Homologous chromosomes contain genes for the same characteristics in an organism, with each copy of the chromosome having the gene for the trait located in the same place. Chromosomes are made up of proteins and DNA, and genes are sections of DNA with more than one version. Homozygous chromosomes have the same allele for a particular gene, while heterozygous chromosomes have different alleles. Somatic cells have two copies of each chromosome, while gametes have only one copy. Human somatic cells have 46 chromosomes, while human gamete cells have only 23 chromosomes.
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 trait located in the same place, or loci. Homologous comes from the Greek where homo means similar and logous means proportional or proportionate to.
Chromosomes are made up of proteins and DNA, deoxyribonucleic acid, which carries instructions for the inheritance of characteristics within organisms. Almost all cells in a single organism have the same amount and type of DNA. DNA is mostly found in the nucleus in tightly coiled strands, which are the chromosomes. Depending on the cell type, different parts of the chromosomes are used to produce different substances, which determine the function of the cell.
The instructions for an organism’s characteristics are actually found within the genes. Genes are sections of DNA, so many genes can be found along the lengths of DNA that make up 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 chromosome pair has the same allele for a particular gene, it is said to be homozygous. If the alleles are different, the cell is said to be heterozygous. How the alleles interact determines the characteristics of the offspring. The alleles can be dominant or recessive, and if different alleles are present, the dominant allele will determine the trait 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 one copy are called haploid cells. Reproductive cells, or gametes, have only 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. Human gamete cells, sperm and ova (eggs), each contain only 23 chromosomes. During fertilization, a sperm and an egg fuse, creating a cell that has the full complement of 46 chromosomes.
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