大学生物专业英语lesson_four.pptVIP

* * The results of Mendel’s experiments on dominant and recessive inheritance led to Mendel’s first law: the law of segregation. 基于显性和隐性遗传的实验结果,孟德尔提出了分离定律。 * * This law states that for a given trait an organism inherits one allele from each parent. 该定律认为:对一个给定的特征而言,个体从每个亲本获得一个等位基因。 Together these alleles form the allele pair. 这些等位基因一起形成等位基因对。 * * When gametes [g?mi:t] are formed during meiosis, the two alleles become separated (halving of chromosome number). 减数分裂期形成配子时两个等位基因分离(染色体数目的减半)。 * * Test cross：a cross between a heterozygote of unknown genotype and an individual homozygous for the recessive genes in question. To gain evidence for his theory Mendel performed test crosses, mating plants of unknown genotype to plants that were homozygous [h?m?zaig?us] recessive [risesiv] for the trait of interest. 为验证此理论，他做了测交实验，即基因型未知的植物个体与纯合隐性的个体杂交。 * * The ratio of dominant phenotypes (if any) in the progeny makes clear whether the unknown genotype is heterozygous [het?r?uzaig?s] , homozygous dominant, or homozygous recessive. 子代显性表型的比例可以明确测得该未知基因型是杂合的, 纯合显性的,还是纯合隐性的。 * * 4. Mendel’s ideas and the law of independent assortment * * Mendel also performed dihybrid [daihaibrid] crosses, which enabled him to consider how two traits are inherited relative to one another. 孟德尔也进行了双因子杂交试验，该实验说明了两个特征相互间是如何遗传的。 * * This work led to the law of independent assortment, which states that the alleles of genes governing different characters are inherited independently. 试验结果产生了自由组合定律，即控制不同性状的等位基因是独立遗传的。 * * An apparent exception to Mendel’s law is incomplete dominance, a phenomenon in which offspring of a cross exhibit a phenotype that is intermediate between those of the parents. 孟德尔定律的一个特例是，不完全显性，即杂交后子代的表型是父母亲本的中间类型。 * * However, incomplete dominance reflects the fact that both alleles for the trait in question exert an effect on the phenotype. 但是，不完全显性说明了两个等位基因可能对表型有影响。 The alleles themselves remain separate. 等位基因本身仍然是分离的。 * * Mendel presented his ideas in 1866