Download How disabilities come to be

How disabilities come to be …
Causes, roots, and beginnings
Reason 1. It’s Genetic!
DNA (deoxyribonucleic acid) is a nucleic acid that is
the most basic unit of genetic information.
Chromosomes are highly organized structures
containing DNA in long strands. Most cells in our body
(except for mature red blood cells) contain a complete
set of 46 chromosomes, or 23 pairs.
< A>
Chromosomes, etc.
The chromosomes are numbered 1-22
(largest to smallest) and the 23rd pair are
the sex chromosomes, which determine
our gender (two "X" chromosomes =
female; one "X" and one "Y" chromosome
= male). We inherit our chromosomes at
the time of conception: one set of 23
from our mother and one set of 23 from
our father. As we grow from a single cell
into a complex human being, our
chromosomes are copied into each new
cell.
Genes
Genes are specific subunits or groups of DNA along the
chromosomes. Just as our chromosomes come in pairs,
so do our genes. Each gene codes for a protein (or
chemical) that has a specific function in the body.
Meiosis
Meiosis – the
formation of
ova and sperm. During
In this process,
chromosomal
mutations may
occur.
Reason 1a. Autosomal
dominant inheritance.
Autosomal dominant conditions affect males
and females equally, and only one gene of the
pair needs to be abnormal for the individual to
have the condition. Every child of an individual
with an autosomal dominant condition has a 50%
chance of inheriting the mutation and having the
disorder.
Autosomal dominant
If the AD gene is
there, it is expressed.
No carriers here.
Children have a
50/50 chance of also
being affected by the
disorder (one parent
affected).
BOTH parents have dominant
gene:
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Statistically, there is a
25% chance that the
child will inherit the two
unaffected genes and
will be unaffected.
There is a 50% chance
that the child will inherit
at least one affected
gene, and will have the
disorder.
There is a 25% chance
that the child will inherit
both dominant genes,
which is usually fatal.
Examples of AD …
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Neurofibromatosis
Tuberous Sclerosis
Marfan Syndrome
Achondroplasia**
Osteogenesis Imperfecta
(there
are 7 distinct types; Type II is NM)
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Noonan syndrome**
Cornelia de Lange syndrome**
Apert syndrome**
Treacher-Collins syndrome
**new mutations originally; AD inheritance
when passed from an affected individual.
Reason 1b. Autosomal
recessive inheritance.
Autosomal recessive conditions affect males and females
equally, but both copies of the recessive gene have to be present
for the individual to have the condition. Autosomal recessive
conditions can be passed on when each parent is a "carrier" for
the condition, and their offspring have a 25% risk of inheriting the
condition. "Carriers" have one abnormal copy of the gene but do
not have clinical symptoms and are not at increased risk to
develop the condition. A family history of a recessive condition
can reveal multiple individuals in a single generation (brothers and
sisters) with the condition; in the case of small families, however,
there may be only one affected individual.
Autosomal recessive
Two carrier parents,
statistically 1
unaffected, 2
carriers, and one
affected. (25%
chance of being
affected by the
recessive gene)
Autosomal recessive – one
carrier parent …
As indicated, if only ONE parent is a carrier
of a recessive trait, then there is no chance
for one of the
children to be
affected;
however, a
50% chance of
being a carrier.

Autosomal recessive – one
affected parent; one non-carrier:
All children will be
carriers, since they
must inherit one
affected (recessive)
gene from the
affected parent, but
one (dominant)
gene from the
unaffected parent.

Autosomal recessive: one affected
parent; one carrier parent:

With the
carrier parent,
the statistics
change to a
50% chance
of being
affected; 50%
chance of
being a
carrier.
Examples of AR …
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Usher syndrome
Retinitis pigmentosa
(AR in
20% to 25% of cases; AD in 15% -20%; XLR in 10% -- 15%)
Tay-Sachs disease
Sickle Cell Anemia
Phenylketonuria
Batten disease
Pendred syndrome
Spinal Muscular Atrophy
Albinism
Torsion Dystonia (or AD)
Reason 1c. X-Linked
Recessive Inheritance
Most X-linked conditions occur in males who
inherit an abnormal copy of the gene from their
mothers. Since males only have one X
chromosome, if it carries an abnormal copy of the
gene, they will suffer from the disorder. These
mothers carry a copy of the altered gene but are
usually unaffected if their other X chromosome
has a normal working copy of the gene.
X-Linked Recessive
An affected male never
transmits the disease to his
sons since the X chromosome
is always passed on from
mother to son. When the
mother carries a copy of a
gene for an X-linked disease,
the chance of inheriting the
altered gene is 1 in 2 in each
pregnancy for both boys and
girls, but only the male
offspring will be affected.
X-linked recessive: affected
father:
In this family, the
father is affected by
an X-linked
(recessive) disorder.
The sons inherit the
Y chromosome from
their father. All
daughters inherit a
recessive gene, i.e.,
are healthy carriers.

X-linked recessive: carrier
mother; affected father:
If the mother is a carrier (XXa) and the father
is affected (XaY), the children will statistically
be: (girls: 50% carrier, 50% affected; boys:
50% unaffected; 50% affected)
Mother: X
Xa
Father: Xa
XXa XaXa

(carrier girl)
Y
(affected girl)
XY
XaY
(unaffected
boy)
(affected boy)
X-linked recessive: affected mother
(XaXa) and father (XaY)

Mother and father are affected by the Xlinked recessive gene: all children are
affected by the recessive gene.
Mother Xa
Xa
Father
Xa
Y
XaXa XaXa
(affected girl)
(affected girl)
XaY
XaY
(affected boy)
(affected boy)
Examples of XLR …
Color Blindness
 Hemophilia
 Fragile X Syndrome
 Duchenne Muscular
Dystrophy
 Spinal Bulbar
Muscular atrophy
 Norrie Disease
 Menkes syndrome

Reason 1d: X-linked Dominant
Inheritance
When an X-linked gene is said to express dominant inheritance, it means that a
single dose of the mutant allele will affect the phenotype of the female. A
recessive X-linked gene requires two doses of the mutant allele to affect the
female phenotype. The following are the hallmarks of X-linked dominant
inheritance:
~The trait is never passed from father to son.
~All daughters of an affected male and a normal female are affected. All
sons of an affected male and a normal female are normal.
~Matings of affected females and normal males produce 1/2 the sons
affected and 1/2 the daughters affected.
~Males are usually more severely affected than females. The trait may be
lethal in males.
~In the general population, females are more likely to be affected than males,
even if the disease is not lethal in males.
X-linked dominant
Offspring of either gender
have a 50/50 chance of
inheriting XLD, if the
mother is affected. If the
father is affected, the
boys will not be affected,
but girls will always.
Examples of XLD …

Aicardi syndrome
(occasional new mutation)
Charcot-Marie Tooth
Disease (type X)
 Hypophosphatemic
rickets
 Rett syndrome
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Reason 1e: Sporadic or new
mutations
A MUTATION is a sudden, permanent
change in DNA.
This change has the potential to cause
disorders or diseases.
A sporadic or new mutation of this sort
has no real risk for recurrence, as a
general rule, and remember rules are
always broken.
Examples of Sporadic/New
mutation …
Trisomy 13 (Patau
syndrome)
 Trisomy 18 (Edwards
syndrome)
 Trisomy 21 (Down
syndrome)
 Cri-du-chat syndrome
 Prader-Willi syndrome
 Rubenstein-Taybi
syndrome
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Here are some syndromes that belong in
this category but could be considered in the
X-linked. But they go here, OK?
XXY syndrome
(Klinefelter syndrome)
 XXX syndrome
 Turner syndrome (XO)
 XYY syndrome
 And other disorders
involving multiple Xs or
Ys.
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Reason 2: Multifactorial
Many reasons exist that might actually
cause these disorders – some genetic,
some environmental, some because of
the interplay between the genes and
environment. So all of those disorders go
HERE.
Examples of multifactorial:
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Spina Bifida
Cancer
Cleft palate
Osteoporosis
Meningitis
Congenital heart defects
Encephalitis
Anencephaly
Microcephaly
Pyloric Stenosis
Hydrocephaly
Reason 3: Perinatal incidents

Conditions, complications, incidents,
accidents, mistakes, the unfolding of
unfortunate events that occur before,
during, or immediately following
childbirth.
Examples of Perinatal …
Jaundice
 Cerebral palsy
 Prematurity
 Respiratory distress
syndrome
 Failure to thrive (maybe …)
 Anoxia
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Reason 4a: Teratogens
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Agents that cause malformations in a
developing embryo. These agents cross the
placental barrier and cause or increase the
incidence of physical malformations and
behavioral and cognitive deficits. Examples are
any medication, chemical, infectious disease, or
environmental agent that might interfere with
the normal development of a fetus; result in
the loss of a pregnancy, a birth defect, or a
pregnancy complication.
Examples of teratogens …
Toxoplasmosis
 Fetal Alcohol
Syndrome
 Cocaineexposed or
addicted
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Reason 4b: Maternal
conditions
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Infections, diseases, or disorders of a
mother during pregnancy, sometimes
having the potential to injure the
unborn child.
Examples of Maternal
condition:
AIDS
 HIV
 Herpes
 Congenital rubella
syndrome
 Cytomegalovirus
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Reason 5: Abuse
The physical or mental injury, sexual abuse, or negligent
treatment of a child under the age of 18 by a person who is
responsible for the child's welfare. The specific types are physical
abuse, physical neglect, emotional abuse, emotional neglect,
sexual abuse, sexual exploitation,
physical endangerment,
abandonment, medical and
psychological neglect,
educational neglect, and
improper ethical guidance.
Examples of Abuse …
Shaken Baby
Syndrome
 Nonorganic Failure to
Thrive
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REASON 6:
Accident, Injury, Illness
Many disabilities with
long-term consequences
are caused by accidents
or illness. Some
disabilities are directly the
result of the accident or
illness; others as a
secondary condition.
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Reason 7: Unknown
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No idea. Well, maybe an idea, but nothing has
panned out so far. But still looking. We’ll let
you know.
Examples of UNKNOWN …
Autism
 Amblyopia
 Sudden Infant
Death Syndrome
 Asperger Syndrome
 Dyslexia
 ADHD
 Epilepsy
 Scoliosis
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