Edexcel生物 遗传 单基因杂交 卡方 血型
遗传学是 Edexcel A-Level 生物里最容易被低估的一章。学生背下了显性与隐性的定义，却在系谱图和卡方检验上反复丢分，原因往往不是知识点没记住，而是没有按考纲要求的语言作答。这篇按 Edexcel 考纲（Specification）的条目顺序，把单基因杂交、系谱图、卡方检验、连锁与 Hardy-Weinberg 串成一条线，也顺手说清生物学 A（Salters-Nuffield，9BN0）与生物学 B（9BI0）在遗传学上的范围差别。
Genetics rewards precision rather than volume in Edexcel A-Level Biology. Plenty of candidates can define dominance but still lose marks on pedigree questions and chi-squared conclusions, usually because the answer is not phrased in the terms the mark scheme expects; this guide follows the specification point by point and separates the genetics demanded by Biology A (Salters-Nuffield, 9BN0) from that demanded by Biology B (9BI0).
一、两套 Edexcel 考纲把遗传学放在不同主题
同样一门 Edexcel A-Level 生物，生物学 A 与生物学 B 的遗传学落点并不一样。生物学 A 的 Topic 2 Genes and Health 用囊性纤维化做背景，考纲条目 2.13 只要求基因、等位基因、基因型、表型、显性、隐性、不完全显性、纯合与杂合这些术语，以及单基因遗传下的系谱图判读。基因连锁与性连锁在 Topic 3 的 3.8，减数分裂产生变异在 3.9，多基因遗传在 3.15，Hardy-Weinberg 方程在 Topic 4 的 4.5。生物学 B 把遗传集中到 Topic 7 Modern Genetics 与 Topic 8 Origins of Genetic Variation，8.2 的术语表里明确出现 codominance 与 multiple alleles，还要求用 Hardy-Weinberg 方程监测种群等位基因频率的变化。很多学生以为两个版本的遗传学一样，其实范围差出一整块。
Biology A folds monohybrid inheritance into Topic 2, where cystic fibrosis supplies the context and point 2.13 covers little more than terminology, pedigree interpretation and single-gene crosses, while linkage and sex linkage appear later at 3.8 and the Hardy-Weinberg equation at 4.5. Biology B gathers the same material into Topics 7 and 8, and its term list at 8.2 explicitly includes codominance and multiple alleles. Treating the two versions as interchangeable is the fastest way to revise content your paper will never ask about.
二、等位基因、基因型与表型的用词边界
考纲 2.13 把 gene、allele、genotype、phenotype、dominant、recessive、incomplete dominance、homozygote、heterozygote 列成一组术语，判分时按字面认。基因是 DNA 上一段编码多肽链氨基酸顺序的碱基序列，等位基因是同一基因座上的不同版本，两者的区别常常被含糊过去。基因型写成 Aa 这类字母组合，表型写实际观察到的性状，做题时把两个词互换是常见的错误。显性不等于数量多，也不等于更常见，它只说明杂合状态下这个等位基因写进了表型。不完全显性下杂合子表型介于两个纯合子之间，例如红花与白花杂交得到粉花，此时不会出现 3 比 1 的表型比。纯合与杂合描述两个等位基因是否相同，写基因型时要与表型分开陈述。
The specification names nine terms at 2.13 and the mark scheme reads them literally, so a gene has to be described as a length of DNA coding for the amino acid sequence of a polypeptide, with an allele as one version of that gene at the same locus. Dominance says nothing about how common an allele is; it only means the heterozygote shows that phenotype, and under incomplete dominance the heterozygote sits between the two homozygotes, which is why a red crossed with a white flower gives pink rather than a 3:1 ratio. Candidates who write phenotype when they mean genotype lose the mark even when the biology underneath is right.
三、单基因杂交与遗传图解的比例写法
单基因杂交的做法是固定的：先写亲本基因型，再列配子，然后组合出子代基因型，最后换算表型比。Aa 与 Aa 杂交，配子各为 A 与 a，子代基因型比 1 AA 比 2 Aa 比 1 aa，表型比 3 比 1。写配子前先确认亲本是纯合还是杂合，纯合子只产生一种配子，这一步漏掉后面的比例全错。测交是把未知基因型的个体与隐性纯合子杂交，子代出现 1 比 1 就说明未知个体是杂合子。两个独立事件同时出现的概率等于各自概率之积，所以 Aa 与 Aa 生出 aa 的概率是二分之一乘二分之一。阅卷时常见的是比例写对了但配子没列出，或者把 1 比 2 比 1 当成表型比。
A monohybrid answer follows a fixed order: parental genotypes, gametes, offspring genotypes, then the phenotype ratio, and skipping the gamete line is what usually breaks the arithmetic. A test cross against a homozygous recessive parent gives a 1:1 split when the unknown parent is heterozygous, and because independent events multiply, two heterozygotes produce an affected child with probability one half times one half. The ratio 1:2:1 belongs to genotypes rather than phenotypes, and mixing the two costs marks even when every number is correct.
四、系谱图判断遗传方式的三条线索
系谱图题先找有没有隔代出现的患病个体，父母正常而子女患病，说明该性状由隐性等位基因控制。接着看患病者是否两性都有，常染色体隐性遗传下男女比例接近，性连锁隐性遗传下男性明显更多。再看父亲患病时女儿是否全部患病，X 连锁隐性遗传里父亲把唯一的 X 传给所有女儿，女儿至少是携带者。系谱图上方块代表男性，圆圈代表女性，涂色代表患病，作答时按题目给的符号来，不要自创。考纲 2.13 只要求在单基因遗传的情境里判读系谱，所以不必推出多基因或线粒体遗传。常见的错误是把携带者当成未患病就忽略过去，携带者在隐性遗传里恰恰是连接两代的个体。
Pedigree questions are solved by three checks in order: an affected child from two unaffected parents shows the allele is recessive, an excess of affected males points to X linkage, and an affected father passing the trait to every daughter confirms it. Squares are males and circles are females, with shading marking the affected phenotype, so reproduce the key exactly as the question prints it. Ignoring carriers is the usual slip, because in a recessive condition they are the individuals that link one generation to the next.
五、减数分裂如何制造配子的多样性
减数分裂的产物是四个单倍体细胞，每个细胞的染色体组合都不完全一样。考纲 3.9 写明不考减数分裂各期的细节，只考它为什么带来变异：同源染色体在中期排布时的独立分配，以及前期同源染色体非姐妹染色单体之间的交叉互换。人类 23 对染色体独立分配可以产生二的二十三次方种组合，约八百多万种，交叉互换再把这个数字放大。随机受精在配子结合时又加一层随机性。很多学生以为变异只来自突变，其实对一个家庭内部的兄弟姐妹来说，独立分配与交叉互换才是主要来源。作答时说清是哪一步产生新组合的等位基因，比笼统写基因重组更能拿分。
Meiosis produces four haploid cells whose chromosome combinations differ, and point 3.9 asks why that happens rather than how each stage proceeds. Independent assortment of the 23 pairs gives roughly eight million combinations before crossing over between non-sister chromatids multiplies that figure, and random fertilisation adds a further layer when gametes fuse. Saying that variation comes only from mutation misses the point, because within one family the reshuffling of existing alleles is what makes each sibling different.
六、卡方检验判断观察比例能否接受
卡方检验处理的是分类计数数据，遗传题里用来判断子代观察数是否符合某个预期比例。公式是各项的观察值减预期值之差的平方除以预期值，再全部相加。自由度等于分类数减一，两个表型类别就是 1，四个类别是 3。生物题统一取百分之五的显著性水平，自由度 1 的临界值是 3.84，自由度 3 的临界值是 7.82。算出的卡方小于临界值，说明差异不显著，接受零假设；大于或等于临界值，拒绝零假设，说明还有别的因素在起作用，比如基因连锁。举个具体例子，总数 800 的 3 比 1 分离，预期 600 与 200，观察到 585 与 215，卡方等于 225 除以 600 加上 225 除以 200，得 1.5，小于 3.84，差异不显著。常见的错误是结论只写结果由偶然造成，没有说明观察值与预期值之间的差异不显著。
The chi-squared test works on counts in categories, and in genetics it asks whether observed offspring numbers fit a predicted ratio. With 800 offspring expected 600 to 200 but observed 585 to 215, chi-squared is 225 divided by 600 plus 225 divided by 200, which gives 1.5 against a critical value of 3.84 at one degree of freedom, so the null hypothesis stands. A conclusion that says the results were due to chance is not the same as saying the difference between observed and expected is not significant, and the second wording is the one that scores.
七、共显性与复等位基因：ABO 血型的写法
共显性指杂合子里两个等位基因都表达，ABO 血型是标准例子：I^A 与 I^B 同时存在时，红细胞表面同时出现 A 抗原与 B 抗原，表现为 AB 型。复等位基因指一个基因座上有三个以上的等位基因版本，但每个人只携带其中两个。ABO 的等位基因写作 I^A、I^B 与小写 i，i 对前两者隐性，所以基因型 I^A i 是 A 型，ii 是 O 型。生物学 B 的 8.2 明确把 codominance 与 multiple alleles 列入术语表，生物学 A 的 2.13 用的是 incomplete dominance，两个词不能互换。很多学生以为 AB 型是中间型，其实两种抗原各自完整表达，这正是共显性与不完全显性的分野。写血型题时先列父母基因型，再枚举配子，最后按抗原而不是按字母顺序判断表型。
Codominance means both alleles are expressed in the heterozygote, and ABO blood groups show it clearly because I^A and I^B together put both antigens on the red cell surface. Multiple alleles describe three or more versions of one locus, of which any individual carries only two, so I^A i gives group A, I^B i gives group B and ii gives group O. Biology B names codominance and multiple alleles at 8.2 while Biology A uses incomplete dominance at 2.13, and swapping the two labels is a marking error rather than a wording preference.
八、两对基因同时考虑时的 9 比 3 比 3 比 1
两对基因位于不同染色体时，等位基因独立分配，双杂合子自交的子代表型比是 9 比 3 比 3 比 1。这个比例的前提是两对基因不连锁，也没有相互作用，一旦基因在同一条染色体上距离很近，实际比例就会偏离。生物学 B 的 8.2 要求能建构遗传杂交，两对基因的题目就落在这里；生物学 A 的 2.13 只写单基因遗传，多写双基因不会加分。写这类题时先分别处理每一对基因，再把概率相乘，比一次画十六格省时间。十六格棋盘只适合检查，考场上按每对基因分开算，出错概率低很多。常见的错误是把 9 比 3 比 3 比 1 直接写进单基因题，或者把独立分配写成自由组合定律却说不清指的是染色体行为。
When two genes sit on different chromosomes, independent assortment gives a 9:3:3:1 ratio from a cross between double heterozygotes, and that figure assumes no linkage and no interaction between the genes. Biology B expects you to construct such crosses at 8.2, whereas Biology A limits the requirement to monohybrid inheritance at 2.13, so extra dihybrid detail earns nothing there. Working each gene separately and multiplying the probabilities is quicker and safer than drawing a sixteen-square grid under exam pressure.
九、基因连锁与性连锁的区别
基因连锁指两个基因座在同一条染色体上，减数分裂时它们倾向于一起进入同一个配子。距离越近，交叉互换打断它们的概率越低，所以连锁会压低重组型配子的比例。性连锁指基因位于性染色体上，哺乳动物的 X 染色体比 Y 染色体大得多，X 上的基因在 Y 上没有对应位置，男性只有一份拷贝。X 连锁隐性疾病因此多见于男性，母亲是携带者时儿子有一半概率患病，父亲患病时女儿全部是携带者。考纲里生物学 A 把 linkage 与 sex linkage 放在 3.8，生物学 B 把相关术语放在 8.2。判断时先看题目给的基因是不是在同一对染色体上，再决定用连锁还是独立分配。
Linked genes sit on the same chromosome and tend to travel into the same gamete, with the recombination frequency falling as the distance between them shrinks. Sex linkage is a separate matter: the X chromosome carries genes with no counterpart on the much smaller Y, so a male has a single copy and an X-linked recessive condition appears far more often in sons of carrier mothers. Biology A places linkage and sex linkage at 3.8 and Biology B at 8.2, and both expect you to establish whether the genes share a chromosome before you choose a ratio.
十、Hardy-Weinberg 方程与携带者频率
Hardy-Weinberg 方程描述理想种群中等位基因频率与基因型频率的关系，形式是 p 加 q 等于 1，p 平方加 2pq 加 q 平方等于 1。p 与 q 是两个等位基因的频率，q 平方是隐性纯合子的频率，2pq 是杂合子的频率。方程成立需要没有选择、没有突变、没有迁移、随机交配且种群足够大，现实种群总有偏离。考纲里生物学 A 把它放在 Topic 4 的 4.5，生物学 B 放在 8.2，用法都是拿它当基准看等位基因频率有没有变化。举个能对上官方数字的例子：英国新生儿中囊性纤维化约每 2500 例 1 例，数据见 gov.uk 的新生儿血斑筛查页面，取 q 平方等于 0.0004，q 等于 0.02，p 等于 0.98，2pq 约等于 0.039，接近二十五分之一，与官方给出的携带者比例一致。算完要把数值的含义写清楚，只留一个数字而不说明它是基因型频率还是等位基因频率，通常拿不到解释分。
The Hardy-Weinberg equation links allele frequencies to genotype frequencies through p plus q equals 1 and p squared plus 2pq plus q squared equals 1, with q squared giving the frequency of the recessive homozygote. Applied to cystic fibrosis, where about 1 in 2,500 babies born in the UK is affected according to the gov.uk newborn blood spot page, q squared of 0.0004 gives q of 0.02, p of 0.98 and 2pq of about 0.039, close to the official carrier figure of 1 in 25. The model assumes no selection, no mutation, no migration, random mating and a large population, so a real deviation is the observation worth commenting on rather than a calculation error.
十一、基因突变怎样变成囊性纤维化的症状
考纲 2.12 把 DNA 复制错误与突变连在一起，2.14 要求说清突变的表达如何损害呼吸、消化与生殖系统。CFTR 基因编码一个氯离子通道，突变后通道功能缺失或下降，氯离子不能正常外流，水随盐走，分泌物变得粘稠。气道里粘液清除能力下降，细菌容易定植，反复感染损伤肺组织。胰腺导管被粘液堵塞，消化酶难以进入小肠，脂肪吸收受影响，体重增长缓慢。生殖系统方面，多数男性患者因输精管先天性缺失而不育，这是同一个通道问题的后果。作答时把分子层面的通道功能与器官层面的症状连起来，只写症状不写机制通常只能拿到一半分。
A mutation in the CFTR gene removes or reduces the function of a chloride channel, so chloride cannot leave the cell normally, water follows the salt and secretions become unusually thick. In the airways that thickness impairs mucus clearance, which encourages bacterial colonisation and repeated infection, while in the pancreas blocked ducts keep digestive enzymes out of the small intestine and fat absorption falls. Biology A asks at 2.14 for the consequences across gas exchange, digestion and reproduction, and the missing vas deferens in most affected males is the same channel defect seen in a different tissue.
十二、遗传筛查与伦理题的作答结构
考纲 2.15 列出三类筛查用途：识别携带者、胚胎植入前遗传学诊断、以及包括羊水穿刺与绒毛膜取样在内的产前检测。羊水穿刺一般在孕十五到二十周进行，绒毛膜取样更早，通常在孕十一到十四周，两者取样的组织不同，流产风险也不同。胚胎植入前遗传学诊断在体外受精的胚胎阶段取样，只移植未携带致病基因型的胚胎。考纲 2.16 要求从多种伦理立场讨论，作答时把支持与反对各写两条并说明理由，比只表态度容易得分。支持方常用减少疾病负担与扩大生育选择，反对方常提胚胎地位、筛查带来的压力与残障权益。很多学生以为伦理题没有标准答案就可以随便写，实际上评分看的是有没有落到具体争点上。
Point 2.15 groups genetic screening into carrier identification, pre-implantation genetic diagnosis and prenatal testing by amniocentesis or chorionic villus sampling, and the two prenatal routes differ in timing, tissue sampled and miscarriage risk. Pre-implantation diagnosis samples embryos created by IVF so that only those without the disease genotype are transferred, which moves the decision earlier than either prenatal test. Point 2.16 asks for discussion from a range of ethical viewpoints, and answers that name a specific tension, such as the status of the embryo against the reduction of disease burden, score better than ones that simply state an opinion.
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