Foundations of Genetics

1.       Early Theories of Inheritance

Early ideas of inheritance included Hippocrates’ theory of pangenesis and August Weismann’s germ plasm theory. Based on experiments with mice, Weismann proposed that hereditary information in gametes transmitted traits to progeny. Both of these early views incorporated the blending theroy: they held that heritable traits of the two parents blend, so that the distinct characteristics of each are lost in offsping.

早期遗传学理论

早期的遗传理论包括Hippocrates的泛生论和August Weismann的种质学说。根据这些用老鼠做的实验，Weismann提出是配子中的遗传信息将特性传递给了子代。这两种早期的观点包括混合理论：他们认为亲本可以混合可遗传的特性，因此每个亲本独有的特征在子代中就消失了。

2.       Gregor Mendel and the Birth of Genetics

Gregor Mendel, an Augustinian monk in the monastery at Brunn, Austria, is known as the “father of genetics”. Having been exposed to theories of the particulate nature of matter while a university student and having a background in mathematics, Mendel carried out a series of carefully planned experiments that demonstrated the particulate nature of heredity. His revolutionary ideas were neither understood nor accepted until many years after Mendel died.

Gregor Mendel and 遗传学的诞生

Gregor Mendel, 奥地利布伦修道院的一位奥古斯丁修道士，被认为是“遗传学之父”。Mendel大学时接触到物质的微粒本质，以及数学知识背景，Mendel做了一系列计划仔细的实验，论证遗传的微粒本质。直到Mendel死后，人们才理解并接受他革命性的的观点。

3.Mendel’s Classic Experiments

Mendel studied genetics through plant-breeding experiments with the garden pea, a plant species that is self-fertilizing and breeds true (each offspring is identical to the parent in the trait of interest). To test the blending theory, he focused his research on seven distinct characters. Each of these characters, 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 pair of parent pea plants, and followed the results of each cross for two generations.

Mendel 的经典实验

Mendel 通过用豌豆做植物培育实验研究遗传学，豌豆是以一种自花受精，繁育纯种后代的植物（每一个子代的性状都和亲代相同）。未验证很和理论，Mendel的实验主要集中研究7个截然不同的性状。每一种性状，像种子颜色和植物高度等，只呈现出两种，明确的可能。他还记录了从每一对豌豆亲本植株产生的类型和所有子代的数量，以及每两代杂交的的结果。

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.

Mendel发现其所研究的的特性中，有一种特性是显型，然而其它的则成隐性。在子二代F2中,显性与隐性的比为3:1。Mendel推断出出现这样的结果可能是每一个个体中只拥有两个可遗传的单元，从来自亲本得到一个。Mendel猜测的这些单元现在被认为是等位基因，基因的替换方式。基因是遗传的基本单元。一个从亲本遗传控制一种特性的含相同的等位基因的有机体被称为纯合型；如果遗传控制一种性状的含不同的等位基因的有机体就是杂合型。

When an organism is heterozygous for a trait, the resulting phenotype for that trait expresses only the dominant allele . Thus organism’s phenotype-its physical appearance and properties-differs from its genotype, which may include both a dominant and recessive allele. A pictorial representation of all possible combinations of a genetic cross is known as a Punnett square.

当一个有机体的一种特性是杂合的，这种特性产生的表现型就只表达显性等位基因额特性。因此，有机体的表现型—它的物理表型和性能—与它的基因型不同，可能含有一个显性和一个隐性等位基因。所有这些基因杂交的可能组合的可以用庞尼特氏方格形象化的说明。

The results of Mendel’s experiments on dominant and recessive inheritance led to Mendel’s first law: the law of segregation. This law states that for a given trait an organism inherits one allele from from each parent. Together these allele form the allele pair. When gametes are formed during meiosis, the two alleles bcome separated(halving of chromosome number). To gain evidence for his theory Mendel performed test crosses, mating plants of unknown genotype to plants that were homozygous recessive for the trait of interest. The ratio of dominant phenotypes(if any) in the progeny makes clear whether the unknown genotype is heterozygous, homozygous dominant, or homozygous recessive.

Mendel 关于隐性和显性的遗传实验结果导致了他提出了Mendel第一个定律：分离定律。这个定律表明对于给定的特性，每一个有机体从亲本那里各遗传一个等位基因。这些等位基因共同形成了等位基因对。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）。为了证明他的理论，Mendel进行了侧交试验，将他所研究的呈纯合隐性，但未知基因型的植株杂交。子代显隐性的比例都很确定，不管未知基因型是杂合的，纯合的，还是纯合隐性。

3.       Mendel’s Ideas and the Law of Independent Assortment

Mendel also performed dihybrid crosses, which enabled him to consider how two traits are inherited relative to one another. This work led 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 pheneotype that is intermediate between those of the parents. However, incomplete cominance reflects the fact that both alleles for the trait in question exert an effect on the phenotype. The alleles themselves remain separate.

Mendel观点以及Mendel第三分离定律

Mendel还进行了双因子杂合试验，两个特征是如何相互影响遗传的。试验结果产生独自分配定律，即控制不同性状等位基因独立遗传。特例是，不完全显性。子代的表型是亲本的中间类型。不完全显性说明了两个等位基因对表型都有影响。等位基因会保持分离。

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，孟德尔在布伦社会自然史上发表了他的科学论文，陈诉了他的观点。不幸的是，他的研究不被当时科学家接受。在1900年，他的著作再被Carl Correns 和Hugo de Vries发现利用。

4.       Chromosome and Mendelian Genetics

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 sexchromosomes 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.

染色体和Mendel遗传学

孟德尔著作被再发现不久，Walter Sutton 和Theodor Boveri独立提出，遗传单位可能存在于染色体上。Tomans Hunt在剑桥大学和他的学生进行了果蝇性染色体研究的实验来证明这种假说。Morgen 的研究也是伴性遗传的第一次探索，还导致了Calvin Bridges在1916年发现了不分离现象，即在减数分裂中，染色体对不分离。