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2017-04-18 09:25:21 | 日記
Mendelian inheritance[edit]

In four o'clock plants, the alleles for red and white flowers show incomplete dominance. As seen in the F1 generation, heterozygous (wr) plants have "pink" flowers—a mix of "red" (rr) and "white" (ww) coloring. The F2 generation shows a 1:2:1 ratio of red:pink:white
Main article: Non-Mendelian inheritance
Mendel explained inheritance in terms of discrete factors—genes—that are passed along from generation to generation according to the rules of probability. Mendel's laws are valid for all sexually reproducing organisms, including garden peas and human beings. However, Mendel's laws stop short of explaining some patterns of genetic inheritance. For most sexually reproducing organisms, cases where Mendel's laws can strictly account for the patterns of inheritance are relatively rare. Often, the inheritance patterns are more complex.

The F1 offspring of Mendel's pea crosses always looked like one of the two parental varieties. In this situation of "complete dominance," the dominant allele had the same phenotypic effect whether present in one or two copies. But for some characteristics, the F1 hybrids have an appearance in between the phenotypes of the two parental varieties. A cross between two four o'clock (Mirabilis jalapa) plants shows this common exception to Mendel's principles. Some alleles are neither dominant nor recessive. The F1 generation produced by a cross between red-flowered (RR) and white flowered (WW) Mirabilis jalapa plants consists of pink-colored flowers (RW). Which allele is dominant in this case? Neither one. This third phenotype results from flowers of the heterzygote having less red pigment than the red homozygotes. Cases in which one allele is not completely dominant over another are called incomplete dominance. In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes.

A similar situation arises from codominance, in which the phenotypes produced by both alleles are clearly expressed. For example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as "erminette", speckled with black and white feathers. Unlike the blending of red and white colors in heterozygous four o'clocks, black and white colors appear separately in chickens. Many human genes, including one for a protein that controls cholesterol levels in the blood, show codominance, too. People with the heterozygous form of this gene produce two different forms of the protein, each with a different effect on cholesterol levels.

In Mendelian inheritance, genes have only two alleles, such as a and A. In nature, such genes exist in several different forms and are therefore said to have multiple alleles. A gene with more than two alleles is said to have multiple alleles. An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population. One of the best-known examples is coat color in rabbits. A rabbit's coat color is determined by a single gene that has at least four different alleles. The four known alleles display a pattern of simple dominance that can produce four coat colors. Many other genes have multiple alleles, including the human genes for ABO blood type.

Furthermore, many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. Polygenic means "many genes." For example, at least three genes are involved in making the reddish-brown pigment in the eyes of fruit flies. Polygenic traits often show a wide range of phenotypes. The broad variety of skin color in humans comes about partly because at least four different genes probably control this trait.

See also[edit]
History of Science portal
icon MCB portal
List of Mendelian traits in humans
Mendelian diseases (monogenic disease)
Mendelian error
Particulate inheritance
Punnett square
Introduction to genetics
Notes[edit]
Jump up ^ Pronunciation: /mɛnˈdiːljən/, /-ˈdiːliən/.
References[edit]
Jump up ^ Grafen, Alan; Ridley, Mark (2006). Richard Dawkins: How A Scientist Changed the Way We Think. New York, New York: Oxford University Press. p. 69. ISBN 0-19-929116-0.
Jump up ^ E. B. Ford (1960). Mendelism and Evolution (seventh ed.). Methuen & Co (London), and John Wiley & Sons (New York). p. 1.
^ Jump up to: a b c Henig, Robin Marantz (2009). The Monk in the Garden : The Lost and Found Genius of Gregor Mendel, the Father of Modern Genetics. Houghton Mifflin. ISBN 0-395-97765-7. The article, written by an Austrian monk named Gregor Johann Mendel...
Jump up ^ See Mendel's paper in English: Gregor Mendel (1865). "Experiments in Plant Hybridization".
^ Jump up to: a b Bailey, Regina (5 November 2015). "Mendel's Law of Segregation". about education. About.com. Retrieved 2 February 2016.
Jump up ^ Bailey, Regina. "Independent Assortment". about education. About.com. Retrieved 24 February 2016.
Jump up ^ Perez, Nancy. "Meiosis". Retrieved 15 February 2007.
Notes[edit]
Peter J. Bowler (1989). The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Johns Hopkins University Press.
Atics, Jean. Genetics: The life of DNA. ANDRNA press.
Reece, Jane B., and Neil A. Campbell. "Mendel and the Gene Idea." Campbell Biology. 9th ed. Boston: Benjamin Cummings / Pearson Education, 2011. 265. Print.
External links[edit]
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