Introduction to Genetics and Genetic conditions

Giving a brief overview of genetics and genetic conditions

Why do we resemble our parents? How do viruses hijack cells? How does a single cell grow into a whole human? Why are some diseases more common in some races than in others? Genetics aims to find the answers.

Genetics is the science of inheritance. It aims to understand the mechanism by which the blueprints for life are passed through generations, and how variations in these blueprints are essential for evolution, yet can cause disease. 

Waves of new technologies have revolutionised genetics over the past thirty years. As a result, we now know the complete sequence of the human genome. Genetics has enabled scientists to examine the very fundamentals of life, health and development and from this position lay the basis for treatments and cures for heritable diseases.

DNA, Genes and the Blueprint for life

Humans, like every other organism, are made up of cells. We all start off as just one cell at the time of fertilisation. This cell contains two sets of genes, one from our mother and one from our father. For ease of storage and access, the genes are packaged up into 46 protein parcels called chromosomes. As the single cell divides, the genes are copied so that every new cell possesses the full complement of genetic material. This mechanism of copying the genes is quite remarkable considering that the human body contains approximately 10 trillion cells! Genes are made of a chemical called DNA. Each cell holds an amazing two metres of DNA (deoxyribonucleic acid)-if the entire DNA contained within the cells of a human being was stretched out, it would reach to the Moon and back eight thousand times.

Humans have approximately 30,000 genes stretched out along their DNA. Each gene acts as a recipe for the production of a protein and together they make up the recipe book or blueprint for you and me. Different genes or recipes are read at different times in different cells in response to the requirements of our bodies.

DNA is composed of two strands of combinations of four chemicals, which face in opposite directions and connect pairwise to form the famous double helix shape. These chemical letters are A, T, C and G (adenine, thymine, cytosine and guanine). A portion of a gene might read "TTCGACGATT" which, on its own does not make a huge amount of sense, until we learn that each three-letter sequence codes for a particular amino acid, which are the building blocks of proteins. A single gene may be many thousand letters long. When this is read by the cell's molecular machinery, a protein is made up out of the amino acids coded for.

The Human Genome Project

The Human Genome Project has now worked out the full human DNA sequence. However, it will not, on its own, uncover the nature of many of the human genes as it is still not known what role these genes play or how they interact with each other. An interesting analogy has been made between the current stage and Shakespeare's Hamlet. Research has nearly now got all the individual words of Hamlet yet they are currently all mixed up in a bucket. The beauty of the original construction will not be fully appreciated until the pieces are lined up again. Another example is if someone opened a book in a foreign language (with the same script as English). That person would recognise all the letters but would not be able to understand the meaning of the words and therefore would be unable to understand the story of the book. At present scientists are able to understand parts of the story (or parts of the DNA and what they are used for), but not all of it - yet.

Causes of Genetic Disorders and How to Measure Them.

Genes that function abnormally cause genetic disorders. Since genes are passed on from one generation to the next, genetic disorders often run in families (see section on inheritance patterns).  The major categories of genetic disorder are those caused by mutations in specific genes or those caused by structural or numerical abnormalities in the chromosomes. The possible effects of mutations in specific genes have already been explained. Chromosome abnormalities are due to a disturbance in the normal group of 46 chromosomes that we inherit from our parents. Individuals may have more (polyploidy) or fewer chromosomes than normal. Down syndrome is an example of the former; Turner syndrome the latter. Other abnormalities may result from structural changes in the chromosomes. At the time of cell division, sections of chromosomes can be detached. This part may be lost completely (deletions), it may re-attach the wrong way round (inversions) or it may attach onto another chromosome (Robertsonian translocation). Alternatively, two sections can become detached and swap positions (Reciprocal translocation).  When a cell does not accurately copy its DNA a mistake or variation in a single DNA letter (known as bases) may arise. The different DNA sequence  is likely to result in a different protein being made and if that protein is vital or crucial for the operations of the human body, disease may occur.