Homologous Chromosomes

This article will explain the concept of homologous chromosomes, and give you an overview of their importance in genetics.
BiologyWise Staff
Genetics is a branch under the vast and fascinating field of biology, which studies genes, heredity, and organism variations. One of the primary units of the study of human genetics is the chromosome. The human body is made up of billions of cells - each human cell is made up of 46 homologous chromosomes, which are paired to make 23 pairs. Of these 23 pairs, 22 are autosomes (non-sex), whereas one single pair is a sex chromosome. In females, all the 23 pairs are identical, that is, each pair is made up of 2 X chromosomes. In males however, the sex chromosome is made up of one X and one Y, which pair up to form an XY sequence. At the genetic level, this is the difference between the male and female of the species.
Homologous chromosomes are the ones with identical gene sequences. In organisms that reproduce sexually, like humans, each parent provides one half of the 46, which corresponds exactly with its matching pair. They pair during meiosis, which needs to occur for the formation of gametes, or reproductive cells. For further clarification, the following are explanations of the above terms:
This is the two-part process of cell division in organisms that reproduce sexually, at the end of which four 'daughter' cells are produced from each 'parent' cell. Meiosis is divided into two stages, meiosis I and meiosis II, each of which contains a prophase, metaphase, anaphase, and a telophase.
Each biological cell contains a nucleus in which an organized structure of DNA and protein combine to form a chromosome. It is the structure that contain the genes.
These are units of heredity that give rise to certain characteristics in offspring. They are sections of DNA carried on the chromosome, and control specifications, like height, hair type, and eye color. Genes can be of many types, two of which are explained below.
Dominant and Recessive Gene Pairing
Each pair of chromosomes in humans contains a code for a biological feature, like eye color or hair type (straight or curly). These codes may be identical (to give rise to the same characteristics in offspring) or different (made up of one dominant and one recessive gene). In conditions where genes are different, the traits in the offspring will correspond to the dominant gene. However in some cases, if both parents have one dominant and one recessive gene for that phenotype or characteristic, two recessive genes may also pair to produce a characteristic different from both parents. The easiest way to explain possible pairing is by using the example of eye color. Each person has two genes that control eye color. These two genes may be a recessive-dominant, a dominant-dominant, or a recessive-recessive combination. For the sake of further explanation, take a look at the possibilities below:
Case 1: Parents with brown eyes have a blue-eyed child. In this case, it's most likely that both sets of parents had one pair of dominant (brown eyes) and recessive (blue eyes) genes as a part of their genetic makeup. The child inherited a recessive-recessive gene pairing, which is why he/she has blue eyes.
Case 2: Children of parents with one parent who has blue eyes and one parent who has brown eyes with a recessive blue eye gene, will exhibit have 50% chance of exhibiting blue eyes.
Case 3: A child who has one blue-eyed parent and one brown-eyed parent without a recessive gene, will have brown eyes.
Crossing Over
Exchange of genes between homologous chromosomes is known as crossing over, or chromosomal crossover. This exchange of genetic material occurs during the prophase stage, one of the different stages in the process of meiosis or cell division. It involves the exchange of gene alleles from one chromosome, with the alternative allele on a corresponding one. This results in a crossover or a recombinant chromosome, which contains a combination of alleles present on the original or parent counterparts.
Genetic Disorders
Sometimes, the information on genes goes through a change or a variation. This is called gene mutation and can occur when the cells are aging, or because of exposure to toxic chemicals or radiation. Most often, cells recognize these variations and repair themselves, but in some cases, the mutated gene replicates, giving rise to a genetic disorder. On reproduction, the mutated gene may replicate in the offspring. Carrying a mutated disease gene does not always mean that you will contract the disease it carries. Since you inherit genes from both parents, you may inherit a normal gene along with a diseased gene. The chances of contracting the disease will mainly arise if the mutated gene is a dominant one. There are various types of genetic disorders, examples of which include color blindness and cystic fibrosis.
Human genetics is making awe inspiring advances in understanding the patterns that give rise to diseases and disorders and uncovering ways to treat them or eliminate their occurrence. I hope this article has helped you gain some understanding some basic facts about the science of genetics.
DNA Background
Chromosome on Blue Background
Illustration of Meiosis