29 March 2012

Genetics: the basics

Ever wondered how you happened to get your mother's wavy hair, or how your father's nose happened to end up on your face?


Ever wondered how you happened to get your mother's wavy hair, or how your father's nose happened to end up on your face? The simple answer is that you inherited it.

You were born with a combination of genes from your two parents. And these determine anything from the colour of your eyes and the size of your nose, to whether or not you will be susceptible to certain diseases in later life.

Blueprint for life

Genes contain instructions for how our bodies grow and develop. They can be thought of as a kind of blueprint for the body, similar to the drawings used to build a building.

Genes dictate in which order amino acids are produced, which in turn determines which proteins are produced. And, it is through regulating proteins that genes wield their crucial power over human biology.

Proteins play an essential role in the functioning and development of the human body. If, however, there is something wrong on a genetic level, the right proteins, or the right quantities of a specific protein, may not be produced and lead to impaired functioning or disease.

Such genetic flaws can be thought of as mistakes on the building plan - the resultant building may be structurally unsound, some rooms may be too small, or there may be problems with the ventilation.


Humans have 46 chromosomes, consisting of 23 pairs. Chromosomes are made of long strands of DNA containing genetic information.

There are two main kinds of chromosomes, sex chromosomes and autosomes. Males have one X and one Y sex chromosome, while women have two X chromosomes. The remaining 22 pairs of chromosomes are all autosomes.

During fertilization a special process of cell division known as meiosis takes place. This pairs 23 chromosomes from the mother with 23 chromosomes from the father, which explains why children inherit characteristics from both parents.

The exact way in which genes, and as a result physical traits, are inherited can, however, be quite complicated.

Basic inheritance

We receive two copies of each gene – one from each parent. Copies of a gene are referred to as alleles.

When you receive two identical alleles, you are homozygous for that gene. If the two alleles are not the same, you are heterozygous.

Kinds of inheritance

Autosomal dominant: In this kind of inheritance one allele 'overpowers' the other one. Thus, a single gene, from either of your parents, will determine the related phenotype (the expression of the gene.) An example of this is Huntington’s disease, where only a single copy of the gene is required for a person to get the disease.

Autosomal recessive: Recessive genes lose out to other genes, which means you need two copies of the recessive gene for it to be expressed. Thus, to get a disease like cystic fibrosis, you will have to get a copy of the cystic fibrosis gene from each of your parents.

X-Linked: In X-linked inheritance, the inherited gene is located on the X chromosome. Because women have two X chromosomes, they usually need two copies of the gene for it to be expressed. Since men have only one X chromosome, they only need one copy of the gene and are therefore much more prone to X-linked genetic diseases.

X-linked inheritance also has dominant and recessive patterns – although the former is very rare. Haemophilia, colour blindness, and certain kinds of muscular dystrophy are examples of X-linked recessive diseases.

More complex patterns

Inheritance can often be much more complicated than the three patterns outlined above. Not just one, but large numbers of genes can be involved in the inheritance of some traits or diseases.

Furthermore, the fact that you have a set of genes that places you at a high risk for developing heart disease or diabetes, does not necessarily mean you will develop heart disease or diabetes. Things are complicated by environmental and lifestyle factors. Thus, if you are lucky, eating right and getting enough exercise may help you dodge the diabetes that has been plagueing generations of your family.

Scientists are increasingly capable of determining which genes you have and what their impact may be on your life. Unlike with buildings, we do not have the full blueprint at our disposal. But scientists are hard at work filling in the blanks.

Through genetic screening, it is already possible to determine whether someone is likely to develop certain heart or blood diseases, long before the symptoms manifest themselves. Specific precaustions can then be taken against developing these diseases.

Not everything can be side-stepped though, and therefore, scientists are at work finding safe ways to change your genetic makeup. So-called gene therapy aims to replace defective genes with corrected copies. Gene therapy has however only had very limited success so far. – (Health24)

Genetics at GlaxoSmithKline
The Human Genome Site

Read more:
Gene therapy: what you should know
DNA tests: for which diseases?


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