Chromosomes are a long strand of DNA in the shape of a double helix containing genes. Chromosomes occur in pairs (except in gametes). Alternative factors for the same characteristics are termed alleles, e.g. tall and short are alleles for height.
For each characteristic there are at least two factors controlling the phenotype (the appearance of the organism).
Factors that are the same are termed homozygous, e.g. TT and tt.
Factors that are different are termed heterozygous, e.g. Tt.
In the heterozygous condition the factor that is fully expressed is termed dominant and the factor that has no noticeable effect is called the recessive.