
responsible for cancer. Cancers may develop as a result of mutations that prevent the tumour
suppressant genes inhibiting cell division. Mutations of both alleles must be present to inactivate
the tumour suppressant genes and to initiate the development of a tumour. Some people inherit
one mutated tumour suppressant gene and are therefore at greater risk of developing cancer.
If a mutated gene is detected by genetic screening, people who are at greater risk of developing
cancer can make informed choices about their lifestyle and any future treatment. They can choose
to give up smoking, lose weight, eat healthier and avoid mutagens (anything that can induce a
mutation) as far as possible. They can also check themselves regularly for signs of cancer, which can
lead to earlier diagnosis and a higher chance of treatment being successful. They may choose to
undergo gene therapy. This treatment is in the developmental stage but trials are taking place on
gene therapy for bladder, brain, liver, ovarian and prostate cancers.
Genetic screening is used in conjunction with genetic counselling. The expert advice provided by a
counsellor helps people to understand the results and implications of the screening and so make
appropriate decisions.
Genetic counselling
Genetic counselling is a special form of social work, where advice and information are given that
enable people to make personal decisions about themselves or their children. One important aspect
of genetic counselling is to research the family history of an inherited disease and to advise parents
on the likelihood of their children inheriting the disease.
Consider a mother who has a family history of sickle cell anaemia. If the mother herself is unaffected
but carries the gene that causes sickle-cell anaemia, she must be heterozygous. Suppose she wanted
to have children with a man who has no history of the condition. In this case it can be assumed that
