Genetics
In the broadest definition, genetics is the study of genes which are units of inheritance. Genes can exist in different versions called alleles that are combined into sets called genotypes. In humans, and many species, one allele is inherited from each parent (although there are many exceptions to this such as the pattern of inheritance of X-chromosomes) and the combination of parental alleles makes a genotype. An individuals genotypes affect their phenotype (appearance and behaviors). There are many genes (tens of thousands) and an effectively unlimited number of ways these can be combined into a persons unique genotypes. Genetics is not only the study of a genes influences on traits in a fully deterministic sense. It is also the study of how non-genetic (environmental) effects interact with a set of genotypes to influence phenotypes.
Genes have a molecular basis as a DNA sequence organized into chromosomes that are shared in identical copies by all cells in an organism (again with exceptions). Many genes are transcribed into an RNA sequence (mRNA or messenger RNA) that is translated into a amino-acid sequence. The amino-acid sequence (a protein or polypeptide) then folds into a complex three dimensional shape with unique physical and chemical properties, which are encoded into the original DNA sequence. Many of these proteins carry out chemical changes needed for a cells function There is a complex process of gene regulation that adjusts gene expression during development and in response to changes in the environment that results in different cells in the body having different patterns of gene expression to make tissues and organs unique despite sharing the same genome.
Genetics is also the study of how genes and alleles change over time. Mutations occur that alter the DNA sequence of a genome every generation. These changes occasionally affect the properties of the genes product creating a new allele and altering phenotypic traits. These changes are sometimes passed on to descendants in a family. Also, natural selection, genetic drift, and other forces result in changes in allele frequencies in a population over multiple generations (such as the removal of lethal allele combinations or the increase in frequency of alleles that confer disease resistance). Over many generations these cumulative changes result in the evolution of a population or even a whole species.
