GCSE生物 遗传学 Punnett方格 测交 3比1比例
遗传学在 GCSE 生物里分值不低，题目却常常只给一张空格子和一句话。Punnett 方格本身不难，难在动笔前的几个判断：显性还是隐性，纯合还是杂合，比例还是概率。这篇把 AQA 与 Edexcel 两条考纲里的单基因杂交要求拆成十二步，每一格都落到能拿分的那句话上。
Genetics carries a decent share of the marks in GCSE Biology, yet the questions often give little more than an empty grid and a single sentence. The Punnett square itself is easy; the difficulty sits in the decisions made before drawing it, such as dominant against recessive, homozygous against heterozygous, and ratio against probability. The twelve steps below follow the monohybrid inheritance requirements in the AQA and Edexcel specifications and end each step on the sentence that earns the mark.
等位基因、基因型与表型这三个词的区别
同一段 DNA 上的同一位置可以有好几种写法，每一种写法就是一个等位基因。两个等位基因放在一起，构成一个人的基因型，比如 AA、Aa、aa。肉眼或化验能看到的那部分特征叫表型，卷子上问表型就是问花是什么颜色、耳垂是不是分离。等位基因里有一个会盖住另一个，大写字母代表显性，小写代表隐性。很多学生以为显性基因更常见，其实显性只表示它只要出现一份就能被看到，跟出现频率没有关系。隐性等位基因要两份都到齐才会表型出来。这三个词如果混着用，后面画格子时字母就会写错。
Every gene has a fixed spot on a chromosome, and the versions of the DNA sequence that can sit there are called alleles. The pair a person carries is the genotype, while the phenotype is whatever can actually be observed, such as flower colour or detached earlobes. A dominant allele shows up with a single copy, which is why it is written in capitals, and a recessive one only shows when both copies match. Students often assume dominant means common, yet the word says nothing about how often the allele appears in a population.
Punnett方格的四格是怎么来的
画格子之前先把双亲的基因型拆成配子，这是唯一需要动脑的一步。减数分裂时两条同源染色体分开，所以 Aa 会产生 A 和 a 两种配子，比例一半一半。把一方的配子写在格子上方，另一方的配子写在左侧，四个格子就是所有可能的组合。每一格取上方一个字母、左侧一个字母，习惯上把大写写在前面。四格代表概率空间，不代表这家真的生四个孩子。常见的错误是漏写配子、直接在格子里凑基因型，结果自己的推理断了线。
Before any grid is drawn, each parent's genotype has to be split into gametes, and that is the only step that requires real thought. During meiosis the two homologous chromosomes separate, so a heterozygote such as Aa produces A and a gametes in equal numbers. One parent's gametes head the columns and the other's run down the rows, giving four boxes that cover every possible combination. Each box takes one letter from its column and one from its row, capitals first by convention; the four boxes describe a probability space rather than four real children.
纯合杂合与字母怎么选
两个等位基因相同叫纯合，AA 和 aa 都算纯合，两个不同叫杂合，只有 Aa 一种。选字母时挑大小写形状差得远的，Aa、Bb、Dd 都可以，Cc、Oo、Ss、Xx 容易看错。同一个字母不能既当显性又当隐性，换题目就换字母没关系，但一份答案里不要换。题目给出亲本表型而不给基因型时，显性个体有两种可能，隐性个体只有一种，隐性那边通常直接写成 aa。这一步写清楚，后面四格填起来就是机械动作。
Two identical alleles make an individual homozygous, which covers both AA and aa, while two different alleles make it heterozygous, and Aa is the only such case. When choosing a letter, pick one whose capital and lower case look quite different, so Aa, Bb and Dd work well and Cc, Oo, Ss or Xx invite misreading. The same letter cannot stand for both a dominant and a recessive allele, and although the letter may change between questions it should stay fixed within one answer. Where a question gives phenotypes instead of genotypes, a recessive individual has only one possible genotype, so that side is usually written as aa straight away.
单基因杂交的四种结果与三比一
Aa 与 Aa 相交是最常考的一种，四格填出来是 AA、Aa、Aa、aa。按基因型算比例是一比二比一，按表型算就是三比一，显性三份、隐性一份。三比一不是必然结果，它只是每一次受精的期望分布，四个孩子里真的出现三个显性一个隐性才会让人意外。AA 与 aa 相交，子代全部是 Aa，表型全为显性，比例谈不上。AA 与 Aa 相交是显性全占，基因型一比一。Aa 与 aa 相交是表型一比一，也是测交的基础。写比例时把基因型和表型分开写，阅卷时两者各自给分。
Crossing two heterozygotes is the version examiners use most, and the four boxes read AA, Aa, Aa and aa. Genotypes therefore come out as one to two to one, while phenotypes give the familiar three to one split of dominant to recessive. That ratio is an expectation for each fertilisation rather than a guarantee, so a family of four is not obliged to produce three and one. A cross between AA and aa gives only Aa offspring, all dominant, and Aa with aa gives equal numbers of the two phenotypes.
把比例换算成概率与百分数
四格里每一格占四分之一，也就是百分之二十五。三比一换成百分数是百分之七十五对百分之二十五，换成小数是 0.75 和 0.25。题目问某个孩子是隐性纯合的概率，直接数格子，不要连着乘。两个孩子都患病的概率才需要相乘，四分之一乘四分之一等于十六分之一。很多学生以为比例会自己平均到每个家庭，实际上每次受精互不影响，前一个孩子的结果不会改变下一次。分数、百分数、小数三种写法在卷面上都算对，先看清题目要哪一种。
Each of the four boxes carries a probability of one quarter, or 25 per cent. The three to one ratio becomes 75 per cent against 25 per cent, or 0.75 against 0.25 in decimals. When a question asks for the chance that one child shows a recessive phenotype, read it straight off the grid instead of multiplying anything. Multiplication only comes in for two children at once, where a quarter times a quarter gives one sixteenth. A common mistake is to assume the ratio will average out inside one family, yet each fertilisation is independent of the last.
测交如何确定显性个体的基因型
显性表型的个体可能是 AA，也可能是 Aa，光看外表分不出来。测交就是把它与隐性纯合个体相交，看子代有没有出现隐性表型。子代全是显性，亲本大概率是 AA；子代里出现隐性，亲本一定是 Aa。用格子验证一遍就知道为什么，aa 只能提供 a，能不能凑出 aa 全看未知亲本。基因型不确定时，测交比自交更省时间，也更适合用在动物上。这一步在卷面上要写出推理过程，只写结论不给分。
An individual showing the dominant phenotype may be AA or Aa, and the appearance alone cannot tell them apart. A test cross mates that individual with a homozygous recessive partner and looks for recessive offspring. If every offspring shows the dominant trait the unknown parent is probably AA, whereas a single recessive offspring proves it carried a. Running the grid makes the logic obvious, since the recessive parent can only donate a, and the exam mark depends on showing that reasoning rather than stating a conclusion.
性别决定的XX与XY一比一
人类有二十三对染色体，其中一对决定性别。女性是 XX，男性是 XY，卵子只带 X，精子带 X 或 Y。XX 与 XY 画成格子是两格 XX、两格 XY，比例一比一。生男生女由父亲那一个精子决定，这一点常被误会成母亲决定。Y 染色体上带 SRY 基因，它启动睾丸发育。题目问某个家庭连生两个女孩之后下一个是男孩的概率，答案仍然是二分之一。
Of the twenty three pairs of human chromosomes, one pair determines sex. Females are XX and males are XY, so every egg carries an X while sperm carry either X or Y. A grid of XX against XY gives two XX boxes and two XY boxes, a one to one ratio. The father's sperm therefore decides the sex of a child, and the SRY gene on the Y chromosome triggers testis development; having two daughters already does not raise the chance of a son, which stays at one half.
隐性遗传病与携带者囊性纤维化
囊性纤维化由隐性等位基因引起，患者基因型是 ff。父母双方都是 Ff 时，孩子有四分之一概率患病，四分之二成为携带者，四分之一完全正常。携带者表型正常，却能把 f 传下去，所以这类病可以在家族里静默传递好几代。近亲结婚提高两个携带者相遇的机会，这也是遗传咨询会问家族史的原因。CFTR 基因的突变影响氯离子通道，黏液变稠堵住气道和胰腺导管。写答案时要把携带者与患者的基因型分开写清楚。
Cystic fibrosis is caused by a recessive allele, so an affected child has the genotype ff. With two carrier parents, one quarter of children are expected to be affected, half are carriers and one quarter are free of the allele. Carriers look healthy yet still pass f on, which is why the condition can travel silently through several generations. Mutations in the CFTR gene disrupt a chloride channel, thickening mucus in the airways and pancreatic ducts, and a family history question usually aims at exactly this hidden carrier route.
显性遗传病多指症与亨廷顿
多指症由显性等位基因引起，只要带一份就会表现。患者常见基因型是 Dd，与 dd 相交时子代一半患病一半正常。显性遗传病不会隔代，患者通常每一代都有人，家系图上是竖着连下来的。两个患者都是 Dd 时仍有四分之一概率生出 dd 的正常孩子。亨廷顿舞蹈症同样按显性遗传，症状多在中年以后出现，所以患者在生育之后才知道自己带病。题目问某个患者的孩子患病概率，落笔前先确定这个患者的基因型。
Polydactyly is caused by a dominant allele, so one copy is enough for the extra digit to appear. An affected parent is usually Dd, and a cross with dd gives half affected and half unaffected children. Dominant conditions do not skip generations, so an affected person normally appears in every generation of a pedigree. Two affected parents who are both Dd can still produce a dd child, and Huntington's disease follows the same pattern, with symptoms appearing in midlife, often after children are born.
系谱图的读法
系谱图里方块是男性，圆圈是女性，涂色表示患病。两个都不患病的父母生出一个患病孩子，这个病一定是隐性，父母双方都是携带者。反过来，患病父亲的所有女儿都患病，要考虑 X 连锁显性。判断显隐性先找有没有无病变有病的一对，这是最快的一条线索。系谱图上不写基因型，答题时要自己补上，并且保证上下代之间不矛盾。画到第三代时，按每一对父母单独推，不要把所有格子混在一起。
In a pedigree, squares are male and circles are female, with shading marking affected individuals. Two unaffected parents who produce an affected child prove the condition is recessive and that both parents are carriers. Where an affected father passes the trait to every daughter, X linked dominance becomes the better explanation. The fastest route is to hunt for an unaffected couple with an affected child, then assign genotypes generation by generation so that no parent ends up with an impossible pair.
常见错误与阅卷扣分点
字母混用是最常见的失分点，一会儿 Aa 一会儿 Bb，格子里的组合就对不上。配子写错也很多，Aa 被写成 A 和 Aa 两种配子。比例和概率混着答，题目要 3:1 却写成百分之七十五。表型描述太笼统，只写显性隐性，没有写具体特征。有些答案把四格的结果当成四个真实后代的预测，忽略了样本大小。阅卷时这些细节各自扣分，思路对但表述错也拿不满。
Mixing letters is the most frequent loss, since switching between Aa and Bb halfway through breaks the grid. Gametes are often written wrongly too, with a heterozygote listed as A and Aa instead of A and a. Answers also blur ratio and probability, offering 3:1 where the question asked for a percentage. Phenotypes get described as dominant or recessive without naming the actual feature, and some candidates treat four boxes as four real offspring. Each of these slips is marked separately, so a correct idea in the wrong format still costs marks.
概率是期望而不是保证
三比一描述的是大量重复受精之后的走向，不是每一窝幼崽的固定分配。样本越小，偏离越大，四个孩子里出现四个显性并不罕见。卡方检验在 A-Level 才正式处理这件事，GCSE 阶段只需要知道小样本可以偏离期望。育种上用大样本估算比例，就是为了让观察值靠近理论值。实验数据与理论比例对不上时，先检查样本量，再怀疑遗传假设。回答预测类题目时，写概率而不是写一定。
A three to one ratio describes what happens over many fertilisations rather than the fixed share in one litter. Small samples drift further from the expectation, so four dominant offspring in a row is not surprising at all. Chi squared tests arrive at A-Level, and at GCSE it is enough to know that small samples can deviate. Breeders use large numbers precisely to pull observed counts towards the theoretical ratio, and prediction questions want a probability rather than a certainty.
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