How Can Homologous Chromosomes Be Identified?

Chromosomes condense and become visible during cell division. Normal cell division, mitosis, duplicates the DNA and then divides it accurately into two so that the new new cells formed are genetically identical.

If you can take a cell at the stage in mitosis where the chromosomes have just condensed, but have not yet duplicated, it is possible to photograph the individual chromosomes with a powerful microscope-linked camera. Homologous chromosomes are the same size and have the same pattern of banding and it is possible to sort the individual chromosome images so that the homologous chromosomes are paired up.

There are some obvious variations between different human chromosomes - the pair of chromosomes that a labelled as chromosome 1 are long, about twice as long as chromosome 8, and about 10 times as long as the short stubby chromosome 21.

Most good biology A level text books will have a picture showing homologous chromosomes from a human cell. A good one is Collins Advanced Science, Biology, by Mike Boyle and Kathryn Senior. Of course I am biased, as I am one of the authors, but it has had some good reviews.

Homologous chromosomes are defined as the chromosomes in a biological cell which pair or synapse during the process of meiosis. They are alternatively defined as those chromosomes which contain information for the same biological features. They also contain the same genes at the same loci, but they may possibly contain different genetic information at those genes. This genetic information is known as alleles.

Two homologous chromosomes may have genes which encode the colour (or colours) of the eyes, but one of the homologous chromosomes may code for a brown eye colour and the other homologous chromosome may code for a blue eye colour.

Non-homologous chromosomes are those chromosomes which represent all the biological features of an organism. There form a set of non-homologous chromosomes. The number of sets in a cell is called a ploidy. In diploid organisms, which include most species of plants and animals, each of the homologous chromosomes is inherited from a different parent. However polyploid organisms are defined as those organisms which have more than two homologous chromosomes.