Homologous Chromosomes
Although homologous chromosomes may differ in the variety of alleles for a particular gene they carry, they are generally identical in gene content, gene loci, overall chromosome size, and centromere placement. Homologous chromosomes synapse (pair) during meiotic division of cells.
Every human somatic cell (i.e., non-sex cell) contains two copies of each chromosome, one received from the father (paternal side) and the other from the mother (maternal side). In humans, this means a set of 23 chromosomes inherited from each parent. Therefore, human somatic cells contain 22 homologous autosomic pairs, and the other two are a pair of X chromosomes (females) or a XY pair (males). What characterizes chromosomal homologies are the similar shape of chromosomes and their genetic content, with each corresponding gene localized in the same position in the two chromosomes. Homologous chromosomes have the same function in the cells.
During gametogenesis, which produces gametes or sex cells, homologous chromosomes are separated during meiosis into different daughter cells, whereas mitosis allocates to each new forming nucleus a complete set of 46 chromosomes. Therefore, somatic cells show a chromosomal diploidy, being thus also termed diploid cells since they contain two chromosomal sets in the nucleus. However, egg cells and spermatozoids are haploid cells, because they contain only one chromosomal set.
Each copy of a given gene, known as allele, may present discrete differences called polymorphisms. Polymorphic variations respond for different versions of a given gene that lead to differences in cellular metabolic rates and a number of individual degrees of susceptibilities to chemical exposure, medicines, pollutants, food, etc.
When the two alleles of a given gene differ the organism is said to be heterozygous because they contain differing alleles at the same loci on homologous chromosomes. The differing alleles on homologous chromosomes exhibit dominant recessive relationships with respect to the alleles carried on the heterozygous alleles. Although not all genes show dominance relationships between alleles, when it does occur there is a differential expression of a particular gene carried on homologous chromososmes.
In mammals, an inherited mutation in one allele may cause an alteration on the function of one of two genes on homologous chromosomes. However, a mutation is often not expressed when the other allele remains capable of normal expression. Such mutation may or may not remain silent, without phenotypic expression. However, depending upon the functional relevance of such gene, one mutated allele may represent a serious impairment that leads to a monogenic disease, such as sickle anemia, hemophilia, or a number of respiratory disorders. Monogenic diseases may result from mutations either in dominant alleles or in recessive ones. Nevertheless, since such mutations cause a dominant phenotype, they are known as autosomic dominant mutations. Other diseases require the mutation of a given gene in the two homologous chromosomes (i.e., loss of heterozygosity) to the pathological phenotype onset, such as cancer.
This is the complete article, containing 468 words
(approx. 2 pages at 300 words per page).
