IB生物 遗传学 DNA复制 减数分裂 卡方检验
IB 生物在 2023 年启用新版考纲，2025 年 5 月首次评估，遗传学内容从旧版 Topic 3 拆分进 Theme D，编号改为 D1.1 到 D3.2 这一串。旧考纲的四个选项（Options）已经删除，Paper 3 随之取消，遗传内容的考查落在 Paper 1A 的选择题、Paper 1B 的数据题和 Paper 2 Section B 的长答题里。这篇文章按知识点顺序走一遍 DNA 复制、转录与翻译、减数分裂、杂交比例、卡方检验、基因连锁和基因编辑，每个部分给定义、给计算、给常见失分点。SL 与 HL 都要掌握前面的复制与遗传比例，基因连锁和基因表达调控只对 HL 提出要求。IBO 官方考纲把 D3.2 Inheritance 的核心放在用概率与统计解释杂交结果，而不是背比例数字。
IB Biology moved genetics into Theme D when the new guide took effect in 2023, with the first assessment in May 2025. The four old Options were removed and Paper 3 disappeared, so genetic content now appears in Paper 1A multiple choice, Paper 1B data questions and Paper 2 Section B extended responses; the numbering runs from D1.1 to D3.2. SL and HL students both need DNA replication and inheritance ratios, while linkage and gene expression are additional higher level material. The aim below is to work through the concepts in the order they are taught, giving each one a definition, a calculation and the error that costs marks.
半保留复制与 Meselson-Stahl 实验
半保留复制（semiconservative replication）指每个子代 DNA 分子都由一条母链和一条新合成的子链组成，这是 D1.1 的核心结论。解旋酶（helicase）先断开两条链之间的氢键，把双螺旋打开成复制叉；DNA 聚合酶（DNA polymerase）只能沿 5' 到 3' 的方向加入核苷酸，所以两条模板链的合成方式并不对称。以复制叉移动方向为准，连续延伸的那条叫前导链（leading strand），另一条必须等模板暴露出一段之后反向分段合成，产生冈崎片段（Okazaki fragments），再由 DNA 连接酶（ligase）把片段连起来。很多学生以为两条链是同时、同速、同方向复制的，画图时把箭头画成一条直线，在标注题上直接丢分。Meselson 与 Stahl 在 1958 年用 15N 标记大肠杆菌 DNA，通过氯化铯密度梯度离心看到第一代全部是中间密度带，第二代出现中间带与轻带各一半，这个结果支持半保留模型而不是全保留模型。IB 考纲要求用图或文字说明过程本身，只背一句结论拿不到分。
DNA replication is semiconservative, so every daughter molecule keeps one parental strand and pairs it with a newly built one. Helicase breaks the hydrogen bonds that hold the two strands together and opens a replication fork; DNA polymerase can only add nucleotides in the 5' to 3' direction, which makes the two template strands behave differently. Synthesis on the leading strand runs continuously towards the fork, whereas the lagging strand is built in short Okazaki fragments that DNA ligase later joins; the enzyme also needs a primer before it can start. Meselson and Stahl tested the model in 1958 by labelling E. coli DNA with heavy nitrogen and separating the products by density gradient centrifugation, and only semiconservative copying explains why the first generation gave a single intermediate band.
转录与翻译：从基因到多肽
转录（transcription）是 RNA 聚合酶以反义链为模板合成 mRNA 的过程，起点是启动子，终点是终止序列，产物序列与编码链相同，只是把 T 换成 U。真核生物的初级转录本还要剪接，内含子被切除、外显子连接，成熟 mRNA 再通过核孔进入细胞质。翻译（translation）在核糖体上进行，核糖体结合 mRNA 的 5' 端并沿 5' 到 3' 方向移动，携带氨基酸的 tRNA 靠反密码子与密码子配对，起始密码子 AUG 同时编码甲硫氨酸，UAA、UAG、UGA 三个终止密码子不编码氨基酸。常见的错误是把模板链与编码链混为一谈，写出与 mRNA 完全一致的模板链序列，这一步在 Paper 2 的序列题上几乎每年都有人错。多肽链合成结束后折叠成二级与三级结构，血红蛋白这类多亚基蛋白还要把几条链组装起来才有功能。IBO 官方考纲 D1.2 要求你在给定 DNA 序列后写出 mRNA 与多肽，所以密码子表必须能熟练查用。
Transcription copies one strand of DNA into mRNA, with RNA polymerase binding at a promoter and stopping at a terminator; the transcript matches the coding strand except that uracil replaces thymine. Eukaryotic transcripts are spliced before they leave the nucleus, so introns are removed and exons are joined into a mature mRNA. Translation happens on ribosomes, which read the message from the 5' end towards the 3' end while tRNAs deliver amino acids through anticodon pairing; AUG starts the chain and UAA, UAG and UGA stop it. A frequent slip is confusing the template strand with the coding strand, so a student writes a template sequence that is identical to the mRNA and loses every subsequent mark.
减数分裂与配子多样性的来源
减数分裂（meiosis）由两次连续的分裂组成，DNA 只复制一次，结果是一个二倍体母细胞产生四个单倍体细胞。第一次分裂的前期 I 中，同源染色体配对形成二价体，非姐妹染色单体之间发生交叉互换（crossing over），交换的片段造成新的等位基因组合；中期 I 的纺锤丝把同源染色体随机拉向两极，这就是独立分配（independent assortment）。两条同源染色体的取向彼此无关，人类 23 对染色体可以产生 2 的 23 次方种组合，约八百四十万种，加上交叉互换与随机受精，同一对父母的后代基因型几乎不可能重复。第一次分裂时同源染色体分开，第二次分裂时姐妹染色单体分开，如果某对同源染色体没有分开，配子会多一条或少一条染色体，受精后形成三体或单体。这一步容易被简化成背阶段名称，真正考的是把交叉互换与独立分配解释成遗传变异的两个来源。IB 考纲把减数分裂放在 D2.1，同时要求你把它与 D3.1 的受精过程连起来。
Meiosis uses two divisions after a single round of DNA replication, turning one diploid cell into four haploid cells. During prophase I homologous chromosomes pair as bivalents and non-sister chromatids exchange segments, so crossing over creates new combinations of alleles; at metaphase I the homologues line up and are pulled to opposite poles independently of each other. With 23 pairs in humans this independent assortment gives over eight million combinations, before crossing over and random fertilisation are counted. Non-disjunction, where a pair fails to separate, produces gametes with an extra or missing chromosome and leads to trisomy or monosomy after fertilisation.
单基因杂交与测交判断基因型
基因型（genotype）是等位基因的组合，表现型（phenotype）是可观察的特征，显性等位基因在杂合状态下也会表达。单基因杂交中用 Punnett 方格或概率乘法推算子代比例，两个杂合个体杂交得到 3:1 的表现型比例与 1:2:1 的基因型比例，这是最常被考查的一组数字。表现型为显性的个体基因型可能是纯合也可能是杂合，用测交（test cross）与隐性纯合个体杂交，子代出现隐性表现型就说明亲本是杂合。等位基因之间不一定只有完全显性，ABO 血型由三个等位基因决定，IA 与 IB 之间是共显性，O 型是隐性纯合。伴性遗传的基因位于性染色体上，人类红绿色盲与血友病的致病等位基因位于 X 染色体，男性只有一条 X，隐性等位基因也会直接表现出来。很多学生把 3:1 当成固定答案，题目一旦给出多个基因或连锁就照搬比例，答案自然对不上。
A genotype is the combination of alleles an organism carries, while the phenotype is what can be observed; a dominant allele shows its effect even in a heterozygote. Monohybrid crosses are worked out with a Punnett grid or with the multiplication rule, and two heterozygotes give a 3:1 phenotypic ratio with a 1:2:1 genotypic ratio. Because a dominant phenotype can be homozygous or heterozygous, a test cross against a homozygous recessive parent reveals the unknown genotype as soon as a recessive offspring appears; ABO blood groups show a different pattern again, with three alleles and codominance between IA and IB. X-linked conditions such as red-green colour blindness appear more often in males, since a single recessive allele on the X chromosome is enough to produce the phenotype.
双基因杂交 9:3:3:1 的计算与偏离
两对位于非同源染色体上的基因独立分配时，双杂合个体自交得到 9:3:3:1 的表现型比例，这个数字来自 3:1 与 3:1 相乘。一次豌豆实验里一共记录 556 株后代，两种显性性状同时出现的 315 株，只有种子形状显性的 101 株，只有子叶颜色显性的 108 株，两种隐性性状同时出现的 32 株，观察值 315:101:108:32 与理论比例接近。计算期望值要把总数乘比例再除以 16，得到 312.75、104.25、104.25 和 34.75 四组期望频数，再用卡方检验判断偏差是否由抽样波动造成。比例偏离 9:3:3:1 常见于两种情形：两对基因位于同一条染色体上发生连锁，或者基因之间存在上位效应使表现型类别合并。IB 题目往往给出实际数据让你判断属于哪种情形，而不是让你默写比例。很多学生拿到数据就直接比大小，忽略样本量，几百株的小样本偏差本来就会比理论值大。
When two genes sit on different chromosomes they assort independently, so a dihybrid self-cross gives the familiar 9:3:3:1 phenotypic ratio, which is simply 3:1 multiplied by 3:1. In one pea experiment 556 offspring were scored: 315 showed both dominant traits, 101 showed only the seed shape dominant, 108 showed only the cotyledon colour dominant, and 32 showed both recessive traits. Expected counts come from multiplying the total by each part of the ratio and dividing by 16, giving 312.75, 104.25, 104.25 and 34.75, and a statistical test then decides whether the difference from those values is random sampling noise. Ratios move away from 9:3:3:1 for two main reasons, linkage between genes on the same chromosome and epistasis, where one gene masks or merges the phenotypic classes produced by another.
卡方检验判断观察值与期望值
卡方检验（chi-squared test）用来判断观察值与期望值的偏差是否达到显著水平，公式是每个类别的观察值减期望值再平方、除以期望值，最后把各类别相加。自由度等于类别数减一，双基因杂交有四个表现型类别，自由度就是 3，查表得到 0.05 显著性水平下的临界值是 7.815。把上面那组数据代入公式，四个类别分别贡献 0.0162、0.1013、0.1349 和 0.2176，合计卡方值 0.47，远小于 7.815，所以不能拒绝原假设，观察结果与 9:3:3:1 的期望值相符。阅卷时最常见的失分点是只写数值不给自由度、不给临界值，或者把结论写成数据正确，规范的写法必须说明在 0.05 水平下差异不显著、原假设成立。原假设（null hypothesis）要写成没有显著差异的中性表述，而不是结果符合孟德尔定律这类带方向的说法。IBO 官方考纲 D3.2 把卡方检验列为技能要求，Paper 2 的数据题会给出临界值表让你查用。
A chi-squared test checks whether the gap between observed and expected values is large enough to matter, using the sum of (O - E)^2 divided by E for each class. The degrees of freedom equal the number of classes minus one, so a dihybrid cross with four phenotypic classes has three degrees of freedom and a critical value of 7.815 at the 0.05 significance level. Substituting the pea data gives contributions of 0.0162, 0.1013, 0.1349 and 0.2176, a total of 0.47, which is far below the critical value. The conclusion therefore states that the difference is not significant at the 0.05 level and the null hypothesis is retained; marking schemes penalise answers that quote the number without the degrees of freedom or without saying what it means.
连锁基因与重组率
位于同一条染色体上的基因不会独立分配，这种关系叫基因连锁（gene linkage），重组频率由交叉互换的频率决定。同源染色体在前期 I 的非姐妹染色单体之间交换片段，交换点两侧的基因组合被重排，得到重组型配子；两基因在染色体上的距离越远，中间发生交换的机会越大，重组率越高。遗传学用重组率作为图距单位，一个单位对应百分之一的重组频率，习惯上把重组率低于 50% 的基因视为连锁。检验连锁要用测交后代的表型计数，把重组型个体数除以总数得到重组频率，再与独立分配的期望比例比较。常见的错误是把交叉互换说成突变，前者只是已有等位基因的重新组合，后者改变的是 DNA 序列本身。连锁分析是 HL 的考查内容，SL 只需理解它会导致 9:3:3:1 失效。这些内容在 IB 考纲里归入 D3.2 的基因连锁部分，与 D2.1 的减数分裂直接衔接。
Genes located on the same chromosome are linked, which means they do not assort independently and their recombinant frequencies depend on how often crossing over occurs between them. Exchange between non-sister chromatids in prophase I rearranges the alleles that sit either side of the crossover point, and the further apart two loci are, the greater the chance that a crossover falls between them. Recombinant frequency is used as a map unit, with one unit equal to one per cent recombination, and genes giving less than 50 per cent recombinants are normally treated as linked. The test is carried out on test-cross offspring by counting recombinant individuals, dividing by the total and comparing the result with the ratio expected from independent assortment.
突变类型与 CRISPR 基因编辑
基因突变指 DNA 序列的改变，碱基替换只把一个碱基换成另一个，由于遗传密码具有简并性，替换可能不改变氨基酸（同义突变），可能改变一个氨基酸（错义突变），也可能把氨基酸密码改写为终止密码子（无义突变）。插入或缺失碱基则会移动阅读框，此后所有密码子都被重新解读，产生的多肽几乎肯定失去功能。镰状细胞贫血来自 HBB 基因第六个密码子的替换，谷氨酸被缬氨酸替代，血红蛋白在低氧条件下聚合成纤维，红细胞变成镰刀形。CRISPR-Cas9 由 Doudna 与 Charpentier 在 2012 年建立，向导 RNA 把 Cas9 核酸酶带到目标序列，在特定位点造成双链断裂，细胞修复时导入的外源片段可以被整合进去，从而实现定点编辑。这项技术带来脱靶效应与伦理边界两个问题，回答讨论题时要区分体细胞编辑与可遗传的生殖细胞编辑。
A gene mutation is a change in the DNA sequence; a substitution swaps one base for another and, because the genetic code is degenerate, it may leave the amino acid unchanged, alter a single amino acid, or create a premature stop codon. Insertions and deletions are more damaging because they shift the reading frame, and every codon downstream is then read incorrectly. Sickle cell anaemia illustrates the effect, since one substitution in the HBB gene replaces glutamic acid with valine and the altered haemoglobin polymerises under low oxygen, distorting red blood cells into a crescent shape. CRISPR-Cas9, developed by Doudna and Charpentier in 2012, uses a guide RNA to direct the Cas9 nuclease to a chosen sequence, where the double-strand break allows a repair template to be inserted; discussion questions in D1.3 usually turn on off-target effects and on the difference between editing body cells and editing cells that pass the change to the next generation.
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