Edexcel IGCSE生物 等位基因 单基因杂交 概率
遗传学是 Edexcel IGCSE Biology (4BI1) Section 3b 里分值稳定、题型固定的一块。考纲 3.19 到 3.27 把范围划得很清楚：等位基因、显隐性、单基因杂交、家族系谱图、性别决定，压轴的多是配遗传图解的 4 到 6 分题。这篇按考点一个个拆，写到能直接照着落笔的程度，也把常见问法和容易掉分的写法一并列出。
Genetics in the Edexcel IGCSE Biology (4BI1) specification is narrow in scope and generous in marks: statements 3.19 to 3.27 cover alleles, dominance, monohybrid crosses, pedigrees and sex determination. Most marks sit in structured questions that ask for a genetic diagram and then for a probability read off that diagram. The sections below follow the statement order and note the wording examiners expect.
基因、等位基因与染色体的成对关系
考纲 3.19 把基因定义为 DNA 分子上的一段，把等位基因定义为同一基因的不同版本。体细胞里染色体成对存在，一对同源染色体在相同位置各带一个等位基因，所以每个基因在体细胞中有两份，到了配子里只剩一份。基因通过编码蛋白质起作用，酶、膜上的载体蛋白、结构蛋白都由基因决定，性状差异最后都落在蛋白质的种类或数量上。很多学生以为基因直接对应某个外貌特征，其实中间隔着蛋白质、细胞活动与发育环境。答题时按「基因编码蛋白质，蛋白质影响性状」的顺序补一句，链条才完整。
A gene is a length of DNA and an allele is one of its alternative forms (3.19), so with chromosomes in pairs each body cell carries two copies of every gene, one per homologue, while a gamete carries one. The link from gene to phenotype runs through protein: enzymes, membrane carriers and structural fibres all depend on the sequence, so inherited differences trace back to those proteins. A frequent slip is to answer with the word gene where the question asks about an allele.
显性、隐性、纯合、杂合与表现型
3.20 要求准确使用六个术语。显性等位基因在杂合状态下也能表现，隐性等位基因要两份都是隐性才表现。基因型是等位基因的组合，通常用同一字母的大写和小写表示，例如 BB、Bb、bb；表现型是能观察到的特征，例如种子圆粒或皱粒。纯合指两份等位基因相同，杂合指两份不同。常见的错误是把 Bb 说成「显性基因型」，或者把表现型写成「高茎的基因」。遇到要求用术语解释的题，给定义加一句实例，比展开描述更稳。
Six terms carry marks here: dominant, recessive, homozygous, heterozygous, genotype and phenotype (3.20). A dominant allele shows in the heterozygote, whereas a recessive allele shows only when both copies are recessive. Genotype is the allele pair written with one letter in upper and lower case, such as BB, Bb and bb, and phenotype is the observable characteristic. Homozygous means the two alleles match and heterozygous means they differ, so pair every definition with a short example from a cross.
单基因杂交的遗传图解怎么写
3.23 要求用遗传图解描述单基因杂交，标准写法分四层。第一层写亲本表现型与基因型，例如高茎 TT 与矮茎 tt；第二层写各自产生的配子，T 与 t；第三层在棋盘格里把配子两两组合，得到子代基因型；第四层写子代表现型与比例。表格外侧必须标出配子，格内写基因型，比例写在图解下方并注明是基因型比例还是表现型比例。阅卷时看到只画格子、不标配子的答案，一般扣一分。最后补一句结论，例如子代全为 Tt，表现型全为高茎。
A genetic diagram follows a fixed layout: parental phenotypes and genotypes, the gametes each parent makes, the offspring genotypes inside the grid, and the ratio underneath, with the gametes labelled outside the grid and the ratio marked as genotype or phenotype. Answers that skip the gametes lose a mark, since they are the evidence that meiosis has been understood. Finish with a short conclusion such as all offspring are Tt and show the dominant phenotype.
从杂交结果算概率
3.25 要求从单基因杂交预测结果概率。两个杂合子杂交，子代基因型比例是 1 AA 比 2 Aa 比 1 aa，表现型比例是 3 显性比 1 隐性。杂合子与隐性纯合子杂交，子代是 1 Aa 比 1 aa，两种表现型各占一半。每次受精独立发生，所以问「两个孩子都是隐性」时要乘两次四分之一，得十六分之一。很多学生把 1 比 2 比 1 直接当成概率写成 1、2、1，或者把 3 比 1 说成「三个高的一个矮的」，这两种写法都不给分。比例用冒号，概率用分数，两者不要混着用。
Probabilities come straight off the grid (3.25). Two heterozygotes give 1 AA to 2 Aa to 1 aa in genotype, which is 3 dominant to 1 recessive in phenotype, while a heterozygote crossed with a homozygous recessive gives 1 Aa to 1 aa. Each fertilisation is independent, so the chance of two recessive children is one quarter multiplied by one quarter. Marks disappear when a ratio is written where a probability is wanted, or when a ratio is quoted in words instead of numbers.
判断显性个体的基因型：测交
题目常给一个表现型为显性的个体，问基因型是 AA 还是 Aa。做法是让它与隐性纯合子杂交，看子代有没有隐性表现型。子代全是显性，说明它只产生显性配子，基因型为 AA；子代出现隐性个体，说明它也能产生隐性配子，基因型为 Aa，比例接近 1 比 1。判断要考虑样本量，子代数量少时不能凭一只个体下结论。这一步最容易犯的错是拿它去和另一个显性个体杂交：出现隐性子代只能说明双方都是杂合子，结论比测交弱得多。
A dominant phenotype with an unknown genotype is settled by a test cross against a homozygous recessive. Any recessive offspring prove that the unknown parent carries a recessive allele and is Aa, and the expected ratio is close to 1 to 1; if every offspring is dominant, the parent is most likely AA. Sample size matters here, so one individual is thin evidence, and crossing with another dominant individual is the weaker design because a recessive child only proves that both parents are heterozygous.
家族系谱图的推断
3.24 要求读懂家族系谱图。先找隐性表现型的个体：两个表现正常的父母生出患病子女，说明致病等位基因是隐性，并且父母都是携带者，这一步同时确定了显隐性的方向。再从患病个体往回推，它的两个等位基因分别来自父母，所以每个表现正常的父母至少带一个隐性等位基因。继续往下推子代时，把每个个体的可能基因型写全，例如 Aa 或 AA，并在旁边标出概率。常见的错误是只写「可能是」而不给概率，或者把性别当成判断基因型的依据。
Pedigrees are solved from the recessive individual outwards (3.24). Two unaffected parents with an affected child prove that the allele is recessive and that both parents are carriers, which fixes the direction of dominance in one step. From an affected individual, each unaffected parent must carry one copy of the allele, so list the possible genotypes for everyone else with a probability attached, since an answer that stops at it could be a carrier rarely earns the mark.
性别决定 XX 与 XY
3.26 与 3.27 讲性别决定。人类第二十三对染色体在女性是 XX，在男性是 XY，母亲的卵子都含 X，父亲的精子一半含 X 一半含 Y。受精时精子带 X 则子代为 XX，带 Y 则为 XY，所以决定性别的是父方提供的性染色体。画遗传图解时把亲本写成 XX 与 XY，配子一侧写 X 与 X，另一侧写 X 与 Y，子代得到 XX 与 XY 各占一半。常见的错误是把比例写成 3 比 1，或者把提供 X 的一方说成决定性别的一方。
Sex is set by one chromosome pair: XX in females, XY in males (3.26, 3.27). Eggs carry an X, while half the sperm carry an X and half a Y, so the sex of the offspring depends on which sperm fertilises the egg. In the diagram the parents are XX and XY, the gametes are X and X against X and Y, and the offspring come out half XX and half XY, a ratio of 1 to 1, so quoting 3 to 1 means a monohybrid ratio has been carried over by mistake.
共显性与 ABO 血型
3.21B 带 B 标记，只在 Paper 2B 出现。共显性指两个等位基因同时表现出来，谁也不掩盖谁。ABO 血型是标准例子：I A 与 I B 共显性，基因型 I A I B 表现为 AB 型，而这两个等位基因对 i 都是显性，所以 A 型可以是 I A I A 或 I A i。做这类题先写等位基因符号体系，再画棋盘格，别把 I A 与 I B 当成上下位关系。上标在手机上显示不佳，答题时正常写上标即可。
Codominance sits under statement 3.21B and is examined on Paper 2B only: both alleles are expressed in the heterozygote, so neither masks the other. The ABO system is the standard case: the A and B alleles are codominant, a genotype carrying both gives blood group AB, and both are dominant to the O allele, which is why group A can be AA or AO. Write the symbols first, then complete the grid, and never rank A above B.
多基因遗传与连续变异
3.22 说明多数性状由多个基因共同决定，而不是一对基因。身高、体重、肤色都属于这一类，每个基因贡献一点，环境再叠加一部分，结果就是连续变异：一群人从低到高排开，中间多、两头少。与之相对的是不连续变异，例如 ABO 血型或豌豆种子的圆与皱，只有几个明确类别。题目给的如果是连续分布的数据，就不要套单基因的 3 比 1；反过来，给出 3 比 1 或 1 比 1 的题，一般就是单基因杂交。
Most characteristics are polygenic, with several genes each adding a small effect (3.22). Height, body mass and skin colour behave this way, and the environment adds a further contribution, so the population shows a continuous distribution with intermediate values common and extremes rare. Discontinuous variation instead puts individuals into a few clear categories such as ABO blood groups or round and wrinkled seeds, so a smooth distribution means a single gene ratio is the wrong model.
环境如何影响表现型
基因型相同的个体，表现型可以不同。同卵双胞胎的基因型完全一致，成年后的身高、体重、某些疾病的发病情况却会分开，差别来自营养、运动、睡眠和暴露过的病原体。植物里的例子更直观：同一批基因型相同的种子，种在光照充足的窗台和阴暗的抽屉里，长出来的叶片宽度与颜色完全不同。写这类题要落到机制上，环境通过影响酶的活性、激素水平和细胞分裂的速度改变性状，而不是改变 DNA 的序列。常见的错误是把「环境改变了基因」写成答案，基因序列没有变，变的是基因被读取和发挥作用的程度。
Identical genotypes can produce different phenotypes: monozygotic twins share every allele yet diverge in adult height, body mass and the age at which some conditions appear, because nutrition, activity, sleep and exposure to pathogens differ. A plant example makes the point quickly, since seeds of one genotype raised on a bright windowsill and in a dark drawer develop leaves of different width and colour, and the mechanism runs through enzyme activity, hormone levels and the rate of cell division rather than a change in the DNA sequence. Writing that the environment changed the gene is the error to avoid.
常见失分点：符号、比例与表述
遗传题的失分点集中且重复。等位基因符号全篇统一，显性用大写、隐性用小写，同一个字母不能中途换掉；基因型写成 BB、Bb、bb，不写成 B 比 B 或 b b。比例与概率分开表述，比例用冒号，概率用分数。表现型描述要具体，写「高茎」而不是「显性」。题目要求解释时，把推理过程写出来，只给结论通常只拿一半分。阅卷时我发现，同一道 4 分题里丢掉的往往不是生物知识，而是符号混用和漏写配子。
Genetics marks are lost in predictable places. Keep one letter per gene throughout, capital for the dominant allele and lower case for the recessive one, and write genotypes as BB, Bb or bb. Separate ratio from probability, using colons for one and fractions for the other, and describe the phenotype by naming the characteristic rather than the word dominant. When the command word is explain, the reasoning has to appear on the page, because a bare conclusion usually earns half marks.
4 到 6 分遗传图解题的落笔顺序
考场上遇到 4 到 6 分的遗传图解题，按固定顺序落笔能省下不少时间。先把题目给的亲本表现型翻译成基因型，再在纸边写出两个亲本各自能产生的配子，接着画棋盘格填子代，最后读格子写基因型比例与表现型比例，回答题目真正问的那一问。写完格子回头核对一遍亲本基因型，多数笔误在这一步就能拦住。这类题的小问通常各自独立，前一问定义写错不影响后面画图的分。学生最容易跳过配子直接写子代，格子里的组合一旦不符合减数分裂，后面的比例和概率跟着出错。
Under timed conditions a fixed order saves marks: translate the parental phenotypes into genotypes, write out the gametes each parent can produce, complete the grid, then read the genotype and phenotype ratios off it and answer the question that was actually asked. The parts of such a question usually stand alone, so a loose definition in part (a) does not carry into the diagram in part (b). Skipping the gametes is what unravels the rest, because the grid then holds combinations that meiosis could not have produced.
卷面结构：Paper 1B 与 Paper 2B 怎么分配时间
4BI1 是线性资格，两份外部笔试。Paper 1B 两小时 110 分，占总分 61.1%，只考不带 B 标记的核心内容；Paper 2B 一小时十五分钟 70 分，占 38.9%，全部内容都可能出现，包括 3.21B 共显性这类带 B 标记的加深内容。两卷合计 180 分，按 9 到 1 的等级评定，都不分层。时间上，110 分的卷子平均每分略多于一分钟，遗传图解题要多留几分钟画格子和标配子。以上结构与分值取自 Pearson Edexcel 官网 4BI1 考纲，现行版本是 2024 年 9 月的 Issue 3，2017 年 9 月首次教学，2019 年 6 月首次考试。
The qualification is linear and assessed by two external papers. Paper 1B runs for two hours, carries 110 marks and 61.1% of the total, and tests only the core statements that carry no B reference, while Paper 2B runs for one hour fifteen minutes, carries 70 marks and 38.9%, and can draw on everything, including B statements such as codominance in 3.21B. The two papers add to 180 marks with grades from 9 to 1 and no tiering, and the structure and weightings follow the Pearson Edexcel 4BI1 specification, currently Issue 3 (September 2024), first taught from September 2017 and first examined in June 2019.
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