Retinitis Pigmentosa Support Group

When dominant retinitis pigmentosa appears in a family, any given member will either be

AFFECTED (that is, manifest the disease and able to pass it to their offspring); or
UNAFFECTED by retinitis pigmentosa (completely free of the disease, and unable to pass it to their offspring).

There are no carriers (those not manifesting the disease, but nevertheless able to
pass it to their offspring) in this form of inheritance.

To understand the mechanism and probabilities involved when considering starting
or enlarging a family in which this form of retinitis pigmentosa appears, we must first
look at some basic concepts of genetics.

The genetic code which determines every characteristic of our body is duplicated;
that is, there are two genetic components responsible for every trait. Normally, should
one of these components prove defective, the other (correct) component will override
the function of the defective one. The distinguishing feature of dominant inheritance
is that, should a trait be dominantly inherited the defective copy will prevail, resulting
in this case in dominant retinitis pigmentosa.

For an affected child to be produced in this form of retinitis pigmentosa one parent
must also be affected.

Each individual child of these parents will have a 5O % (2 in 4) chance of
inheriting the abnormal genetic component. They will manifest the disease
and be affected. They will also be capable of passing the defective genetic
material to their offspring.

Each individual child of these parents will have a 5O% (2 in 4) chance of not
inheriting the abnormal genetic component. They will be unaffected by retinitis
pigmentosa. They will neither manifest the disease nor be able to pass it to
their offspring.

Please Note: A common error in interpreting genetic mechanisms when they are
expressed in terms of percentages or fractions is to assume that if one has a first child
affected through dominant inheritance (in a situation reflecting a 50% or a 2 in 4 risk
factor), that the next son will not be endangered. THIS lS NOT TRUE! AII offspring of
the same father and mother run the same risk.

It is currently possible to ascertain whether an individual is a affected by this form of
retinitis pigmentosa with electroretinographic testing. We hope this brief introduction
to dominant inheritance will help you to understand this genetic pattern of retinitis
pigmentosa.

When recessive retinitis pigmentosa appears in a family, it frequently occurs that
both parents of an affected child have good vision and appear (ophthalmologically)
essentially normal. What is the explanation of this situation?

The genetic code which determines every characteristic of our body is duplicated;
that is, there are two genetic components responsible for every trait. lt sometimes
happens that one component will be normal while the second proves defective.
Normal genetic codes override the function of defective ones, so in this case the
individual will not exhibit any symptoms but will be a carrier of the trait, in this case,
retinitis pigmentosa.


Each individual child of these parents will have a 25% (1 in 4) chance of
inheriting two defective genes. They will manifest the disease and be
affected. They will also be capable of passing the defective genetic material
to their offspring.


Each individual child of these parents will have a 50% (2 in 4) chance of
inheriting one normal and one defective gene. They will become carriers of the
disease, like their parents. They will not manifest symptoms of the disease, but
will be able to pass the defective genetic maierial to their offspring.

Each individual child of these parents will have a 25% (1 in 4) chance of
inheriting two normal genes. They will be unaffected by retinitis pigmentosa.
They will neither manifest the symptoms of the disease nor be able to pass it
to their offspring through their genetic material.

Please Note: A common error in interpreting genetic mechanisms when they are
expressed in terms of percentages or fractions is to assume that if one has a first child
affected through recessive inheritance (in a situation reflecting a 25%, or a 1 in 4 risk
factor), that the next son will not be endangered. THIS lS NOT TRUE! All offspring of
the same father and mother run the same risk.

We estimate that 1 out of 80 people in this country are carriers of recessive retinitis
pigmentosa.

The chance that an affected patient will marry a carrier is about 1 in 80;

The chance that a carrier will pass on the genetic defect is 1 in 2;

Therefore, the chance that an affected patient will have an affected child is about 1 in
80 times 1 in 2, or 1 in 160 for each individual chitdbirth.

All offspring of an atfected patient will be carriers. The situation when these
carrier offspring come to marry will be the same as that described at the beginning:

The chance that a carrier offspring will marry a carrier is about 1 in 80;

The chance that these carrier parents will produce an affected child is
1 in 4 for each individual childbirth;

Therefore, the chance that an unaffected, carrier offspring will have an affected child
is about 1 in 80 times 1 in 4, or 1 in 320 for each individual childbirth.

No test is available at present to detect carriers of this form of retinitis pigmentosa.
We hope this brief introduction to recessive inheritance will help you to understand
this genetic pattern of retinitis pigmentosa.

When X-linked retinitis pigmentosa (or RP) appears in a family, any given member will be

AFFECTED (that is, manifest the disease and able to pass it to
their offspring); a

CARRIER (those who, while not manifesting the disease are
nonetheless able to pass it to their offspring); or

UNAFFECTED as regards retinitis pigmentosa (completely free
of the disease, and unable to pass it to their offspring).

To understand the mechanism and probabilities involved when considering starting
or enlarging a farnily in which this form of retinitis pigmentosa appears, we must first
look at some basic concepts of genetics.

The genetic code which controls each characteristic of our body is governed by
forty-six chromosomes, which we receive from our parents- half from our mother and
half from our father. Of these forty-six chromosomes, forty-four are (ideally) identical,
whether they come from the father or the mother. But two chromosomes are special-
they contain the genetic coding for all sexual differentiation. There are two types of
these special chromosomes, referred to as Xs or Ys on account of their shapes.
Females always have two Xs, but males have one X and one Y.  A mother can only
contribute an X chromosome to her child. Daughters are produced when each parent
passes on an X chromosome. Sons are produced when the father passes on his
Y chromosome, rather than his X. For every birth, therefore, the probability of
producing a son or a daughter is exactly equal- 50%. The deciding factor is which of
the father's sexual chromosomes is passed on, the X or the Y.

The defective genetic code which results in this form of retinitis pigmentosa is
always found on the X chromosome. This is why this type of the disease is sometimes
referred to as "X-linked". Because these X chromosomes are responsible for
gender, a disease resulting from their faulty genetic coding will occur in a way directly
related to gender. This is why the same type of retinitis pigmentosa is sometimes
referred to as "sex-linked".

lf the mother is a Carrier and the father (as regards retinitis pigmentosa) is Normal:

Each individual son of these parents wili have a 5O% (2 in 4) chance of
inheriting the defective X chromosome. They will manifest the disease and
be affected. They will also be capable of passing the defective genetic
material to their daughters.

Each individual son of these parents will have a 50% (2 in 4) chance of
inheriting two normal chromosomes. They will be unaffected by retinitis
pigmentosa. They will neither manifest the symptoms of the disease nor be
able to pass it to their offspring through their genetic material.

Each individual daughter of these parents will have a 50% (2 in 4) chance
of inheriting one normal and one defective chromosome. They will become
carriers of the disease, like their mother. They will not manifest symptoms of
the disease, but will be able to pass the defective genetic material to their
offspring.

Each individual daughter of these parents will have a 50%" (2 in 4) chance
of inheriting two normal chromosomes. They will be unaffected by retinitis
pigmentosa. They will neither manifest the symptoms of the disease nor be
able to pass it to their otfspring through their genetic material.

lf the mother is Unaffected and the father is Affected by retinitis pigmentosa:

Each and Every Son of these parents will inherit two normal chromosomes.
They will be unaffected by retinitis pigmentosa. They will neither manifest
the symptoms of the disease nor be able to pass it to their offspring through
their genetic rnaterial.

Each and Every Daughter of these parents will be a carrier of the disease.
They will not manifest symptoms of the disease, but will be able to pass the
defective genetic material to their offspring.


Please Note: A common error in interpreting genetic mechanisms when they are
expressed in terms of percentages or fractions is to assume that if one has a first son
affected through sex-linked inheritance (in a situation reflecting a 50% or a 2 in 4 risk
factor), that the next son will not be endangered. THIS lS NOT TRUE! All offspring of
the same father and mother run the same risk.

It is currently possible to ascertain whether an individual is a carrier of this form of
retinitis pigmentosa to a 96% degree of certainty through electroretinographic testing.
We hope this brief introduction to sex-linked inheritance will help you to understand
this genetic pattern of retinitis pigmentosa.

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