Autosomal Inheritance
Autosomal inheritance describes the patterns of genetic transmission related to genes located on autosomal chromosomes. Accordingly, autosomal dominant or autosomal recessive inheritance patterns are based upon transmission of genes found on chromosomes other than the sex chromosomes. Autosomal inheritance patterns account for approximately half of genetic diseases and abnormalities.
All chromosomes found in human cells, except the X and Y sex chromosomes, are autosomes. Human cells carry 22 pairs of homologous autosomal chromosomes. Autosome pairs are numbered, largest to smallest, from 1 to 22. In normal human cells, one of the homologous chromosomes in each pair comes from the mother, the other is donated by the father. Homologous autosomal chromosomes contain similar, but not identical genes. Although normal homologous autosomes contain the same genes (e.g., the gene for a particular trait) each may carry a different form of the gene (allele).
Autosomal dominant inheritance occurs when an allele of one the chromosomes of a homologous pair (i.e., derived from one parent) of autosomal chromosomes is able to express itself regardless of what allele is present on the homologous chromosome. With autosomal dominant inheritance, if an abnormal gene located on an autosomal chromosome is a dominant gene, it is able to cause the expression of a trait or disease even though the corresponding gene on the homologous chromosome is normal.
Autosomal recessive inheritance occurs when an allele of one the chromosomes of a homologous pair (i.e., derived from one parent) of autosomal chromosomes is incapable of expressing the trait or disease it codes for unless another copy of the recessive gene is present on the homologous chromosome. With autosomal recessive inheritance, if an abnormal gene located on an autosomal chromosome is a recessive gene, it is only able to cause the expression of a trait or disease if a copy of the abnormal gene is also present on the homologous autosomal chromosome.
When homologous pairs carry a dominant and a recessive gene (heterozygosity), the dominant gene is expressed. Such heterozygous individuals are, however, capable as acting as carriers of the disease or trait because they may pass on the abnormal allele to their children. With autosomal recessive inheritance, if both parents are heterozygous carriers of an abnormal recessive gene located on an autosome, there is 25% statistical chance (.25 statistical probability) that they will have a child that inherits both abnormal recessive genes. Because both parents contribute one chromosome to a pair of homologous autosomal chromosomes, there is a 50% chance that a child will inherit at least one of the abnormal genes and become a carrier of the recessive gene or trait.
The gene for cystic fibrosis, a disease that can result in chronic or lethal obstructions of the airway, are passed from parents to children by autosomal recessive inheritance. Both parents must carry the cystic fibrosis genes for a child to be affected with the disease. At present, more than 400 genes are associated with cystic fibrosis transmission, all located on autosomal chromosome seven.
Although the presence of a dominant autosomal gene results in the expression of an often adverse (deleterious) trait such genes are maintained in a population due to variable expressivity, high mutation rates, onset of the trait after the reproductive years have passed (late onset), and incomplete penetrance. Variable expressivity results when individuals exhibit only a portion of the range of symptoms associated with a particular disease or when the severity of such expression is variable. High mutation rates, such as those associated with Achondroplasia, persist in reintroducing the same deleterious alleles into a population, even when strongly selected against at the phenotypic level (e.g., fetal death). Late onset diseases, such as Huntington disease, do not usually produce adverse symptoms until after the reproductive years have passed (i.e., an individual has an opportunity to pass on deleterious genes before being adversely affected. Incomplete penetrance (along with variable expressivity, relevant only in cases of autosomal dominant inheritance), allows an individual to carry an unexpressed dominant gene that may once again be expressed as a dominant gene in his or her children.
During meiosis in organisms with a full set of chromosomes (diploid organisms), the homologous autosomal chromosomes align and allow a crossing over of genetic material that causes a reassortment of maternal and paternal genes (genes from the mother and father).
This is the complete article, containing 708 words
(approx. 2 pages at 300 words per page).
