How is pleiotropy exemplified in Drosophila?
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Universality of Mendel’s laws
Although Mendel experimented with varieties of peas, his laws have been shown to apply to the inheritance of many kinds of characters in almost all organisms. In 1902 Mendelian inheritance was demonstrated in poultry (by English geneticists William Bateson and Reginald Punnett) and in mice. The following year, albinism became the first human trait shown to be a Mendelian recessive, with pigmented skin the corresponding dominant.
The routine monitoring of blood pressure levels is an important part of assessing an individual's health. Blood pressure provides information about the amount of blood in circulation and about heart function and thus is an important indicator of disease.
human disease: Heredity and environment
The many causes of cancer include intrinsic factors, such as heredity, and extrinsic factors, such as…
In 1902 and 1909, English physician Sir Archibald Garrod initiated the analysis of inborn errors of metabolism in humans in terms of biochemical genetics. Alkaptonuria, inherited as a recessive, is characterized by excretion in the urine of large amounts of the substance called alkapton, or homogentisic acid, which renders the urine black on exposure to air. In normal (i.e., nonalkaptonuric) persons the homogentisic acid is changed to acetoacetic acid, the reaction being facilitated by an enzyme, homogentisic acid oxidase. Garrod advanced the hypothesis that this enzyme is absent or inactive in homozygous carriers of the defective recessive alkaptonuria gene; hence, the homogentisic acid accumulates and is excreted in the urine. Mendelian inheritance of numerous traits in humans has been studied since then.
In analyzing Mendelian inheritance, it should be borne in mind that an organism is not an aggregate of independent traits, each determined by one gene. A “trait” is really an abstraction, a term of convenience in description. One gene may affect many traits (a condition termed pleiotropic). The white gene in Drosophila flies is pleiotropic; it affects the colour of the eyes and of the testicular envelope in the males, the fecundity and the shape of the spermatheca in the females, and the longevity of both sexes. In humans many diseases caused by a single defective gene will have a variety of symptoms, all pleiotropic manifestations of the gene.
Allelic interactions
Dominance relationships
The operation of Mendelian inheritance is frequently more complex than in the case of the traits recorded by Mendel. In the first place, clear-cut dominance and recessiveness are by no means always found. When red- and white-flowered varieties of four-o’clock plants or snapdragons are crossed, for example, the F1 hybrids have flowers of intermediate pink or rose colour, a situation that seems more explicable by the blending notion of inheritance than by Mendelian concepts. That the inheritance of flower colour is indeed due to Mendelian mechanisms becomes apparent when the F1 hybrids are allowed to cross, yielding an F2 generation of red-, pink-, and white-flowered plants in a ratio of 1 red : 2 pink : 1 white. Obviously the hereditary information for the production of red and white flowers had not been blended away in the first hybrid generation, as flowers of these colours were produced in the second generation of hybrids.
The apparent blending in the F1 generation is explained by the fact that the gene alleles that govern flower colour in four-o’clocks show an incomplete dominance relationship. Suppose then that a gene allele R1 is responsible for red flowers and R2 for white; the homozygotes R1R1 and R2R2 are red and white respectively, and the heterozygotes R1R2 have pink flowers. A similar pattern of lack of dominance is found in Shorthorn cattle. In diverse organisms, dominance ranges from complete (a heterozygote indistinguishable from one of the homozygotes) to incomplete (heterozygotes exactly intermediate) to excessive or overdominance (a heterozygote more extreme than either homozygote).
Another form of dominance is one in which the heterozygote displays the phenotypic characteristics of both alleles. This is called codominance; an example is seen in the MN blood group system of human beings. MN blood type is governed by two alleles, M and N. Individuals who are homozygous for the M allele have a surface molecule (called the M antigen) on their red blood cells. Similarly, those homozygous for the N allele have the N antigen on the red blood cells. Heterozygotes—those with both alleles—carry both antigens.
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