Video Transcript
In this video, we will learn about
some diseases that a person or a couple may be genetically or medically tested
for. We will learn about different types
of genetic tests that are carried out before a child is conceived or born. We will also understand why some
people and couples will be offered genetic or medical screening.
Diseases are an everyday part of
human life, whether it’s due to pesky pathogens infecting our bodies or genetic
disorders that can be inherited. Humans have to live alongside a
range of diseases. One of the things we can do to
limit the spread of disease and help understand them more is to screen people for
certain diseases. Screening is a term that’s used to
refer to the process of studying a person’s DNA or general health to identify
infectious diseases, genetic disorders, or any susceptibility to them.
Let’s start by having look at
genetic screening. Genetic screening works by studying
a person’s DNA. DNA, or deoxyribonucleic acid, is
our genetic material. It controls our characteristics,
such as eye color and natural hair color, and is also the unit of inheritance. A section of DNA that codes for a
particular characteristic is called a gene. Sometimes there’s mistakes in these
genes, called mutations, that can cause certain diseases. For instance, a mutation in the
HEXA gene on chromosome 15 can cause Tay–Sachs disease. Tay–Sachs is a genetic disorder
which causes the destruction of nerve cells in the brain and spinal cord. Children with Tay–Sachs tend to
have serious seizures, problems with their movement, and unfortunately it is highly
likely to result in an early death. Tay–Sachs is caused by a recessive
allele. This means a person must inherit
two copies of the allele to inherit the disease.
Let’s take a closer look at how
this might happen. If someone has a recessive allele
for Tay–Sachs but also has the corresponding dominant allele that doesn’t cause
Tay–Sachs, then they are referred to as a carrier. This means that they have the
allele that causes a certain disease or disorder, but they don’t have the disease
themselves. We can display this as a written
genotype using the letters uppercase T for the nonmutated allele and lowercase t for
the allele that causes Tay–Sachs.
Let’s imagine this person has
children with another person who is also a carrier. When the male’s sperm fertilizes
the female’s egg, they pass on their alleles to any offspring they have. If the offspring inherit the
uppercase T and uppercase T allele, then the child will be unaffected and not a
carrier. If they inherit an uppercase T and
a lowercase t, the child will be a carrier. Likewise, if they inherit the
lowercase t and uppercase T, like this, the child will also be a carrier. And if they inherit the lowercase t
and lowercase t, then the child will have Tay–Sachs.
By carrying out genetic screening
on potential parents, we can better understand the risk they have of passing genetic
disorders on to their offspring. This then allows them to make
informed choices and prepare themselves for any potential diseases or disorders that
may arise in their children. Now let’s take a look at some
different types of genetic screening.
Carrier testing is a form of
genetic screening. It is offered to people who have a
family history of genetic disease but they themselves do not display symptoms. This type of screening would be
best for a couple that has a family history of a disease like Tay–Sachs, which we
saw in the previous example.
Preimplantation testing is another
type of genetic screening. It can be used to test embryos that
are being used in IVF, or in vitro fertilization. This is done before they are placed
in the mother’s uterus. This is why it’s
preimplantation. Again, this will most likely be
offered to couples who have a family history of genetic disorders.
Prenatal testing is carried out on
pregnant mothers before the baby is born. It’s most likely to be offered to
expectant mothers that are in at-risk groups. For example, the chances of a baby
developing the chromosomal disorder Down syndrome increases with age. So expectant mothers over the age
of 35 are often offered a prenatal test for Down syndrome.
Newborn testing is carried out on
newborn babies. This is often to screen genetic
disorders that can be treated early in life. This way, these disorders can be
treated before developing into something more debilitating. An example of this is the genetic
disorder PKU. If left untreated, PKU can lead to
severe brain damage. But if identified and treated early
in life, the babies will go on to lead a PKU-free life.
Finally, presymptomatic testing can
be used for people that have a family history of a certain disorder that they are
not displaying symptoms for yet. This type of test is most common
for disorders that tend to develop later on in life. For example, Huntington’s disease
is a neurological disease which usually starts to display its symptoms when a person
is in their 40s. By being tested earlier for
Huntington’s when they are younger, a person may be able to prepare themselves for
this disease if it will affect them later on in life.
It’s important to note that while
genetic screening is useful and informative, it is not 100 percent certain. And as some genetic disorders still
have no cure, the choice to have genetic screening is a personal choice that must be
made by the individuals involved.
Now let’s have a quick look at
medical screening. Medical screening for couples
planning on having children is often used to prevent the transmission of infectious
disease rather than genetic or inherited disease. This helps prevent the transmission
of infectious disease between couples and between parents and their children. Types of diseases that can be
tested for include sexually transmitted diseases like HIV or hepatitis or
others. These can potentially be passed on
to the fetus or newborn, so screening is very important.
Now that we’ve learned more about
genetic and medical screening, let’s try a practice question.
All of the individuals described
below are planning on having children in the near future. Person A has a grandparent who is
an identical twin. Person B has a family history of
thalassemia, a disorder affecting the hemoglobin of the blood. Person C lives in a small community
and is marrying their first cousin. And person D already has two girls
and would like a boy. Which of the people listed above
would most likely be offered genetic screening?
Genetic screening is the process by
which a person’s DNA is studied to identify any genes that could cause a genetic
disorder. It is commonly offered to those
with a family history of a specific illness or disease. Genetic screening can be carried
out either before a couple tries to conceive or during pregnancy. For high-risk parents or those that
are related, these tests are offered in the prepregnancy stage. One type of genetic screening is
called carrier screening. This is used for people with a
history of a genetic disease but who do not have symptoms. So it is unclear if they carry the
allele for that disease. We can see that this description
applies to one of our options. Person B has a family history of
thalassemia.
Genetic screening may also be
offered to those that live in isolated communities or communities that have a
history of a particular genetic disorder. This is because, typically, these
communities are made up of family groups and individuals are highly related. This means that if this grandparent
was a carrier for a recessive genetic disease like Tay–Sachs, they could pass this
on to their children. Their children, shown here, are now
carriers for the disease. But they could pass the allele on
to their own children. If these two individuals who are
cousins were now to have their own children, they have a much higher risk of passing
on two recessive alleles to their children. This means that their offspring are
much more likely to inherit Tay–Sachs disease. This means that genetic testing is
likely to be offered to another one of our individuals, person C, who lives in a
small community and is marrying their first cousin.
Genetic screening is only offered
to those who are at risk of passing on genetic diseases to their offspring. This helps prevent the spread of
disease and allows parents to prepare themselves mentally, physically, and
financially for having a child with a genetic disorder. It is not offered to people who
would like to choose the sex of their child, so we can exclude person D. And we can also exclude person A,
as genetic screening will not be offered to people who are expecting or who would
like twins.
So the correct answer to which
people would be offered genetic screening is person B and person C.
Now let’s take a moment to go over
the key points that we covered in this video. Genetic screening in conception and
pregnancy is used to limit the inheritance of genetic disorders and diseases. Different groups or individuals may
be recommended for genetic screening based on their family history or other
factors. Some examples of genetic disorders
that can be screened for include Tay–Sachs, which is screened for before pregnancy;
Down syndrome, which is screened for during pregnancy; and Huntington’s disease,
which is screened for after pregnancy and typically much later in life. Another type of screening is
medical screening. And this helps prevent the spread
of infectious diseases like HIV and hepatitis.