My book says "alleles of the same gene always occur at the same locus, but on a pair of homologous chromosome" Please how is it possible since genes of particular character always occur on the same chromosome? When we know that alleles combine to form a gene, how comes then do they occur on homologous chromosome?
Homologous chromosomes refers to the "twin" chromosomes in your cells (one from your mother and one from your father). Each somatic cell has at least one nucleus containing all your chromosomes*2. Having two sets of all chromosomes is referred to as diploid. On a chromosome, genes are encoded through different nucleotide sequences in DNA, and thus have a specific spot on the chromosome. The spot where a gene is located is called the locus, and because you have two chromosomes in your cells that contain the same genes (though maybe different alleles!), the genes will be in the same locus (same spot) on the both of the chromosomes.
The alleles do not actually combine to form a gene. An allele is simply one variant of a gene, and there can be many alleles for one gene. If a person has the same allele on both chromosomes they are considered homozygous for that allele and if they have different alleles they are considered heterozygous for it. Genes can also be considered either dominant or recessive. If they are dominant, they can be expressed with just one copy in the cell, but if they are recessive the same allele must be present on each chromosome. The alleles do not "combine to form a gene", they are simply separate copies of the same gene with possible variations on them. Homologous chromosome simply means the other copy (e.g. a gene on the 3rd chromosome of your mother will be present on the 3rd chromosome from your father although it could be a different allele due to a variation in nucleotide sequence). I hope this helps, although I would suggest doing some more research on your own, perhaps through https://www.khanacademy.org/science/biology. Their videos tend to be very descriptive.