Living organisms such as humans, cats and plants etc. are built up by cells (skin cells, blood cells, brain cells, etc.). Cells, in turn, are made up of proteins. What proteins the cell contains and in what form and order they are placed determines how the cell look and what it does.
Genes encode proteins. Which proteins are formed in a cell and in which order depend on the genetic composition of the organism (the genome). Genes are made up of DNA (= a piece of a DNA strand forms a gene). The entire DNA thread with the different genes is packed (when not used) into a chromosome. Each chromosome exists in a double set and is matched to its equal in pairs as an "x" (the exception is the sex chromosome of men, consisting of an X and Y (ie not identical)). Different organisms have different numbers of chromosomes, cats have 38 chromosomes (= 19 pairs).
The chromosome, in turn, is stored in the nuclei of the cells. All cells (except germ cells) contain all of the organism's genomes but only a certain part of the genome is expressed by the cell due to what kind of cell it is. How a cell "chooses" which genes / DNA to be expressed and in what conditions are called epigenetics and I will review this in later sections.
Whole properties of an organism e.g. coat color may be due to differences in only one gene (monogenetic trait) or the outcome of several different genes (polygenetic trait), of which the latter is most common. A gene may occur in several different forms (= different alleles). Because the organism have a double-set of the genes, it may have either two different alleles (hetrozygotes) or two identical alleles (homozygotes) for each gene. The genome of two different Ragdolls (and cats in general) is identical. The difference of two individuals is due to the different alleles of the genes they are carrying. Thus, it is the pursuit of the "right" composition of alleles that drives a breeders job. For example the pursuit of the right coat color, length quality etc.
DNA - Carries the genetic information of an organism
Gene - A bit of a DNA strand encoding a particular protein
Alleles - Different variants of the same gene
Homozygot - Both alleles the organism carries are identical for the gene in question
Heterozygot - The organism carries two different alleles for the gene in question
Genome - The entire gene set of a particular organism (for example a cat)
Chromosom - Packed DNA thread
Chromosom pair - Two chromosomes with identical gene set
Epigenitics - The doctrine of how DNA is read and expressed
Monogenetic - Property that occurs due to only one gene
Polygenetic - Property that occurs in a collaboration between several different genes