Edexcel GCSE生物 DNA 减数分裂 等位基因
遗传学是 Edexcel GCSE 生物里名词最密的一章：Paper 1 的十道题里，Topic 3 至少占一道大题，而且几乎每一问都要先写准定义。减数分裂、等位基因、基因型与表型、显性与隐性，任何两个写反都会连带整道题失分。下面按考试里实际出现的顺序，把繁殖方式、DNA 结构、蛋白质合成、遗传图解、血型与突变串成一条线，每一段都落到官方 spec 的条目号上，复习时可以对着条目查漏。
Genetics is the densest topic in Edexcel GCSE Biology, and it sits inside Paper 1 alongside the other four topics rather than in a paper of its own. Meiosis, alleles, genotype and phenotype, dominant and recessive: swap any two of them and a whole question collapses. This walk-through follows the order the content appears in the specification, from reproduction and DNA structure through protein synthesis, genetic diagrams, blood groups and mutation, so each section can be checked off against an item number as you revise.
Paper 1 里 Topic 3 的分值位置
按 Pearson 官方 spec 1BI0（2016 年 9 月起施教，2018 年首次认证，现行版本为 Issue 4），Paper 1 的卷号是 1BI0/1F 与 1BI0/1H，时长 1 小时 45 分钟，满分 100 分，占最终成绩的 50%，覆盖 Topic 1 到 Topic 5，共十道题，计算器允许带入考场，Foundation 对应 1 到 5 级，Higher 对应 4 到 9 级，两卷之间还有 27 分的重叠题。Topic 3 Genetics 的内容条目从 3.1B 排到 3.23，条目号后面带 B 的是 Higher tier 单独要求，例如蛋白质合成（3.8B）、编码区与非编码区变异（3.9B、3.10B）、ABO 血型（3.17B）与性连锁遗传（3.18B）。Foundation 考生不做这些条目，但同一道大题的前几问照样考减数分裂与庞尼特方格。阅卷时最常见的空白是把 3.3 写成「形成配子」就收笔，漏掉四个子细胞与染色体数目减半这两个得分点。
Paper 1 in the Pearson Edexcel 1BI0 specification, first taught from September 2016 and first certificated in 2018, runs 1 hour 45 minutes for 100 marks and 50% of the qualification, covers Topics 1 to 5 in ten questions and allows a calculator; Foundation targets grades 1 to 5 while Higher targets 4 to 9, and 27 marks are overlap questions shared by both tiers. Topic 3 Genetics carries items 3.1B to 3.23, where the letter B flags Higher-tier-only content such as protein synthesis at 3.8B, variants in coding and non-coding DNA at 3.9B and 3.10B, ABO blood groups at 3.17B and sex-linked disorders at 3.18B. Foundation candidates never meet those items, yet the opening parts of the same question still test meiosis and Punnett squares, and answers that stop at making gametes lose the two marks sitting on four daughter cells and a halved chromosome number.
有性繁殖与无性繁殖的取舍
无性繁殖不经过配子融合，子代的遗传信息与亲本相同，好处是不必寻找配偶、繁殖周期短、速度快，草莓的匍匐茎、马铃薯的块茎、水螅的出芽与细菌的二分裂都属于这一类；代价是种群中没有新的等位基因组合，一旦环境骤变或出现新病原体，整片个体可能同时受影响。有性繁殖需要两个配子融合，亲本各提供一半遗传信息，子代彼此之间存在差异，自然选择才有可选的素材，代价是寻找配偶的时间、更慢的繁殖周期，以及产生大量配子与后代所消耗的资源。3.1B 与 3.2B 的题目要求利弊各写一条并落到具体生物，只写「能增加变异」通常给一分，补上「变异让部分个体在后代环境中存活下来」才拿到第二分。很多学生以为无性繁殖的后代完全一样，环境造成的差异不算遗传变异，这句话在概括题里会被扣分。
Asexual reproduction skips gamete fusion, so offspring carry the same genetic information as the parent, which is cheap and quick because no mate has to be found; strawberry runners, potato tubers, budding hydra and binary fission in bacteria all work this way, and the price is a population with no new combinations of alleles, so a new pathogen or a sharp change in conditions can hit every individual at once. Sexual reproduction fuses two gametes so each parent supplies half the genetic information, giving the variation natural selection works on, at the cost of finding a mate, a slower cycle and the resources poured into gametes and offspring. Questions on items 3.1B and 3.2B mark the advantage or disadvantage and the explanation separately, so a general claim about variation scores once and naming which offspring might survive the change scores twice.
减数分裂产生四个单倍体配子
减数分裂经过两轮分裂，第一轮把同源染色体分开，第二轮把姐妹染色单体分开，一个母细胞最终产生四个子细胞，每个子细胞的染色体数目是母细胞的一半，因此被称为单倍体（haploid）。人的体细胞有 23 对染色体共 46 条，减数分裂后的配子各带 23 条；受精时两个配子融合形成受精卵（zygote），染色体数目回到 46 条，恢复成二倍体。四个子细胞的遗传组成各不相同，这是有性繁殖变异的来源之一。官方 spec 的 3.3 明确写了分裂的各个阶段不需要记忆，要记的是四个子细胞、染色体数目减半、遗传上互不相同这三件事，考试出题也几乎只考这三处。常见的错误是写成「产生两个子细胞」或者「染色体数目不变」，这两种写法在 1BI0 里都拿不到分。
Meiosis runs through two divisions and ends with four daughter cells from one parent cell, each carrying half the chromosome number and therefore described as haploid; a human body cell holds 23 pairs, so gametes carry 23 chromosomes and fertilisation restores 46 in the zygote. The four cells differ genetically from one another, which is one source of the variation sexual reproduction depends on, although the specification states that the stages of meiosis are not required. The marks sit on the number four, the halving and the genetic difference, so an answer describing two daughter cells or an unchanged chromosome number scores nothing at all.
DNA 的双螺旋与碱基互补配对
DNA 是聚合物，每个单体叫核苷酸，一个核苷酸由一分子糖、一分子磷酸和一个碱基组成。两条链互相缠绕形成双螺旋，链上的碱基按互补规则配对，腺嘌呤配胸腺嘧啶、胞嘧啶配鸟嘌呤，碱基对之间靠弱的氢键连在一起。氢键弱这件事在提取实验里正好用得上：果实先捣碎，加洗涤剂破坏细胞膜与核膜，加盐改变 DNA 的溶解性，加蛋白酶去掉与 DNA 结合的蛋白质，最后沿管壁缓慢加入冰乙醇，出现的白色絮状物就是析出的 DNA。3.4 的得分点落在两条链、碱基互补、糖加磷酸加碱基这三处，只写「双螺旋结构」通常拿一分。错得最多的是把碱基之间的氢键写成共价键，或者把磷酸与碱基的位置写反。
DNA is a polymer built from nucleotides, and each nucleotide combines a sugar, a phosphate group and one of four bases; two strands coil round each other into a double helix, held together by weak hydrogen bonds between complementary base pairs, adenine with thymine and cytosine with guanine. Those weak bonds are what makes DNA extraction work, since mashed fruit is treated with detergent to break the cell and nuclear membranes, salt to alter the solubility of DNA, protease to strip away bound proteins, and finally ice-cold ethanol layered down the side of the tube so the DNA comes out as white strands. Marks in item 3.4 sit on the two strands, the complementary pairing and the sugar-phosphate-base nucleotide, so double helix on its own earns one mark, and naming a covalent bond between the bases loses the rest.
基因组、基因与蛋白质形状
基因组是某个生物全部 DNA 的集合，基因是 DNA 分子上编码某种特定蛋白质的一段。碱基序列决定氨基酸序列，氨基酸序列决定蛋白质折叠成什么形状，形状决定它能做什么，酶之所以只催化特定反应，就是因为它的活性部位与底物在形状上互补。基因数目并不等于性状数目，大多数表型特征由多个基因共同决定，3.19 要求考生说明这一点。人类基因组计划的测序工作在 2003 年完成，之后的应用方向包括定位致病基因、开发针对性药物与个体化治疗，也带出知情同意与基因信息保密这类伦理问题。答题时把序列决定结构、结构决定功能这条链写完整，比罗列基因组计划带来的好处更容易拿分，因为前者对应条目里的因果表述。
A genome is the entire DNA of an organism, and a gene is a section of a DNA molecule that codes for a specific protein; the order of bases decides the order of amino acids, the amino acid chain decides how the protein folds, and the folded shape decides what the protein can do, which is why an enzyme catalyses only one kind of reaction. Most phenotypic features come from several genes rather than a single one, a point item 3.19 asks candidates to state. Sequencing through the Human Genome Project was completed in 2003, and the applications that followed run from locating disease genes to targeted drugs and personalised treatment, alongside questions about consent and the privacy of genetic information.
转录与翻译的六个环节
3.8B 要求写清蛋白质合成的过程。RNA 聚合酶先结合到基因前方的非编码 DNA 上，随后以编码链为模板合成一条互补的 mRNA，mRNA 离开细胞核并附着到核糖体上，mRNA 上每三个碱基构成一个密码子，对应一个特定氨基酸，tRNA 把氨基酸运到核糖体，氨基酸逐个连接成多肽链。变异出现在哪一段，后果差别很大：非编码区的变异影响 RNA 聚合酶的结合，从而改变蛋白质的产量；编码区的变异改变氨基酸序列，进而改变蛋白质的活性，这两条正是 3.9B 与 3.10B 的分界，写混了分就丢了。很多学生以为一个密码子对应三个氨基酸，实际上是三个碱基对应一个氨基酸，方向写反整张图解都读不通。
Protein synthesis starts when RNA polymerase binds to non-coding DNA in front of the gene and builds a complementary strand of mRNA from the coding DNA; the mRNA then leaves the nucleus and attaches to a ribosome, where each triplet of bases, called a codon, specifies one amino acid carried in by tRNA, and the amino acids join into a polypeptide chain. Where a variant sits decides the effect, because a change in non-coding DNA alters how well RNA polymerase binds and therefore how much protein is made, while a change in coding DNA alters the amino acid sequence and so the activity of the finished protein. Candidates who read a codon as three amino acids rather than three bases will misread every diagram in the question.
等位基因、基因型与表型
3.13 的十一个术语要一次背准：染色体、基因、等位基因、显性、隐性、纯合、杂合、基因型、表型、配子、受精卵。人的体细胞里染色体成对存在，一对同源染色体上的相同位置可能藏着两个不同的等位基因，等位基因的说法就是这么来的。显性等位基因只要出现一个就表现出来，隐性等位基因要两份才表现；基因型记录的是等位基因的组合，表型记录的是可以观察到的特征。书写习惯也要统一，同一个基因的显性等位基因用大写字母，隐性用同一个小写字母，两个字母并排写，杂合写作 Bb 而不是 bB。学生最常把表型与基因型写反，或者把杂合误解成「带有两个不同的基因」，两种说法的差别在于杂合讲的是同一个基因的两个等位基因不同。
The eleven terms in item 3.13 have to be exact: chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote; body cells carry chromosomes in pairs, and the same position on a pair of homologous chromosomes may hold two different alleles, which is where alleles come from. A dominant allele shows its effect when one copy is present while a recessive one needs two, and the genotype records the alleles while the phenotype records the observable feature. Marks go missing when the two words are swapped or when heterozygous is read as carrying two different genes rather than two different alleles of one gene, so write the dominant allele in capitals and the recessive in the same letter, lower case.
庞尼特方格与单基因杂交
单基因杂交画庞尼特方格最稳。两个杂合子交配，也就是 Bb 与 Bb，两边各写 B 与 b，方格里的四个组合是 BB、Bb、Bb、bb，基因型比例是 1:2:1，表型比例是 3:1，出现隐性表型的概率是 25%，也就是四分之一。两个携带隐性等位基因的健康父母，每一次生育生出患病孩子的概率都是四分之一，前面几个孩子的结果不会影响下一次，这一点在概率题里经常被追问。囊性纤维化是隐性遗传的例子，多指是显性遗传的例子；遗传系谱图题要求先由父母正常而孩子患病这类信息判断隐性，再回推每个人的基因型。答案里同时写出比例与概率最保险，题目问 ratio 就给 3:1，问 percentage 就给 25% 或 75%。
A monohybrid cross is safest drawn as a Punnett square: with two heterozygotes, Bb by Bb, the four boxes hold BB, Bb, Bb and bb, giving a genotype ratio of 1:2:1, a phenotype ratio of 3:1 and a 25% chance of the recessive feature. Two healthy carriers of a recessive allele face a one in four chance of an affected child at every pregnancy, and earlier children do not change the odds for the next one, which is exactly what probability questions probe; cystic fibrosis illustrates recessive inheritance while polydactyly illustrates dominant inheritance. Read a pedigree by finding the pairing that produces an affected child from unaffected parents, since that pattern only fits a recessive allele, then work back to each genotype before writing the ratio.
性别决定与 ABO 血型
性别的决定写在性染色体上。女性是 XX，男性是 XY，母亲的卵子都带一条 X，父亲的精子一半带 X、一半带 Y，受精的结果是 XX 或 XY，两种各占一半，所以决定后代性别的是父亲提供的精子，母亲提供的卵子并不影响性别。ABO 血型是共显性与复等位基因的经典例子：这个基因有三个等位基因，比一对等位基因的标准情形多出一个，前两个对第三个都是显性，而当它们同时存在时两个都表现出来，形成 AB 型。AB 型与 O 型的子女只可能是 A 型或 B 型，比例各半，不会出现 O 型或 AB 型。写基因型时等位基因要写全，同一个表型可能来自两种不同的基因型组合，混写会在解释题里失分。
Sex is decided by the sex chromosomes, since females are XX and males are XY; every egg carries an X while half the sperm carry X and half carry Y, so fertilisation gives XX or XY in roughly equal numbers and the father's gamete settles the sex of the child. The ABO blood group gene carries three alleles instead of the usual pair, and because two of them are dominant to the third while showing together when both are present, an AB parent crossed with an O parent produces only A and B children in equal numbers, never O or AB. Write the alleles in full, because one phenotype can arise from more than one genotype combination, and sloppy notation loses marks in explanation questions.
门德尔、突变与变异的来源
门德尔用豌豆做了近十年的杂交实验，时间从 1856 年持续到 1863 年，他用大量后代的计数算出比例，1866 年发表结果，当时没有人把他的比例与染色体联系起来，直到 1900 年前后才被重新发现。他成功的关键在于选了对性状、做了大量重复、并把每一代的比例记录下来，这三条在今天做遗传题时同样管用。3.20 把变异分成遗传变异与环境变异，遗传变异来自突变与有性繁殖，环境变异来自生物所处的环境，比如同一种植物在向阳处与背阴处长成不同高度。多数突变对表型没有影响，一部分有轻微影响，极少数一次突变就明显改变表型。整章复习时把定义表、庞尼特方格与血型题各过一遍，比重新抄一遍笔记有效。
Mendel crossed pea plants for nearly a decade, from 1856 to 1863, used counts of large numbers of offspring to work out ratios and published in 1866, yet nobody linked his ratios to chromosomes until his work was rediscovered around 1900; he succeeded by choosing clear-cut features, running many repeats and recording the ratio in every generation. Variation is split into genetic variation, arising from mutation and sexual reproduction, and environmental variation, arising from where an organism lives, such as one plant species growing to different heights in sun and in shade. Most mutations have no effect on the phenotype, some have a small effect and a rare few change it markedly, so a final revision pass over definitions, Punnett squares and blood group crosses pays better than recopying notes.
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