such as seed color and plant height, present only two, clear-cut possibilities. He also recorded the type and number of all progeny produced from each paia plants, and followed the results of each cross for two generations.孟德尔通过花园中的豌豆开展了植物育种试验而研究遗传学，豌豆是自花授粉植物和纯品系。为验证融合理论，他将研究重点放在7个明显的特征上。这些特征的每一个，例如，种子颜色和植株高度，代表了仅有两个且易于区分的可能性。他记录了产生的每一个子代的类型和数量，以及两代杂交的结果。
For each of the characters he studied, Mendel found that one trait was dominant while the other was recessive. In the second filial (F2) generation, the ratio of dominant to recessive was 3:1. Mendel deduced that this result was possible only if each individual possesses only two hereditary units, one from each parent. 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 allele.Thus, the organism’s phenotype—its physical appearance and properties-differs from its genotype, which may include both a dominant and a recessive allele. A pictorial representation of all possible combinations of a genetic cross is known as a Punnett square.对于他研究的每一种特性，要么显性，要么隐性。在子2代中显形与隐性比为3∶1。孟德尔推断只有在每个个体仅拥有两个研究遗传单元，并每个单元来自一个亲代时，实验结果才成立。孟德尔假设的单位如今称为等位基因（基因二中选一的形式）。基因是遗传的基本单位，一些有机体继承了来自父本、母本的同一性状的两个相同的等位基因，被称为某一性状的纯合体。如果
继承的是某一性状的不同的等位基因，有机体被称为某一性状的杂合体。当有机体是某一性状的杂合体时，它的表型由显性基因决定。因此，有机体的表型（它的物理形状和特性）与它的基因型不同，基因型同时包括一个显性和一个隐性等位基因。遗传杂交的所有可能的组合的图示形式被称为庞纳特方格。
The results of Mendel's experiments on dominant and recessive inheritance let to Mendel's first law: the law of segregation.This 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 become 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 recessive.孟德尔关于显性和隐性遗传实验的结果称为孟德尔第一定律：分离定律。这一定律认为对于某一特定形状，有机体继承了每一个亲本的等位基因。这些等位基因在一起形成了等位基因对。当减数分裂形成配子时，两个等位基因分离（染色体数目减半）。为验证此理论，孟德尔做了测交实验，即基因型未知的植物与某一隐性纯合体的植物杂交。根据后代中显性基因型的比例可以搞清未知基因型是杂合、纯合显性还是纯合隐性。
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 Mendel's laws is incomplete dominance, a phenomenon in which offspring of a cross exhibit a phenotype that is intermediate between those of the parents. However, incomplete dominance reflects
the fact that both alleles for the trait in question exert an effect on the phenotype. The alleles themselves remain separate. 孟德尔也开展了双因子杂合体杂交实验，双因子杂合试验，使得他考虑两个性状如何有关联的遗传。试验结果产生独自分配定律，即控制不同性状的等位基因独立遗传。孟德尔定律的一个明显例外情况是不完全显性。杂交后代的表型是亲本的中间类型。然而，不完全显性说明了两个等位基因对表型都有影响。等位基因自身仍然是独立的。
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，孟德尔在布鲁恩自然历史学会出版的科学报上发表了他的观点。不幸的是，他的研究不被当时科学家接受。在1900年，他的著作被 Carl Correns 和Hugo de Vries 重新发现。 。
Soon after Mendel'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. Morgan's studies were also the first exploration of sex-linked 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. 孟德尔的工作被重新发现不久，Walter Sutton 和Theodor Boveri分别提出，遗传单位可能位于染色体上。这一假设被哥伦比亚大学的Thomas Hunt Morgan 和他的学生用果蝇的性染色体实验进行了证实。摩根的研究也是性连锁特性的首次探索。它导致了1916年Calvin Bridges 的不分离现象的发现，即在减数分裂中，染色体对不分离。
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 one-gene-one-enzyme hypothesis-the 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 complex metabolic pathways.In 1949, in research on the role of hemoglobin in sickle cell anemia, Linus Pauling helped refine the one-gene-one-enzyme hypothesis into the one-gene-one-polypeptide hypothesis. Archibald Garrod爵士是第一个研究基因如何影响表型的科学家，他对尿黑酸症的研究揭示了基因与酶之间的关系。Beadle 和Ephrussi在三十年发现关于特定基因和控制果蝇中复眼颜色的生物合成反应之间的关系。接着，在一系列关于面包霉粗糙脉孢霉突变效应的经典实验中，Beadle 和Tatum 探究了一基因一酶假设，这一假设认为每一个基因编码一个特定的酶。他们的工作为其他研究者以精确方式阐明酶影响复杂代谢途径铺平了道路。 在1949年，在研究了镰刀形红细胞贫血症中血红蛋白的作用后，Linus Pauling 帮助将一基因一酶假设精炼成为一基因一多肽假设。
Nuclei acid, originally isolated by Johann Miescher in 1871, was identified as a prime constituent of chromosomes through the use of the red-staining method developed by Feulgen in the early 1900s. Frederick Griffith'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
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