【正文】 arent. The units Mendel hypothesized are today known as alleles, alternative forms of genes. Genes are the basic units of heredity. An organism that inherits identical alleles for a trait from each parent is said to be homozygous for that trait。 if different alleles for a trait are inherited, the organism is heterozygous for that trait. When an organism is heterozygous for a trait, the resulting phenotype for that trait expresses only the dominant , the organism’s phenotype—its physical appearance and propertiesdiffers from its genotype, which may include both a dominant and a recessive allele. A pictorial representation of all possible binations of a genetic cross is known as a Punnett ，要么顯性，要么隱性。在子2代中顯形與隱性比為3∶1。孟德?tīng)柾茢嘀挥性诿總€(gè)個(gè)體僅擁有兩個(gè)研究遺傳單元，并每個(gè)單元來(lái)自一個(gè)親代時(shí)，實(shí)驗(yàn)結(jié)果才成立。孟德?tīng)柤僭O(shè)的單位如今稱為等位基因（基因二中選一的形式）。基因是遺傳的基本單位，一些有機(jī)體繼承了來(lái)自父本、母本的同一性狀的兩個(gè)相同的等位基因，被稱為某一性狀的純合體。如果繼承的是某一性狀的不同的等位基因，有機(jī)體被稱為某一性狀的雜合體。當(dāng)有機(jī)體是某一性狀的雜合體時(shí)，它的表型由顯性基因決定。因此，有機(jī)體的表型（它的物理形狀和特性）與它的基因型不同，基因型同時(shí)包括一個(gè)顯性和一個(gè)隱性等位基因。遺傳雜交的所有可能的組合的圖示形式被稱為龐納特方格。 The results of Mendel39。s experiments on dominant and recessive inheritance let to Mendel39。s first law: the law of law states that for a given trait an organism inherits one allele from each parent. Together these alleles form the allele pair. When gametes are formed during meiosis, the two alleles bee separated (halving of chromosome number).To gain evidence for his thMendel performed test crosses, mating plants of unknown genotype to plants that were homozy recessive for the trait of interest. The ratio of dominant phenotypes (if any) in the progeny makes clear whether the unkno genotype is heterozygous, homozygous dominant, or homozygous ：分離定律。這一定律認(rèn)為對(duì)于某一特定形狀，有機(jī)體繼承了每一個(gè)親本的等位基因。這些等位基因在一起形成了等位基因?qū)Α．?dāng)減數(shù)分裂形成配子時(shí)，兩個(gè)等位基因分離（染色體數(shù)目減半）。為驗(yàn)證此理論，孟德?tīng)栕隽藴y(cè)交實(shí)驗(yàn)，即基因型未知的植物與某一隱性純合體的植物雜交。根據(jù)后代中顯性基因型的比例可以搞清未知基因型是雜合、純合顯性還是純合隱性。 Mendel also performed dihybrid crosses, which enabled him to consider how two traits are inherited relative to one another. This work let to the law of independent assortment, which states that the alleles of genes governing different characters are inherited independently. An apparent exception to Mendel39。s laws is inplete dominance, a phenomenon in which offspring of a cross exhibit a phenotype that is intermediate between those of the parents. However, inplete dominance reflects the fact that both alleles for the trait in question exert an effect on the phenotype. The alleles themselves remain separate. 孟德?tīng)栆查_(kāi)展了雙因子雜合體雜交實(shí)驗(yàn)，雙因子雜合試驗(yàn)，使得他考慮兩個(gè)性狀如何有關(guān)聯(lián)的遺傳。試驗(yàn)結(jié)果產(chǎn)生獨(dú)自分配定律，即控制不同性狀的等位基因獨(dú)立遺傳。孟德?tīng)柖傻囊粋€(gè)明顯例外情況是不完全顯性。雜交后代的表型是親本的中間類型。然而，不完全顯性說(shuō)明了兩個(gè)等位基因?qū)Ρ硇投加杏绊?。等位基因自身仍然是?dú)立的。Mendel presented his ideas in 1866 in a scientific paper published by the Brunn Society for Natural History. Unfortunately, the meaning of his research was not understood by other scientists of the day. His work was rediscovered in 1900 by Carl Correns and Hugo de Vries. 1866年，孟德?tīng)栐诓剪敹髯匀粴v史學(xué)會(huì)出版的科學(xué)報(bào)上發(fā)表了他的觀點(diǎn)。不幸的是，他的研究不被當(dāng)時(shí)科學(xué)家接受。在1900年，他的著作被 Carl Correns 和Hugo de Vries 重新發(fā)現(xiàn)。 。Soon after Mendel39。s work was rediscovered, Walter Sutton and Theodor Boveri independently proposed that the hereditary units might be located on chromosomes. Experiments to prove this hypothesis were carried out by Thomas Hunt Morgan and his students at Columbia University, in research on the sex chromosome of fruit flies. Morgan39。s studies were also the first exploration of sexlinked traits. It also led to the discovery in 1916 by Calvin Bridges of the phenomenon of nondisjunction, in which a chromosome pair fails to segregate during meiosis.孟德?tīng)柕墓ぷ鞅恢匦掳l(fā)現(xiàn)不久，Walter Sutton 和Theodor Boveri分別提出，遺傳單位可能位于染色體上。這一假設(shè)被哥倫比亞大學(xué)的Thomas Hunt Morgan 和他的學(xué)生用果蠅的性染色體實(shí)驗(yàn)進(jìn)行了證實(shí)。摩根的研究也是性連鎖特性的首次探索。它導(dǎo)致了1916年Calvin Bridges 的不分離現(xiàn)象的發(fā)現(xiàn)，即在減數(shù)分裂中，染色體對(duì)不分離。The first scientist to investigate the question of how genes affect phenotype was Sir Archibald Garrod, whose studies of alkaptonuria implied a relationship between genes and enzymes. Thirty years later Beadle and Ephrussi showed a relationship between particular genes and biosynthetic reactions responsible for eye color in fruit flies. Next, in a series of classic experiments on the effects of mutations in the bread mold Neurospora crassa, Beadle and Tatum explored the onegeneoneenzyme hypothesisthe idea that each gene codes for a particular enzyme. Their work paved the way for other researchers to elucidate the precise ways in which enzymes affect plex metabolic 1949, in research on the role of hemoglobin in sickle cell anemia, Linus Pauling helped refine the onegeneoneenzyme hypothesis into the onegeneonepolypeptide hypothesis. Archibald Garrod爵士是第一個(gè)研究基因如何影響表型的科學(xué)家，他對(duì)尿黑酸癥的研究揭示了基因與酶之間的關(guān)系。Beadle 和Ephrussi在三十年發(fā)現(xiàn)關(guān)于特定基因和控制果蠅中復(fù)眼顏色的生物合成反應(yīng)之間的關(guān)系。接著，在一系列關(guān)于面包霉粗糙脈孢霉突變效應(yīng)的經(jīng)典實(shí)驗(yàn)中，Beadle 和Tatum 探究了一基因一酶假設(shè)，這一假設(shè)認(rèn)為每一個(gè)基因編碼一個(gè)特定的酶。他們的工作為其他研究者以精確方式闡明酶影響復(fù)雜代謝途徑鋪平了道路。 在1949年，在研究了鐮刀形紅細(xì)胞貧血癥中血紅蛋白的作用后，Linus Pauling 幫助將一基因一酶假設(shè)精煉成為一基因一多肽假設(shè)。 Nuclei acid, originally isolated by Johann Miescher in 1871, was identified as a prime constituent of chromosomes through the use of the redstaining method developed by Feulgen in the early 1900s. Frederick Griffith39。s experime with the R and S stains of pneumococci showed that an as yet unknown material from one set of bacterial could alter the physical traits of a second set. In the 1940s the team of Avery, MacLeod, and McCarty showed that this unknown material was DNA. At about the same time . Levene discovered that DNA contained four nitrogenous bases, each of which was attached to a sugar molecule and a phosphate groupa bination Levene termed a nucleotide.核酸最早由Johann Miescher 在1871年分離。在20世紀(jì)早期，采用由Feulgen 建立的紅染方法，核酸被鑒定為組成染色體的基本成分，F(xiàn)rederick Griffith 采用R和S菌株對(duì)肺炎雙球菌進(jìn)行染色，表明來(lái)自于一個(gè)細(xì)菌的未知物質(zhì)能夠改變另一細(xì)菌的物理性狀。在20世紀(jì)40年代，Avery、MacLeod和McCarty 組成的團(tuán)隊(duì)，表明這一未知物質(zhì)是DNA。大約是同一時(shí)代，. Levene 發(fā)現(xiàn)DNA 包括四種含氮堿基，每一種堿基與一個(gè)核糖分子和一個(gè)磷酸基相連，Levene稱這種組合為核苷酸。Disagreement over whether DNA could carry plex genetic information was ended in the early 1950s by Martha Chase and Alfred Hershey, whose work with E. coli showed clearly that DNA, and not protein, is the bearer of geneti