第四章孟德尔式遗传分析.ppt

If the sperm and egg came from genetically different plants, the hybrid zygote would inherit two different alleles, one from the mother and one from the father. Such an offspring is said to be heterozygous. The different alleles that are present in heterozygote must coexist even through one is dominant and the other recessive, and that each of these alleles would have an equal chance of entering a gamete when the heterozygote reproduces. Furthermore, random fertilizations with a mixed population of gametes—half carrying the dominant allele and half carrying the recessive allele—would produce some zygotes in which both alleles were recessive. Thus, we could explain the reappearance the recessive characteristic in the progeny of the hybrid plants. Mendel used symbols to present the hereditary factors that he postulated—a methodological breakthrough. The genotypeis the genetic constitution of a cell, an organism, or an individual (i.e. the specific allele make up of the individual) usually with reference to a specific character under consideration. 基因型是一个细胞、一个生物或一个个体的遗传构成形式(比如组成个体的特定等位基因)，通常与其控制的特殊性状相关。 A phenotypeis any observable characteristicor traitof an organism: such as its morphology, development, biochemical or physiological properties, or behavior. 表型是生物可以被观察到的特征或性状：如形态、发育、生化或生理学的性质或行为。 Phenotypes result from the expression of an organism’s genes as well as the influence of environmental factors and possible interactions between the two. The allele for dwarfness, being recessive, is symbolized by a lowercase letter d; the allele for tallness, being dominant, is symbolized by the corresponding uppercase letter D. As the parental strain, the tall and dwarf pea plants form the P generation of the experiment. Their hybrid progeny are referred to as the first filial generation, or F1. Upon self-fertilization, the two kinds of gametes produced by heterozygotes can unite in all possible ways. Thus, they produce four kinds of zygotes: DD, Dd, dD, and dd. However, bec