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Genetic drift
Genetic bottleneck
Genetic drift is a random change in gene frequency arising through chance alone. It tends to be more apparent in small populations allele frequencies drift away from their starting values, increasing, decreasing or fluctuating up and down. It occurs because there is an element of chance in which individuals and which gametes will produce offspring and which individuals will die before reproducing. Random changes in allele frequency can lead to fixation and the progressive loss of genetic vWahreinataiopnopfurloamtitohne upnodpeurlgaoteisona.n abrupt contraction in numbers, this is accompanied by a change in gene frequencies and a decline in the total genetic variation. Genetic drift in the small population during the bottleneck results in the loss of genetic variation. Although population numbers may recover, genetic variation will remain low for many generations.
O1 遗传的变异
要点
基因和等位 基因
基因是带有可产生特定蛋白的遗传密码的DNA片段。一 个基因可有许多等位基因，而个体可有纯合或杂合的 基因型。表达的表现型取决于等位基因是显性的、隐 性的还是共显性的。种群中存在的所有基因组和等位 基因叫做基因库。
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O1 GENETIC VARIATION
Key Notes
Genes and alleles
Genes are pieces of DNA which contain the genetic code necessary to produce a specific protein. A gene can have many alleles and individuals can have a homozygous or heterozygous genotype. The phenotype expressed depends on whether alleles are dominant, recessive or codominant. The total set of genes and alleles present in a population is known as the gene poo精l品. 课件
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遗传漂变 遗传瓶颈
遗传漂变是基因频率的随机变化，仅偶然出现。在小 种群中，遗传漂变更明显。基因频率“漂离”起始值， 增加、减少或上下波动。发生遗传漂变是因为在个体然要 素。基因频率的随机变化导致来自种群的遗传变异的 固定和逐渐丧失。
当种群数量突然减少时，基因频率也会发生变化，总 的遗传变异下降。瓶颈发生的时候，小种群的遗传漂 变导致遗传变异丧失。尽管种群数量可以恢复，遗传 变异在以后许多代仍会保持低水平。
多型
多型指的是种群中等位基因的存在。一个种群或物种 可能颜色是多型的，如蜗牛，或者一些生化特性是多 型的，如植物的毒性。一些多型可能是由自然选择保 持下来的，但其他是由于许多基因控制多型性状的结 果。
测定遗传变异
种群和物种内的遗传变异可直接由DNA或蛋白（别构酶） 变异来估计。蛋白或DNA片段可通过凝胶电泳分离，呈 现可刻划的带来决定个体的基因型。对个体DNA的详细 调查产生了一种独特的遗传指纹，对建立父子关系很 有用。
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polymorphism
Measuring genetic
variation
The term polymorphism refers to the presence of alleles in a population. A population or species may be polymorphic for color, as in snails, or for some biochemical function, as in plant toxicity. Some polymorphisms are maintained by natural selection, but others appear to result from the effect of many genes controlling the Gpeonleytmiocrpvhairciacthiaornacwtietrh.in populations and species can be estimated directly from the DNA or from protein (allozyme) variation. Proteins or DNA fragments are separated by gel electrophoresis to give bands that can be scored to determine the genotype of individuals. Detailed investigation of the DNA of individuals yields a unique genetic fingerprint, useful for establishing paternity.