Introduction to genetics

Genetics is the study of how living things receive common traits from previous generations. These traits are described by the genetic information carried by a molecule called DNA. The instructions for constructing and operating an organism are contained in the organism's DNA. Every living thing on earth has DNA in its cells.

A gene is a hereditary unit consisting of DNA that occupies a spot on a chromosome and determines a characteristic in an organism.

Genes are passed on from parent to child and are believed by many to be an important part of what decides looks and behavior.

Glossary
It helps to know the technical words used:
 * DNA is a long molecule that has the form of a "double helix". It resembles a ladder that has been twisted. In eukaryotes such as animals and plants, the DNA is stored inside the cell nucleus, while in prokaryotes such as bacteria the DNA is in the cell's cytoplasm.
 * Nucleotides form the rungs of the DNA ladder. There are four types of nucleotides, and the sequence of nucleotides carries the information in the DNA.
 * A chromosome is a package for carrying the DNA in the cells. Different species of plants and animals have different numbers of chromosomes.
 * A gene is a segment of a DNA molecule on a chromosome. The genes are like sentences built up of the "letters" of the nucleotide alphabet, and between them the genes direct the physical development and behavior of the organism.
 * Alleles are the different forms of a given gene that an organism may possess. For example, in humans, one allele of the eye-color gene produces blue eyes and another allele of the eye-color gene produces brown eyes.
 * A population is a localized group of individuals belonging to the same species. For example, all the trout of the same species sharing a single stream is a population.
 * A gene pool is the sum of all the alleles shared by members of a single population.

Mendel’s contribution
Darwin’s theory of natural selection laid the groundwork for evolutionary theory. However, it was the emergence of the field of genetics, pioneered by Gregor Mendel (1822-1884), that provided the missing information on how evolution works in practice. Mendel’s experiments with peas led him to realise that heredity in sexual reproduction works by the mixing of separate factors, not by the blending of inherited characters. This combination of Darwin's theory and our current understanding of heredity led to the birth of the scientific area called "population genetics".

Genetics of pathologies
New studies are regularly being published that expose the role that the genetics play in various disorders. Many disorders are primarily hereditary, while some disorders have a hereditary as well as an environmental component. Even complex disorders, such as Leukemia have been shown to have some genetic component.