Overview

Some diseases run in the family, which means, one or more members of the family from different generations may have been diagnosed with the same. Further, genetic mutation can happen in some individuals which are then passed on to a child or grandchild eventually. The terms genetic, inherited and hereditary are all used interchangeably for such conditions, and to a layman, they all appear the same. However, there are subtle differences between them. In this article, we will explore the differences and briefly look at such ailments.

Introduction

Our bodies are constantly changing from generation to generation, depending on the gene mix, or the varied genes that we inherit from parents, and grandparents on both sides. The upside of this is that, we get the best of genes, or inherit positive aspects from our elders, which moulds various aspects of our personality. The downside is that genetic mutations or unhealthy changes in genes or genetic material can also be passed down over generations and affect our lives. Genetic mutations happen due to environmental and lifestyle factors.

Diseases, ailments and infections can cause subtle changes to our genetic material. Environmental factors like pollution or exposure to toxins can do the same. Addictions, substance abuse, workplace conditions – all these have an ability to cause changes in our genetic material. Unhealthy diets and lifestyle choices (exercise or the lack of it) can all do likewise. Genes are made of DNA which contains instructions for cell function, and also decides various aspects of our personality. We inherit half the genes from both parents, so the likelihood of inheriting a genetic mutation from either parent is as high as 50%.

Again, all hereditary diseases are genetic (they have passed from a parent or grandparent). But all genetic disorders are not hereditary. One or more generations may not develop this disorder, due to the healthy ‘gene mix’ over generations. Or the condition may show up after a couple of generations, if the parent/grandparent from the other family also carry such genes.

There are 4 types of genetic disorders.

• Single gene or monogenic inheritance disorders
• Multifactorial or polygenic inheritance disorders
• Chromosomal abnormalities
• Mitochondrial inheritance

Also Read: Cystic Fibrosis – Symptoms and Treatment

Single gene or monogenic inheritance disorders

As the name implies, genetic mutation or changes in the DNA sequence of a single gene causes such conditions. Also called Mendelian inheritance. Again, depending on the pattern of inheritance, these disorders are of 3 types.

• Autosomal dominant: Just 1 copy of the defective gene, that is, from either parent, is enough for the child to develop the condition.
• Autosomal recessive: The child must inherit 2 copies of the defective gene, or one each from both parents, in order to develop the condition.
• X-linked: The defective gene is present on the X-chromosome (female). Again, such inheritance can be dominant or recessive.

There are thousands of single-gene disorders, but the 7 most common ones are:

• Cystic fibrosis: Here, the genetic mutation affects cells that produce mucus, sweat and digestive juices. These fluids become thick and sticky, causing problems in the respiratory, excretory and digestive systems. The inheritance is autosomal-recessive, which means, the child must inherit one copy of the mutated gene from both parents to develop this condition.
• Alpha- and beta-thalassemia: In Thalassemia, the genetic mutation causes a shortfall in the production of haemoglobin. There are not enough RBCs in the blood, or they die quickly, resulting in lower counts of healthy RBCs in the blood. So, there is not enough oxygen-rich blood reaching cells and organs of the body, so they start dying out eventually.
• Sickle cell anaemia/disease: Here, the mutation has happened in the genes that encode the haemoglobin protein. Red-blood-cells with the mutated haemoglobin become sickle-shaped or curved in shape. Such cells cannot pass through narrow blood-vessels smoothly, thereby causing blocked or inadequate blood-flow.
• Marfan syndrome: Connective tissue is an important component found in the human body. It supports and anchors various tissue and organs in the body, thereby giving it the desired position and structure. In Marfan syndrome, the genetic mutation affects the connective tissue causing damages in the skeleton, heart, eyes, blood-vessels etc. The person is very tall and thin, with long arms, legs, fingers and toes.
• Fragile X syndrome: Here, the genetic mutation affects parts of the brain, leading to cognitive impairment, learning disabilities and awkward physical features. The person has a long face, very flexible fingers, large ears, jaw, forehead, and enlarged testicles in males. The person may develop autism spectrum disorder and/or attention deficit disorder (ADD).
• Huntington’s disease: Here, the genetic mutation causes nerve cells in the brain to breakdown. As a result, there is a progressive degeneration of brain and muscle function in the body. The condition develops in the middle-age and inheritance is of dominant type. This means, even if one parent has the defective gene, there is 50% chance the child will develop the condition.
• Hemochromatosis: Here, a genetic mutation causes the body to absorb too much iron from the food or beverages we consume. The excess iron is stored in the pancreas, liver and heart causing life-threatening ailments in these organs.

Multifactorial or Polygenic inheritance disorders

Here, multiple genes mutated over time and passed down through generations can cause various health conditions. The 7 most common ones are:

• A range of heart conditions including coronary artery disease
• Hypertension or high blood pressure
• Cancer in one or more parts of the body
• Obesity and hypercholesteremia
• Alzheimer’s disease
• Type-1 and type-2 Diabetes
• Arthritis and osteoporosis

Also Read: Hypertension: The Silent Killer

Chromosomal abnormalities

Unlike the above conditions caused by changes in the DNA, there are other conditions caused by an abnormal number or structure of chromosomes. This typically happens when there is a problem in healthy cell-division. Down’s syndrome, Turner syndrome, Klinefelter syndrome and Cri du chat syndrome are examples of conditions caused by chromosomal abnormalities.

Mitochondrial inheritance disorders

 Mitochondria are small, rod-shaped components found in the cytoplasm of plant and animal cells. They are called the lungs of cells as they are responsible for respiration of cells. Each mitochondrion contains 5 to 10 round-shaped pieces of DNA. These are the non-nuclear DNA of mitochondria. Mutations in these DNA causes a host of conditions called Mitochondrial inheritance disorders. During fertilization, only the egg cells retain their mitochondria while sperm cells do not. So, these disorders are always inherited from the mother.

Examples of such disorders are:

• MELAS, which stands for ‘Mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes’, a rare form of dementia
• An eye ailment called Leber’s hereditary optic atrophy (LHON)
• A type of epilepsy called MERRF (Myoclonic epilepsy with ragged red fibres)

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