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Mental Disability

Genetic Abnormalities

The individual factors of inheritance, which are carried by the chromosomes, are called genes and each chromosome in a pair (except for the X and Y) has a similar complement of genes. Thus one gene of each pair has come from the father and the other from the mother.

Genetic disorders can be inherited in much the same way a person can inherit other characteristics such as eye and hair colour, height and intelligence. Children inherit genetic or hereditary information by obtaining genes from each parent. There are three common types or modes of inheritance: dominant, recessive and X-linked (or sex-linked). Sometimes a pair of genes is necessary to produce an effect, then the genes are called recessive; sometimes only one is necessary and then the gene producing the effect is called dominant. Where several genes are involved then the term multifactorial is used.

Each of us has thousands of genes and each gene is responsible for the direction of a specific protein or component of our bodies. During inheritance a gene may undergo a change and such a change is called a mutation. A gene that carries a mutation is unable to instruct the body to grow correctly, causing a disruption in normal development and functioning. This could be the cause of mentally challenged in people. X Rays and other forms of radiation during pregnancy can cause mutations and genetic damage.

Genetically determined conditions may fall under these four headings:

Autosomal Dominant Inheritance

Autosomal Recessive Inheritance

Sex Linked Recessive

Polygenetic inheritance

1. Autosomal Dominant Inheritance:

Dominant inheritance occurs when one parent has a dominant, disease-causing gene, which causes abnormalities, even if coupled with a healthy gene from the other parent. Dominant inheritance means that each child has a 50 percent chance of inheriting the disease-causing gene. An example of dominant inheritance associated with mentally challenged is tuberous sclerosis.

Tuberous sclerosis is a genetic disorder that causes benign tumours to form in many different organs - primarily in the brain, eyes, heart, kidney, skin and lungs. A person with this gene, is usually Mentally Challenged and may have a facial rash. Mental and physical deterioration may occur.

2. Autosomal Recessive Inheritance

Recessive inheritance occurs when both parents carry a disease-causing gene but outwardly shows no signs of any disease. Parents of children with recessive conditions are called "carriers" since each parent carries one copy of a disease-bearing gene. They show no symptoms of having a disease gene and remain unaware of having the gene until they have a child who is affected. When parents who are carriers give birth :

Each child has a 25 percent chance of inheriting both disease genes and being affected.

Each child also has a 25 percent chance of inheriting two healthy genes and not being affected.

Each child has a 50 percent chance of being a carrier of the disorder, like their parents.

It may also skip a generation and may reappear after one or more generation.

Examples of disorders, which are inherited recessively and are also associated with mentally challenged, include:



Tay-Sachs disease

Hurlers Syndrome


PKU (phenylketonuria), in its "classic" form, is a rare, inherited metabolic disease that results in mentally challenged and other neurological problems when treatment is not started within the first few weeks of life. When a very strict diet is begun early and well-maintained, affected children can expect normal development and a normal life span. PKU is carried through an "autosomal recessive" gene. This means that both parents are "silent carriers" of the gene. When they conceive a child, there is a one in four (or 25%) chance for each pregnancy that the baby will have PKU. The incidence of carriers in the general population is approximately one in fifty people, but the chance that two carriers will mate is only one in 2500.Carrier tests are available only through PKU treatment programmes.

The disease arises from the absence of a single enzyme. This enzyme normally converts the essential amino acid, phenylalanine, to another amino acid, tyrosine. Failure of the conversion to take place results in a buildup of phenylalanine. Through a mechanism that is not well understood, the excess phenylalanine is toxic to the central nervous system and causes severe problems normally associated with PKU. Not every child has the same degree of enzyme deficiency. However, some have enough enzyme activity that the diet can be quite liberal, while others must have a very strict diet. An experienced PKU treatment programme must determine the nature of the diet for an individual child.

The condition can be treated with a high degree of success if diagnosed shortly after the birth. The treatment is based on a low phenylalanine diet.


It is a rare hereditary disease leading not only to cirrhosis in infants, but more seriously, to early devastating illness if not diagnosed quickly.

This disease is caused by elevated levels of Galactose (a sugar in milk) in the blood resulting from a deficiency of the liver enzyme required for its metabolism (breakdown). A child will only get the disease if inherited from both the parents.

The signs of the disease usually appear in the initial days of life following the ingestion of breast milk or formula milk. Vomiting, liver enlargement and jaundice are often the earliest signs of the disease (all children who get jaundice at birth do not, however, have Galactosemia) but bacterial infections (often severe), irritability, failure to gain weight and diarrhea may also occur. If not recognized in the newborn, the disease may produce liver, brain, eye and kidney damage. It may lead to mentally challenged in the child.

Blood tests can help at arriving at a diagnosis. Treatment is based on elimination of Galactose or formula milk from the diet. This may be done in the early neonatal period by stopping breast-feeding and by the administration of diets that contain no lactose or Galactose. This diet should be compulsively followed and continued for years and possibly for life. The red blood cell levels of Galactose or its metabolites, may be used as a monitor to gauge the adherence to the diet and restriction of Galactose. It is also recommended that mothers of affected infants be placed on a Galactose-free diet during the subsequent pregnancy. This may somewhat modify symptoms present at birth. With early therapy, any liver damage which occurred in the first few days of life will nearly completely heal.

Tay-Sachs Disorder

Tay-Sachs and most of the allied diseases are autosomal recessive disorders; that is, they are transmitted through the genes in the same way as eye colour is passed from parent to child. Even though they are inherited conditions, most families are not aware that they carry genes for the disease until the birth of their child, who is affected by it.

Almost all of our genes come in pairs, one inherited from each of our parents. A carrier of a genetic condition is a person who has one copy of the same gene. A recessive condition like Tay-Sachs results when a child inherits two copies of an altered gene, one from each parent. Both parents must be carriers of the same recessive disease gene in order to affect their children. Carriers themselves do not have the disease and carrier status does not affect the mother or the father physically, mentally or in any other way. The only consequence of being a carrier of a recessive genetic trait is the possibility of transmitting the particular genetic trait to a child.

High-risk couples, in which the man and the woman are carriers of the same genetic condition :

Have a 25% chance with each pregnancy of conceiving a child with that condition.

Have a 50% chance of producing a child who is a carrier like the parents.

Have a 25% chance that the child will be neither a carrier nor affected with the disease.

Tragically, there is no cure and no treatment that will prevent the disease from running its course. Affected children can only be made as comfortable as possible.

Hurler Syndrome

Hurler Syndrome is a genetic disorder caused by a 'bad' chromosome pair that does not contain the information needed to produce a certain enzyme. This enzyme normally breaks down the Mucopolysaccharides (MPS) in the body. People with MPS can't break them down and they are stored in cells in the body causing progressive damage. This is why Hurler Syndrome is also known as a 'storage disorder'. The child appears normal at birth. They progressively get worse as the Mucopolysaccharides builds up. The skeletal system and internal organs, including the brain, is affected. As the disorder progresses, the organs are enlarged and the skeletal deformities cause problems in walking and other areas such as the hands becoming claw like (the fingers begin to curl in and won't straighten out). The brain is damaged by the build-up leading to severe challenge. The affected child may have cardiac or respiratory problems.

Genetic counselling is important for prospective parents with a family history of Hurler syndrome. In addition, tests are available for the prenatal diagnosis of Hurler syndrome. The test consists of an amniocentesis and collection of amniotic fluid, which are then cultured and activity in the cells is determined.

3. Sex Linked Recessive:

In this type of genetic condition :

Males suffer when they inherit the recessive gene on their X chromosome.

There is a constant risk for the male child of a carrier mother suffering from the condition. Females only carry the condition.

All female children of male sufferers will be the carriers of the condition.

Male children of male sufferers will neither be sufferers nor carriers.

A number of genetically determined conditions associated with mental handicap are inherited in an X (sex) linked manner. Males are affected, while females are carriers of the conditions.

If a mother has a female child, the child has a 50 percent chance to inherit the disease gene and be a carrier and pass the disease gene on to her sons .On the other hand, if a mother has a male child, he has a 50 percent chance of inheriting the disease-causing gene since he has only one X chromosome. Consequently, males cannot be carriers of X-linked recessive disorders. If a male inherits an X-linked recessive disorder, he is affected. Some examples of X-linked inheritance associated with mentally challenged include Fragile X Syndrome, Hunter Syndrome, Lesch Nyhan Syndrome and Duchenne Muscular Dystrophy.

An example of sex linked recessive inheritance is the Hunter's Syndrome. It is defined as a Lysosomal storage disorder characterized by the accumulation of acid Mucopolysaccharides in the Central Nervous System and peripheral tissues. This is a form of Mucopolysaccharidosis (Gargoylism), which affects males. There is a slow rate of physical and mental deterioration and sufferers usually survive into adulthood. The degree of mentally challenged has been found to vary from borderline to profound, with the majority being moderate to severe.

Fragile X Syndrome occurs when there is an abnormal area on one of the X-chromosomes. Men and women can be affected, although more frequently females are carriers and pass the abnormal chromosome to their male offspring. It is possible for the diagnosis to be made in pregnancy and carrier women can also be detected.

The affected people have lax joints, a long face and large ears, large testicles, cardiac abnormalities and a large head with a large nose. They have speech abnormalities, anxiety, hyperactivity and depression.

4. Polygenetic Inheritance / Multifactorial Factors:

A number of congenital conditions are thought to be caused by a number of genes acting together. The risk of inheritance is much less than in a single gene inheritance. But the risk is not constant. The risk increases with the number of first degree relatives who suffer from the particular condition. Combinations of the multiple genes and the environmental factors leading to mentally challenged are called multifactorial disorders. They are inherited but do not share the same inheritance patterns typically found in single-gene disorders. It is unclear exactly why they occur. Their inheritance patterns are usually much more complex than those of single gene disorders because their existence depends on the simultaneous presence of heredity and environmental factors. For example, weight and intelligence are traits inherited in this way. Other common disorders, including cancer and hypertension, are examples of health problems caused by the environment and heredity. Multifactorial disorders are very common and are responsible for a majority of the birth defects.

Examples of multifactorial disorders include heart disease, diabetes, spina bifida, anencephaly, cleft lip and cleft palate, clubfoot and congenital heart defects.