The small area of a chromosome that does not stain with basic dyes during mitosis and meiosis; at interphase it is single while the rest of the chromosome is made up of two chromatids. metaphase （分裂）中期

Second stage in mitosis, during which the chromosomes align at the equatorial plane. spindle 纺锤体

An array of microtubutes extending from pole to pole and used in the movement of chromosomes. metaphase plate 赤道板，中期板

The grouping of the chromosomes in a plane at the equator of the spindle during the metaphase stage of mitosis. anaphase 分裂后期

The third stage of mitosis, during which the centromeres split and the chromosomes move to the poles. telophase （分裂）末期

Last stage in mitosis during which daughter nuclei are formed.

Division of the cytoplasm of cytokinesis 胞质分裂

one cell into two new cells.

cell plate 细胞板

A plant cell structure that begins to form in the center of the cell and proceeds to the cell membrane, resulting in cytokinesis.

Foundations of Genetics

Early Theories of inheritanee

Early ideas of inheritance included Hippocrates' theory of pangenesis and August Weismands 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 theory: they held that

heritable traits of the two parents blend, so that the distinct characteristics of each are lost in offspring.

遗传学的早期理论包括泛生说和种质理论。基于小鼠实验，维丝曼提出遗传信息储存在配子中并将遗传信息传递给后代。这两个早期观点合起来形成融合理论：子代拥有父母本混合的遗传特征，而不完全象亲代。

Gregor Mendel and the Birth of Genetics

Gregor Mendel, an Augustinian monk in the monastery at Brunn, Austria, is known as the \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.

孟德尔，众所周知的遗传学之父，是一名修道士。当他还是大学生时就提出了物质的粒子属性。孟德尔进行了一系列周密安排的实验来证实遗传的颗粒性。直到他去世后，他的理论才被理解和接受。

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.

孟德尔通过豌豆实验研究遗传学，豌豆是自花授粉植物和纯品系。为验证融合理论，他的研究主要集中在7个特征上。例如，种子颜色，植株高度，这些特征只有两个明确的可能性。他记录了产生的每一个子代类型和数量，在杂交产生子2代。

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.

对于每个特征而言，要么显形，要么隐性。在子2代中显形与隐性比为3∶1。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。此遗传单元就是今天共识的等位基因。两个一样的等位基因决定一个特征，称纯合。相反，称杂合。当生物是杂合时，它的表型由显性基因决定。因此，生物的表型与基因型是不同的。旁纳特方格可以陈列所有可能的遗传组合。

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 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. 分离定律，生物只遗传父母本等位基因对的一个等位基因。减数分裂期形成配子时两个等位基因分离。为验证此理论，他做了测交实验，即基因型未知的植物与纯合的隐性基因植物杂交。子代显性表型可以明确测得杂合基因或纯合基因的基因型。

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 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.

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，即等位基因独立遗传。特例是，不完全显性。子代的表型是父母本的中间类型。不完全显性说明了两个等位基因对表型都有影响。，等位基因会继续分离。

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，他的著作再被发现利用。

Chromosomes and Mendelian Genetics

Soon after Mendel's work was rediscovered, Walter Sutton and Theodor Boveri independently proposed that the hereditary units