Single Gene Inheritance-
Genetic conditions caused by a mutation in a single gene follow predictable patterns of inheritance within families. Single gene inheritance is also referred to as Mendelian inheritance as they follow transmission patterns he observed in his research on peas. There are four types of Mendelian inheritance patterns:
Autosomal recessive X-linked recessive
Autosomal: the gene responsible for the phenotype is located on one of the 22 pairs of autosomes (non-sex determining chromosomes).
X-linked: the gene that encodes for the trait is located on the X chromosome.
Dominant: conditions that are manifest in heterozygotes (individuals with just one copy of the mutant allele).
Recessive: conditions are only manifest in individuals who have two copies of the mutant allele (are homozygous).
Dominant conditions are expressed in individuals who have just one copy of the mutant allele. The pedigree on the right illustrates the transmission of an autosomal dominant trait. Affected males and females have an equal probability of passing on the trait to offspring. Affected individual's have one normal copy of the gene and one mutant copy of the gene, thus each offspring has a 50% chance on inheriting the mutant allele. As shown in this pedigree, approximately half of the children of affected parents inherit the condition and half do not.
Most diseases have multifactorial inheritance patterns. As the name implies, multifactorial conditions are not caused by a single gene, but rather are a result of interplay between genetic factors and environmental factors. Diseases with multifactorial inheritance are not genetically determined, but rather a genetic mutation may predispose an individual to a disease. Other genetic and environmental factors contribute to whether or not the disease develops.
Numerous genetic alterations may predispose individuals to the same disease (genetic heterogeneity). For instance coronary heart disease risk factors include high blood pressure, diabetes, and hyperlipidemia. All of those risk factors have their own genetic and environmental components. Thus multifactorial inheritance is far more complex than Mendelian inheritance and is more difficult to trace through pedigrees.
Mitochondrial Inheritance -
Mitochondria are organelles found in the cytoplasm of cells. Mitochondria are unique in that they have multiple copies of a circular chromosome. Mitochondria are only inherited from the mother's egg, thus only females can transmit the trait to offspring, however they pass it on to all of their offspring. The primary function of mitochondria is conversion of molecule into usable energy. Thus many diseases transmitted by mitochondrial inheritance affect organs with high-energy use such as the heart, skeletal muscle, liver, and kidneys.
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