Edexcel生物 减数分裂 碱基配对 血型 突变
遗传这一章在 Edexcel 1BI0 的 Paper 1 里分量不小，考纲把 Topic 3 单独列成一块，从减数分裂一路排到人类基因组计划。很多学生以为背完棋盘格就够了，实际上 Paper 1 的遗传题常把术语、概率与实验情境拧在一起问。这篇按 Edexcel 官方 specification（1BI0，Issue 4，2024 年 3 月版）逐条梳理 3.1B 到 3.23 的考点，标 B 的条目属于 Separate Science 内容，Combined Science 的同学可以跳过不背。
Topic 3 is examined on Paper 1, a 1 hour 45 minute paper worth 100 marks and half of the qualification. This walkthrough follows the current specification point by point, from meiosis to the Human Genome Project. Points marked B are separate science only, so combined science students can leave them out.
无性生殖与有性生殖的取舍
无性生殖只需要一个亲本，后代由有丝分裂产生，基因与亲本完全一致。它的好处很直接：不用花时间找配偶，繁殖周期短，种群数量能在短时间内上去。代价是种群内部没有变异，一旦遇到新病害或环境剧变，整群可能一起倒下。有性生殖反过来，两个亲本各出配子，受精后把两套等位基因混在一起，后代彼此不同。变异让部分个体在环境变化里活下来，代价是要找配偶，耗时间也耗能量。这两条在 3.1B 与 3.2B 里永远是成对出题，答题时把好处与代价各写一条，再补一句后果。很多学生以为写「无变异」三个字就够了，阅卷时更想看的是变异缺失带来的后果，比如整群对新病害都没有抵抗力。
Asexual reproduction needs one parent and produces offspring genetically identical to it, so a population can grow quickly without the cost of finding a mate. The trade-off is uniformity: if a disease or a sudden change in conditions arrives, every individual shares the same weakness. Sexual reproduction mixes alleles from two parents, which gives variation and lets some offspring survive a change that kills others, but finding a mate takes time and energy. Examiners want that consequence, not just the word variation.
减数分裂的四个单倍体配子
减数分裂发生在生殖器官里，一个亲本细胞连续分裂两次，最后得到四个子细胞。每个子细胞只有亲本一半的染色体数，人类体细胞 46 条，配子就是 23 条。这四个子细胞彼此不同，也与亲本细胞不同，因为染色体在分裂过程中会片段互换并随机分配。Edexcel 2024 年版考纲写明减数分裂的各阶段不考，只考结果：四个、单倍体、基因各不相同。常见的错误是把四个写成两个，或者把减数分裂的后代与有丝分裂搞混，有丝分裂出的是两个基因相同的二倍体细胞。受精时两个配子融合成受精卵，染色体数回到 46 条，一半来自父方，一半来自母方。写遗传图谱时先把这一步交代清楚，后面棋盘格上的配子才有出处。减数分裂产生的配子若是 23 条，两个配子相加正好补齐 46 条，数字对不上就是概念没弄明白。
Meiosis happens in the reproductive organs, where one parent cell divides twice to give four daughter cells. Each carries half the chromosome number of the parent, so a human body cell with 46 chromosomes produces gametes with 23. The four cells differ from each other and from the parent because chromosomes swap sections and are then distributed at random. The specification states that the stages of meiosis are not required, only the outcome: four genetically different haploid gametes.
DNA 双螺旋与碱基互补配对
DNA 是一种聚合物，重复单位叫做核苷酸。每个核苷酸由一个糖、一个磷酸基团，加上四种碱基中的一种构成。两条链盘绕成双螺旋，链与链之间靠互补碱基对连接，用的是弱氢键。配对规则固定：腺嘌呤对胸腺嘧啶，胞嘧啶对鸟嘌呤，所以一条链写 ATGC，另一条必然是 TACG。很多学生以为氢键很强才稳，实际上正因为它是弱键，双链才能在复制与转录时被拉开。常见的错误是把氢键说成连接糖与磷酸的键，真正的共价键在核苷酸内部，氢键只出现在两条链的碱基之间。数碱基的题还要注意一点，如果一条链上腺嘌呤占三成，整段 DNA 里腺嘌呤也占三成，不是一成半。
DNA is a polymer built from repeating nucleotides, each made of a sugar, a phosphate group and one of four bases. Two strands coil into a double helix and are held together by complementary base pairs joined by weak hydrogen bonds. Adenine always pairs with thymine and cytosine with guanine, so one strand predicts the other: ATGC on one means TACG on the other. Those bonds have to be weak, otherwise the strands could not separate during replication or transcription.
基因组、基因与从水果提取 DNA
基因组指一个生物体的全部 DNA，不只是编码蛋白质的那些片段。基因则是 DNA 分子上的一段，编码一种特定的蛋白质。两个定义的分界在考试里被反复考，写反方向就丢分，把基因组说成「全部基因」也会被判错。提取水果 DNA 的实验步骤同样是考点：先用洗涤剂破坏细胞膜与核膜，让 DNA 释放出来；再用盐溶液帮助 DNA 与蛋白质分开；最后加冰乙醇，DNA 在乙醇层析出成白色絮状沉淀。这里必须是冰乙醇，室温乙醇的沉淀效果差很多。搅拌要轻，DNA 链很长，剧烈搅动会把它打断，絮状物就看不到了。
The genome is all the DNA of an organism, while a gene is a section of a DNA molecule that codes for a specific protein. Keeping those two apart matters, because the exam regularly asks for one and accepts only that wording. In the fruit extraction practical, detergent breaks the membranes, salt helps the DNA separate from protein, and ice-cold ethanol makes the DNA appear as a white precipitate. Stir gently, since long DNA strands break easily and then no precipitate forms.
从碱基顺序到蛋白质形状
基因里的碱基顺序决定了氨基酸的排列顺序，氨基酸连成链后再折叠成特定形状。形状决定功能，酶之所以能催化特定反应，靠的正是活性位点的形状。蛋白质合成分两步：转录在细胞核里进行，RNA 聚合酶结合到基因前面的非编码 DNA，以编码链为模板做出互补的 mRNA。翻译在核糖体上进行，mRNA 上三个碱基组成一个密码子，对应一个氨基酸，tRNA 把氨基酸运到核糖体，逐个连成多肽。这两步的场所与模板最容易被互换着考。常见错误是把 RNA 聚合酶写成 DNA 聚合酶，或者把密码子说成由四个碱基组成。还要分清 mRNA 与 DNA 的碱基差异，RNA 里没有胸腺嘧啶，换成尿嘧啶。
The base sequence of a gene sets the order of amino acids, and the chain then folds into a shape that decides what the protein can do. Enzymes show this clearly, since catalysis depends on the shape of the active site. Transcription happens in the nucleus, where RNA polymerase binds to non-coding DNA in front of the gene and builds a complementary mRNA strand. Translation happens on the ribosome, where each codon of three bases specifies an amino acid that tRNA delivers.
编码区与非编码区的变异
变异落在 DNA 的不同位置，后果完全不同。落在编码区的变异会改变氨基酸顺序，可能让蛋白质活性下降甚至完全失效，这是 3.10B 的口径。落在非编码区的变异不改变氨基酸，却能影响 RNA 聚合酶结合的难易程度，于是蛋白质的产量变了，这是 3.9B 的口径。这两条经常配对出题，题干给一个具体变异，让判断属于哪一类、后果是什么。答题时先定位变异在哪个区，再说改变的是数量还是结构，两句话对上就给分。很多学生以为所有变异都会改变蛋白质结构，这一条正好是命题人常用的陷阱。
A variant in the coding DNA changes the amino acid sequence, which can reduce or destroy the activity of the protein. A variant in the non-coding DNA leaves the amino acids alone but changes how easily RNA polymerase binds, so the quantity of protein produced rises or falls. Questions often give one variant and ask which category it belongs to, so locate the region first, then say whether the amount or the structure has changed.
孟德尔与当时为何难被接受
孟德尔用豌豆做了多年杂交实验，1866 年发表结果，提出性状由成对的遗传因子控制，显性与隐性会按固定比例分离。他的结论当时没被接受，原因不难理解：那时人们不知道染色体与 DNA 的存在，也无法观察细胞里发生的事情。比例的统计要靠大量个体，豌豆恰好合适，换成别的材料未必能看出规律。直到 1900 年前后，三位科学家分别用不同植物重复出相似结果，孟德尔的工作才被重新发现。答题时把「缺乏机制解释」与「材料选择合适」两点写出来，比只写「不被重视」扎实得多，这也是 3.11B 的分数落点。
Mendel crossed pea plants for years and published his results in 1866, arguing that inherited factors come in pairs and separate in fixed ratios. His work was ignored at the time because chromosomes and DNA were unknown, so no mechanism could explain the ratios. Peas happened to be a good choice, since large numbers of offspring made the pattern visible. Only around 1900, when three scientists repeated similar crosses in other plants, did his conclusions gain acceptance.
等位基因与十一个必背术语
同一种基因的不同版本叫等位基因，等位基因不同，性状才出现差异。3.13 列了十一个必须能准确使用的术语：染色体、基因、等位基因、显性、隐性、同型合子、异型合子、基因型、表现型、配子、受精卵。这些词在答案里互换一次就要扣分，最容易换错的是基因型与表现型、同型合子与异型合子。写遗传图谱时用大写字母表示显性等位基因，小写表示隐性，同一字母代表同一个基因。显性等位基因在杂合状态下也表现出性状，隐性等位基因要两份都在才表现。很多学生以为显性就是更常见，实际上显性只描述表达关系，与频率没有关系。受精卵这个词也别忽略，它是配子融合后的第一个二倍体细胞。
Alleles are different versions of the same gene, and different alleles are what produce differences in inherited characteristics. Specification point 3.13 expects precise use of eleven terms, from chromosome and allele through to genotype, phenotype, gamete and zygote. Genotype and phenotype are the pair most often swapped, and so are homozygous and heterozygous. Write the dominant allele in capitals and the recessive in lower case, using one letter for one gene.
单基因杂交的棋盘格与概率
单基因杂交只看一对等位基因，用棋盘格把两个亲本的配子两两组合。两个杂合子杂交，子代基因型是 1 AA 比 2 Aa 比 1 aa，表现型是 3 显性比 1 隐性。概率的说法要能互换：显性表现型占 75%，隐性占 25%，杂合子占 50%。家族系谱图考的是反向推理，已知子代有隐性个体，双亲必然各带一份隐性等位基因。测交用来判断显性个体的基因型，把它与隐性纯合子杂交，子代出现隐性个体就说明它是杂合子。书写要求很细，两个亲本的配子要分行列出，等位基因不能挤在同一行里糊成一团。概率题如果问「再生一个孩子」是多少，答案与前一胎无关，仍然是四分之一。
A monohybrid cross follows one pair of alleles, and a Punnett square pairs every gamete from one parent with every gamete from the other. Two heterozygotes give a 1 to 2 to 1 genotype ratio and a 3 to 1 phenotype ratio, which works out as 75 per cent dominant, 25 per cent recessive and 50 per cent carriers. Pedigrees run the same logic backwards, since a recessive child proves both parents carry the recessive allele. A test cross with a homozygous recessive shows whether a dominant individual is homozygous or heterozygous.
性别由受精那一刻决定
人类的性别由受精那一刻决定，与母亲无关。女性有两条 X 染色体，产生的卵细胞都带一条 X。男性有一条 X 和一条 Y，精子一半带 X，一半带 Y。带 X 的精子与卵结合得到 XX，是女性；带 Y 的精子与卵结合得到 XY，是男性。所以每次怀孕生男生女的概率各占一半，染色体组合写成 XX 与 XY，不要只写 X 与 Y。常见的错误是把父亲说成决定性别的一方，图上却把卵子画成两种，两句话自相矛盾。题目若问「概率多少」，写 50% 或 1 比 1 都可以，写 25% 就是没分清性别与单基因杂交。
Sex in humans is fixed at fertilisation, and the father's sperm decides it. A woman has two X chromosomes, so every egg carries an X. A man has one X and one Y, so half his sperm carry X and half carry Y. An X-bearing sperm gives XX and a Y-bearing sperm gives XY, which is why each pregnancy carries an even chance of either sex.
ABO 血型与伴性遗传
ABO 血型由三个等位基因决定，书写为 IA、IB 与 i，其中 A 与 B 是上标，一个人只带其中两个。IA 与 IB 之间是共显性，两者同时存在时表现为 AB 型，两者对 i 都是显性。所以 A 型可以是 AA 或 AO，B 型可以是 BB 或 BO，AB 型只有 AB，O 型只有 OO。伴性遗传考的是 X 染色体上的隐性基因，男性只有一条 X，带一个隐性等位基因就发病，女性要两条都带才发病。携带者母亲与正常父亲生育，儿子有一半概率患病，女儿不患病但可能成为携带者。画图时先把父母的性染色体写出来，再分别列配子，最后合成子代。红绿色盲与血友病是常用例子，两个都由 X 染色体上的隐性基因引起。
The ABO blood groups come from three alleles written IA, IB and i, and one person carries only two of them. IA and IB are codominant, so both together give group AB, and each is dominant over i. Group A is therefore AA or AO, group B is BB or BO, group AB is AB and group O is OO. Sex-linked inheritance works differently, because a recessive allele on the X chromosome shows in a male after a single copy while a female needs two.
多基因性状、变异与人类基因组计划
多数性状不是单个基因决定的，身高、体重这类特征由多个基因共同作用，还叠上环境因素。变异分两类：遗传变异来自突变与有性生殖，环境变异来自生物体所处环境造成的获得性差异。种群内部通常存在大量遗传变异，这些变异的最终来源是突变。大多数突变对表现型没有影响，少数有小影响，很少见的单个突变会显著改变表现型。人类基因组计划测出整个人类基因组的序列，结果在医学上可用于寻找致病基因、设计诊断与开发新药。这一节的名词最多，答题时按来源、类别、后果三层写，分数一般都能拿稳。连续变异与不连续变异的区别也考，身高体重是连续，血型与花色是不连续。
Most characteristics are not controlled by a single gene, since height and body mass depend on many genes acting together with the environment. Variation comes from two sources: genetic variation, produced by mutation and sexual reproduction, and environmental variation, which is acquired during life. Within a population there is usually plenty of genetic variation, and mutation is where it comes from. Most mutations have no effect on the phenotype, some have a small effect, and only rarely does a single one change it significantly.
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