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Molecular Basis of Heredity: Part 2. Genomes

Author(s): Raye L. Alford, PhD

The Human Genome (I)

The haploid human genome consists of 23 different chromosomes. The diploid human genome consists of 23 pairs of chromosomes. One of each pair of chromosomes is inherited from each parent. For example, each of us carries two copies of chromosome 1: one from our mother, the other from our father. The first 22 pairs of human chromosomes are called autosomes. The 23rd pair of human chromosomes determine gender. Human females carry two X chromosomes: one inherited from the mother, one from the father. Human males carry one X chromosome and one Y chromosome. Males inherit their X chromosome from their mothers and their Y chromosome from their fathers.

In addition to the nucleus, the mitochondria of human cells also carry one or more copies of a small, circular DNA molecule. The mitochondrial chromosomes are inherited through the egg. Sperm do not contribute mitochondria to embryos. As a result, genetic diseases caused by mutations in genes on the mitochondrial chromosome are passed through the maternal lineage, from mothers to all their children. Children of fathers affected by mitochondrially transmitted genetic disorders are not at risk for inheriting the diseases. However, because mitochondria also import proteins encoded by nuclear genes, the inheritance of diseases of the mitochondria do not always follow this pattern. Examples of mitochondrially transmitted genetic diseases include MERFF (myoclonic epilepsy with ragged red [muscle] fibers), LHON (Leber hereditary optic neuropathy), a form of dementia called MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes), MIDD (maternally inherited diabetes and deafness) syndrome, and genetic susceptibility to aminoglycoside ototoxicity (aminoglycosides are a group of antibiotics that include gentamicin, streptomycin, tobramycin, kanamycin, neomycin, amikacin, and others). In general, observable effects of mitochondrial mutations are believed to be a reflection of how sensitive particular tissues (central nervous system, skeletal muscles, heart, kidney, and liver) are to energy metabolism.