Genetic Disorders
Major Cause of Childhood Morbidity


Recognized genetic disorders are over 15,500. More and more new ones are being discovered as a result of advancement in knowledge of genetics and on going mutations.

Gross genetic defects are noted in about 1 percent of births. With intricate genetics involved in fetal development, it is hardly a surprise that congenital disorders do occur.

The incidence of genetic disorders:

The incidence of genetically determined disorders from the western world is estimated to be 2-4 percent of all births. This number rises to 7 percent at one year of age, as some of the genetic diseases present later.

Nearly 5 percent of teenagers and youth under 25 years of age develop serious disease attributable to a genetic component.

Gene mutation is a major cause of genetic disorders

Genetic diseases occur due to gene mutations and chromosomal abnormalities. Their effects vary in severity, and can present at any age. Some of them present even before birth.

50 percent of the fetuses from early spontaneous abortions have demonstrable chromosomal abnormality. However, all chromosomal abnormalities seen are not due to inherited genetic aberrations.

Genetic defects with clinical presentation at birth or during early childhood are the major cause of lifelong chronic morbidity or even early death.

In developed countries, it has been noticed that 30-50 percent of the hospitalized children have birth defects or genetic disorders.

Classification of Birth Defects

Depending on underlying genetic disorder

1. Single gene disorder

  • Single gene disorder occurs in 1 percent of live born.

  • It is caused by a single gene mutation, also called point mutation. Mutation alters the genetic message carried by the gene, with the result it often losses its function.
  • The gene mutation is copied every time body cells divide, and the genetic changes are passed down via the egg cells from one generation to another.

  • It has all three mode of transmissions; autosomal dominant, autosomal recessive, and X linked.

  • The relatives are at high risk of inheriting the disease.
    Some isolated cases due to new dominant mutations may also occur.

2. Autosomal Dominant inheritance

  • Trait appears in every generation

  • Each child of an affected parent has 1 in 2 chances of being affected

  • Males and females are equally affected

  • Male to male transmission occurs

3. Autosomal recessive inheritance

  • Trait appears in siblings, not in their parents or in their offspring.

  • Each sibling has 25% chance of being affected.

  • The chances of a normal sibling of an affected individual of being a carrier.cannot be predicted.

  • Males and females are likely to be affected.

  • Consanguineous marriage may precipitate rare trait.

4. X-linked Recessive inheritance

  • Incidence of trait is higher in males than in females.

  • Trait is passed from carrier mother.

  • Each daughter has 1 in 2 chances of being a carrier.

  • Each son of a carrier mother has 1 in 2 chances of being affected.

  • Trait is transmitted from affected males to all their daughters.

  • It is never transmitted from father to son

  • It may skip generations, because the trait can be passed through multiple carrier females.

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Multi-factorial genetic disorders

  • The most common cause of morbidity and mortality.

  • There is no clearly defined mode of inheritance, but these diseases do have familial predisposition.

  • Polygenic inheritance: The affected people inherit several genes from both the parents, which are predisposed to certain diseases under the environmental and lifestyle trigger. 

  • The disease predisposition affects only one organ system.

  • Common examples are diabetes mellitus, asthma, cleft lip and palate and neural tube defects.

Chromosomal disorders

  • The defect in the structure or number of chromosomes. 

  • Thousands of genes may be affected. 

  • The error occurs in meiosis during the production off egg cells.

  • Chromosomal abnormalities occur approximately in 1 of 10 fertilized ova, but only 1 in 200 of live births.

  • Most chromosomal abnormality conceptions are not viable.
    50 percent of spontaneous abortions are due to chromosomal abnormalities.

  • Both autosomal and sex chromosomes may be affected.

  • They cause multiple congenital malformations, dysmorphic features and mental retardation

Types of chromosomal abnormalities

  • Abnormality in number of chromosomes
    There may be either one chromosome from pair missing or there may be more than two chromosome.

  • Structural abnormalities: Altered form of chromosome -
  1. Deletion
  2. Duplication
  3. Trans-location
  4. Inversion
  5. Ring formation
  6. Isochromosom formation (mirror image copy of chromosome)

External links of interest:

  1. Animations of Genes Mutation causing Genetic Disorders

  2. Click on any chromosome to see a list of selected traits and disorders associated with that chromosome.

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Related pages of interest are indexed in the right column

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Genetics in Fetal Development

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Genetics

Chrmosome and Genes

Modes of Gene Expression

Meiosis and Mitosis

Monozygotic Twins

Modes of Gene Expression

Genetic Disorders

Incidence

Genetic Mutations

Single Gene Disorders

Multifactorial Disorders

Chromosomal Disorders

Genetic Screening

Prenatal Screening

Types of Genetic Screening

Indications

Quadruple Screen Test

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Amniocentesis

Chorionic villus sampling

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Risk of Mosaicism: Why?

Possible Flaws in Diagnosis

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Application of Stem Cells

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