Are the 'alleles' same as the word homologous?
Alleles are different forms of same gene and are located on homologous chromosomes at same location.
Let us take an eukaryotic diploid cell: there are chromosomes present within nucleus of each cell. Every chromosome is made of a linear DNA molecule.
Thing is that there are two sets of chromosomes in each cell: which means each chromosome is represented twice inside the diploid nucleus. Thus there are homologous pairs of chromosomes in eukaryotic cell. For example, in each somatic cell of human body, there are 23 pair of chromosomes.
In the above diagram, you can see homologous chromosomes lying side by side: these are similar in length and there are same genes located on each homologous chromosome .
Genes are hereditary units, arranged in linear manner on chromosome and each gene controls a particular trait. Now you consider the fact that there is a gene on chromosome 9 which determines blood group type in human; but you also know that there could be four different blood group types. This is because the gene for blood group exists in three different forms: and one person can possess two different forms of the same gene.
- Different forms of same genes are called alleles and a diploid organism can alsways have two alleles of a gene: because chromosomes appear in duplicate within cell.
- These two alleles could be same or different, but are always located on homologous chromosomes.
So allele and homologue, these words are not synonymous ; but yes, alleles are always present on homologous chromosomes. One member of a homologous pair comes from father while another member comes from mother.