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Genetics is the science of studying the genes, the genetic variation and the inheritance of the characteristics from one generation to the other. Every living organism is defined by its genetics. Genetics is all around… Our knowledge on genetics first started during the pre-historic period, as humans domesticated animals and started breeding specific plant species, acknowledging that some characteristics were passed from the parents to the next generation.

Nowadays, we know that all physical inborn characteristics that combine a human being, depend on genes…..Genes- this magic word- are inherited by the parents to their children and define all of their features. Eye and hair color, height, weight and many more characteristics are a result of the combination of many genes. Even mental skills and natural talents are affected by genes in close interaction with the environment. On the other hand, many disorders and diseases are caused by genes.

Nowadays, we know that all the genetic information is enclosed in the inner of our cells, specifically in the cell nucleus. The genetic information could be compared to a book, whose half of the chapters derive from the mother and the other half from the father. This book contains sentences- the chromosomes, which consist of words- the genes.

Human beings have 23 pairs of chromosomes in each somatic cell, which in total add up to 46 chromosomes.  Half of these 46 chromosomes are of maternal origin and the other half are of paternal origin. Some chromosomes have a large number of genes, while other carry only a small number. Chromosomes consist of what we all have heard as “DNA”, which refers to the chemical structure of Deoxyribonucleic Acid. Chromosomes of the pairs 1-22 are called autosomes and are common for both sexes. The last pair consists of chromosomes X and Y, which are called sex chromosomes and are differently distributed between males and females: a male human has one Y and one X chromosome (symbolized XY), while a female human has two X chromosomes (symbolized XX).

The chromosomes that form a pair (homologous chromosomes) are identical in terms of having the same kind of genes at the same location. However, genes of the same location in the two homologous chromosomes may differ on the nature of their genetic information. These genes are called allelic (there is a gene that defines a characteristic, the allelic gene A results in phenotype A and the allelic gene B results in phenotype B. Both genes are in the same location on the homologous chromosomes and control the same characteristic, but in a different way).

Every human inherits one chromosome of each homologous pair from the mother and one from the father. Especially, regarding the sex chromosomes, all humans inherit one X chromosome from their mother, who has two X chromosomes (XX). The second sex chromosome is inherited by the father and could be either X or Y, as he has an X and an Y chromosome (XY). As a result, sex is defined by the Y chromosome and all females have as sex chromosomes a pair of XX, while all males have a pair of XY.

Every word- gene- consists of letters- bases- which are adenine (A), thymine (T), guanine (G) and cytosine (C). These four bases can be combined in multifarious ways, defining the function of the body cells. Genes could be affected by age and may develop errors- mutations and malfunctions due to environmental factors and endogenous toxins. If a mutation occurs inside a gene, then alters the base sequence and could eventually change or destroy the genetic information for the synthesis of a protein. In such cases, depending on the importance of the non-functioning protein, various diseases could emerge.

Genetic mutations can cause three different kind of disorders:

1) Chromosomal abnormalities

2) Multifactorial genetic diseases/ disorders

3) Monogenic diseases

Monogenic diseases result from alterations in a single gene. We all have two copies of each gene (allelic genes); one comes from our mother and the other from our father. Monogenic diseases may result by a mutation in a single gene. A monogenic disorder example is Achondroplasia, which results in dwarfism.

In other cases of monogenic diseases, mutations in both the allelic genes have to coexist for someone to be a patient. Such examples of monogenic diseases are Thalassemia a, Thalassemia b, or cystic fibrosis. If the mutation exists only in one of the two allelic genes, then this person is not considered patient, but a carrier of the disease. Carriers do not show symptoms of the disease. However, if both the partners are carriers of the same gene mutation, they have 25% chances of having a child suffering from the disease.

Chromosomal abnormalities

Chromosomal abnormalities are cases of irregularities in the structure or the number of the chromosomes. Therefore, there are structural or numerical chromosomal abnormalities. These abnormal situations are present when someone has one or more chromosomes with detriment. A numerical chromosome abnormality could exist due to surplus chromosomes (trisomy or polysomy) or on the opposite, due to lack of some of them (monosomy).  The most characteristic case of trisomy- existence of one surplus chromosome- is Down syndrome. People with Down syndrome have one extra 21 chromosome, instead of just a pair of chromosomes 21, and for this reason the syndrome is also called trisomy 21. Regarding monosomies, the most characteristic case is Turner syndrome or X0. People showing Turner syndrome are women, who lack one X chromosome.

Multifactorial genetic diseases

Multifactorial genetic diseases may arise from the interaction of genes with a combination of environmental factors and lifestyle. Usually, there are many implicating genes in the determination of a disease and not just one. Such examples are various cancer types, heart conditions, Alzheimer disease and diabetes.

In these cases, a genetic analysis can relief, provide therapy or help in the prevention of a disease. Moreover, information of a genetic examination can help other family members as well, since there may be predisposition to a genetic disease. Such an example is breast cancer. Women with a family history of breast cancer are advised to check for BRCA1 and BRCA2 genes, in order to evaluate the chances of showing the disease. A positive result in a genetic analysis of this kind may seem portentous in the beginning, but the right consultation can lead to prevention and prompt therapy.

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