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A beginner's guide to genetics: the basics

BMJ 2004; 329 doi: (Published 01 September 2004) Cite this as: BMJ 2004;329:0409316
  1. Adrién J Gonzélez, intern1,
  2. Heidy R Arrieta, research assistant1,
  3. Osvaldo M Mutchinick, chief of genetics department1
  1. 1National Institute of Medicial Science and Nutrition “Salvador Zubirén,” Mexico

In the first of our new series about clinical genetics, Adrién J Gonzélez, Heidy R Arrieta, and Osvaldo M Mutchinick explain the fundamental principles of medical genetics, which provide the basis of prevention and treatment of genetic diseases

Founded by Gregor Mendel in the 19th century, genetics is the scientific discipline that probably has the greatest potential to change the way we practise medicine in the future. Genetics affects many aspects of life--in horse racing, where certain characteristics, such as speed and strength, are chosen to be passed on to the next generation; in crop selection, to allow better vegetables to feed more people; and in recombinant DNA technology, to produce insulin and other chemicals to improve disease treatment.

For medics, the most important area of genetics is human genetics, particularly medical genetics, which deals with the interactions between the human genome and the environment in which we live and how these interactions affect our life in terms of health and susceptibility to disease.

Molecules of life

Genetic information in most living organisms, including humans, is organised in chromosomes. In human cells, chromosomes are inside the nucleus and are in pairs. Except for gametes (oocytes and spermatozoids), which have 23 chromosomes (the haploid number of chromosomes), our cells have 46 (the diploid number of chromosomes). Among the 23 pairs of chromosomes, one pair is the sex chromosomes--XX in females and XY in males.

The most important constituent of chromosomes is deoxyribonucleic acid (DNA). DNA is made of sugar (deoxyribose) and phosphate as a backbone and four nitrogenated bases: two purines, adenine (A) and thymine (T), and two pyrimidines, cytosine (C) and guanine (G), which are organised along the DNA thread in a fixed pattern.

In 1953, Watson and Crick described the structure of the DNA as a double strand of polynucleotides …

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