In genetics, homologous chromosomes contain genes for the same characteristics of an organism. More specifically, each copy of the chromosome has the gene for the characteristic located in the same place, or loci. Homologous comes from the Greek where homo means like and logous means proportional or in proportion to.
Chromosomes are made up of protein and DNA, deoxyribonucleic acid, which carries instructions for the inheritance of characteristics within organisms. Almost all cells of an individual organism have the same amount and type of DNA. The DNA is found mostly in the nucleus in tightly coiled threads, which are the chromosomes. Depending on the cell type, different parts of the chromosomes are used to make different substances, which determine the cell’s function.
The instructions for an organism’s characteristics are actually located within the genes. Genes are sections of DNA, so many genes can be found along the lengths of DNA making up the chromosomes. All genes have more than one version, so when chromosomes are inherited from parent cells, the version of the gene, or allele, is not always the same. Although homologous chromosomes have the same genes, they do not have to have the same allele for each gene.
If each chromosome of the homologous chromosomes pair has the same allele for a particular gene, it is said to be homozygous. If the alleles are different, then the cell is referred to as heterozygous. The way in which the alleles interact determines the characteristics of the offspring. Alleles can be dominant or recessive and, if different alleles are present, the dominant allele will determine the characteristic of the offspring.
Within the somatic, non-reproductive, cells of each organism, there are two copies of each chromosome. Cells that have two copies of each chromosome are called diploid cells and those that have only a single copy are called haploid cells. Reproductive cells, or gametes, only have one copy of each chromosome, so they are haploid. This ensures that the cells that are formed through the fusion of two gamete cells have the correct number of chromosomes after fertilization.
For example, human somatic cells have 46 chromosomes within their nuclei, or 23 pairs of homologous chromosomes. One of each pair comes from the mother and one from the father. The human gamete cells, sperm and ova (eggs), each contain only 23 chromosomes. During fertilization, one sperm and one ovum fuse creating a cell that has the full complement of 46 chromosomes.