Chromosomes contains genetic information in an cell, which is in a nucleus of a cell.
Chromosomes are made up of a molecule called DNA (Deoxyribonucleic acid) and this forms a shape known as double helix. Sections of these molecules is called genes.
One Chromosome will have possible thousands of genes. Each gene carries the information for the construction of a protein. This gives the characteristic associated to the gene. For instance, blood group.
Different animals have different number of chromosomes: cat has 38, a chicken has 78, chimpanzees have 42 and we have 46 per cell.
Chromosomes operates in homologous pairs , and the homologous nature depends on the length of the homologous pairs.
If we look at a gene, the position is known as a gene loci. If we go to the same positiion on the homologous pair we find the same gene loci. = same gene. Therefore we have 2 versions of each gene for one characteristic. This version is called alleles.