Genetics is the science of genes, heredity, and the variation of organisms. The word "genetics" was first suggested to describe the study of inheritance and the science of variation by the prominent British scientist William Bateson in a personal letter to Adam Sedgwick, dated April 18, 1905. Bateson first used the term "genetics" publicly at the Third International Conference on Genetics (London, England) in 1906.
Human beings have cells with 46 chromosomes --2 sex chromosomes and 22 pairs of autosomal (non-sex) chromosomes. Males are "46, XY" and females are "46, XX". These chromosomes are made up of extremely long DNA molecules in combination with chromosomal proteins.
Genes are defined by intervals along one of the DNA molecules. The location of the gene is called the locus. Most genes carry information which is necessary to synthesize a protein.
The pairs of autosomal chromosomes (one from the mother and one from the father) carry basically the same information. That is, each has the same genes, but there are slight variations in the DNA sequence of nucleotide bases in each gene.
These slight variations occur in less than 1% of the DNA sequence and produce different variants of a particular gene that are called alleles.
The information contained in the nucleotide sequence of a gene is transcribed to mRNA (messenger RNA) by enzymes in the cell's nucleus and then translated to a protein in the cytoplasm. This protein may be a structural constituent of a given tissue. It may be an enzyme which catalyzes a chemical reaction, or it may be a hormone. There are also many other potential functions for proteins.
If a gene is abnormal, it may code for an abnormal protein or for an abnormal amount of a normal protein. Since the autosomal chromosomes are paired, there are 2 copies of each gene. If one of these genes is defective, the other may code for sufficient protein, so that no disease is clinically apparent. This is called a recessive disease, and the gene is said to be inherited in a recessive pattern.
In the case of a recessive disease, if one abnormal gene is inherited, the child will not show clinical disease. However, each child of these parents will stand a 50% chance of inheriting the abnormal gene. If one abnormal gene produces disease, this is called a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from mom or dad, the child will likely show the disease.
A person with one abnormal gene is termed HETEROZYGOUS for that gene. If a child receives an abnormal recessive disease gene from both parents, the child will show the disease and will be HOMOZYGOUS for that gene.
If two parents are each heterozygous for a particular recessive disease gene, then each child has a 25% chance of being homozygous for that gene and therefore, of showing the disease. If one parent is homozygous and the other heterozygous, then each child has a 50% chance of being homozygous.