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Thanks to these experiments, he was able to determine the fundamental law of independent assortment and the law of dominance. The law of independent assortment states that traits controlled by different genes are inherited independently. [3] Mendel was able to determine this law because he could determine the four possible phenotypes in his crosses. The law of dominance states that when a dominant allele is inherited, the dominant phenotype is expressed. [3] The importance of Mendel`s work did not gain a broad understanding until 1900, after his death, when Hugo de Vries and other scholars rediscovered his research. William Bateson, a proponent of Mendel`s work, coined the word genetics in 1905. [20] [21] (The adjective genetic, derived from the Greek word genesis – γένεσις, “origin,” is older than the noun and was first used in the biological sense in 1860.) Bateson acted both as a mentor and was largely supported by the work of other researchers at Newnham College, Cambridge, particularly the work of Becky Saunders, Nora Darwin Barlow and Muriel Wheldale Onslow.[22] [23] Bateson popularized the use of the word genetics to describe the study of heredity in his inaugural address to the Third International Conference on Plant Hybridization in London in 1906. [24] Non-Mendelian inheritance is a model in which traits are not separated according to Mendel`s laws. These laws describe the inheritance of traits associated with individual genes on cell nucleus chromosomes. In Mendelian inheritance, each parent contributes one of two possible alleles to a trait. If the genotypes of both parents are known in a genetic cross, Mendel`s laws can be used to determine the distribution of expected phenotypes for the offspring population. There are several situations in which the proportions of phenotypes observed in offspring do not agree with the predicted values. Another form of non-Mendelian inheritance is known as infectious inheritance.
Infectious particles such as viruses can infect host cells and continue to reside in the cytoplasm of those cells. If the presence of these particles leads to an altered phenotype, this phenotype can then be transferred to the offspring. [12] Since this phenotype depends only on the presence of the invader in the cytoplasm of the host cell, heredity is determined only by the infectious status of the maternal parent. This leads to a uniparental transfer of the trait, just as in the non-nuclear inheritance. To explain his findings, Mendel formulated a hypothesis that included the following: In the organism, there are a pair of factors that control the appearance of a certain characteristic. (They are called genes.) The organism inherits these factors from its parents, one of each. A factor is passed down from generation to generation as a discrete and immutable entity. (The r-factor in the F2 generation went through the round F1 generation. Nevertheless, the rr seeds of the F2 generation were no less wrinkled than those of the P generation.) When gametes are formed, the factors separate and are distributed in units to each gamete.
This statement is often referred to as Mendel`s segregation rule. If an organism has two different factors (called alleles) for a trait, one can be expressed to the exclusion of the other (dominant vs. recessive). The principle of dominant inheritance discovered by Mendel states that in a heterozygous, the dominant allele causes the recessive allele to be “masked”, i.e. not expressed phenotype. Only when an individual is homozygous with respect to the recessive allele is the recessive trait expressed. Therefore, a cross between a dominant homozygous organism and a recessive homozygous organism results in a heterozygous organism whose phenotype has only the dominant trait. Non-random chromosome segregation is a deviation from the usual distribution of chromosomes during meiosis and, in some cases, mitosis. The process of sexual reproduction alternates between forms that contain single copies of the genome (haploid) and double copies (diploid).
[46] Haploid cells fuse and combine genetic material to form a diploid cell with paired chromosomes. Diploid organisms make haploids by dividing without replicating their DNA to create daughter cells that inherit one of each pair of chromosomes.
Last Update : 17 พฤศจิกายน 2022