GCSE生物 遗传学 Punnett Square 单基因杂交
遗传计算题是 GCSE 生物考卷的固定题型,而 Punnett Square(庞纳特方格)是解这类题的核心工具。无论 AQA、OCR 还是 Edexcel,单基因杂交的比例与概率题都会在 Paper 1 和 Paper 2 中轮流出现,单题通常值 4 到 6 分,掌握画格与读格的方法就能稳定拿分。本文用中英双语把方格法拆成四步,配合显隐性、伴性遗传和 ABO 血型三类高频例子,并附谱系图反推基因型的方法,帮你一次过关。
Inheritance questions appear on every GCSE Biology paper, and the Punnett square is the essential tool for solving them. Whether you sit AQA, OCR or Edexcel, monohybrid ratio and probability questions rotate between Paper 1 and Paper 2, and each is worth 4 to 6 marks. This bilingual guide breaks the grid method into four steps, works through dominant and recessive alleles, sex-linked inheritance and ABO blood groups, and finishes with pedigree analysis, so you can bank these marks every time.
1. 孟德尔遗传与核心术语
孟德尔用豌豆实验发现,生物的每个性状由一对等位基因控制,一个来自父方,一个来自母方。基因型(genotype)是这对等位基因的组合,表型(phenotype)是实际表现出的性状。显性等位基因(dominant allele)用大写字母表示,只要存在就会表现;隐性等位基因(recessive allele)用小写字母表示,只有两份同时存在才表现。纯合(homozygous)指两个等位基因相同,如 TT 或 tt;杂合(heterozygous)指两个不同,如 Tt。配子(gamete)通过减数分裂产生,每个配子只携带一对等位基因中的一个。
Mendel's pea experiments revealed that each characteristic is controlled by a pair of alleles, one inherited from each parent. Genotype means the combination of those two alleles; phenotype means the observable feature they produce. A dominant allele, written as a capital letter, shows whenever it is present, while a recessive allele, written in lower case, only shows when two copies are inherited. Homozygous means both alleles are the same, such as TT or tt; heterozygous means they differ, such as Tt. Gametes are formed by meiosis, so each gamete carries only one allele from the pair.
判断显隐性是解题第一步:如果两个正常表型的亲本生出患病的后代,则该病由隐性等位基因控制。这一条规律在后面的谱系题里反复使用。孟德尔本人的数据也验证了 3 : 1:他在 F2 共统计 1064 株豌豆,高茎 787 株、矮茎 277 株,比值约 2.84 : 1,与理论预测高度吻合。
Deciding which allele is dominant is the first step of any question: if two unaffected parents produce an affected child, the condition must be recessive. This single rule unlocks most pedigree questions you will meet. Mendel's own data confirmed the 3:1 ratio: he counted 1064 F2 pea plants, 787 tall and 277 dwarf, a ratio of about 2.84:1 that matches the theoretical prediction closely.
2. Punnett Square 四步画法: 以 Tt × Tt 为例
方格法只需要四个动作。第一步,写出亲本基因型并确认配子:两株高茎豌豆均为 Tt,各自产生两种配子,分别是 T 和 t。第二步,画一个 2 × 2 的方格,父方配子写在顶端,母方配子写在左侧。第三步,把行与列的配子两两合并,填满四个格子。第四步,统计基因型比例与表型比例。
The grid method takes four moves. First, write down the parental genotypes and work out their gametes: two tall pea plants are both Tt, and each produces two types of gamete, T and t. Second, draw a 2 × 2 grid, placing the father's gametes across the top and the mother's down the side. Third, combine the row and column gametes to fill all four boxes. Fourth, count the genotype and phenotype ratios.
Tt × Tt 方格结果: TT, Tt, Tt, tt
基因型比 1 TT : 2 Tt : 1 tt,表型比 3 高茎 : 1 矮茎
矮茎概率 1/4 即 25%;另一高频例:囊性纤维化携带者 Ff × Ff,患病孩子 ff 的概率同为 1/4,携带者 Ff 的概率为 1/2
注意杂合子 Tt 与纯合显性 TT 表型相同,所以表型比是 3 : 1 而不是基因型比 1 : 2 : 1。答题时先问自己:题目要的是基因型比例还是表型比例?另外,画格前要确认亲本基因型能否直接写出:题干出现"纯合"或"true-breeding(纯育种)"字样才能直接确定;只写"高茎"这类表型时,需要结合后代比例反推亲本。
Notice that heterozygous Tt plants look identical to homozygous TT plants, so the phenotype ratio is 3:1 even though the genotype ratio is 1:2:1. Before answering, always ask yourself whether the question wants genotypes or phenotypes. Also check whether the parental genotype can be written directly: only the words homozygous or true-breeding allow that, whereas a phenotype like tall forces you to deduce the genotype from the offspring ratios.
3. 三种单基因杂交的经典比例
把高频组合背熟能省下大量考场时间。第一种,纯合显性 × 纯合隐性,如 TT × tt:所有后代基因型均为 Tt,表型全部为显性,比例直接写 100%。第二种,杂合 × 杂合,如 Tt × Tt:基因型 1 : 2 : 1,表型 3 : 1,这是考得最多的一组。第三种,杂合 × 隐性纯合,如 Tt × tt,称为测交(test cross):后代一半 Tt 一半 tt,表型比 1 : 1。
Memorising the classic crosses saves real time in the exam. First, homozygous dominant × homozygous recessive, such as TT × tt: every offspring is Tt and shows the dominant phenotype, so the answer is simply 100%. Second, heterozygous × heterozygous, such as Tt × Tt: genotype ratio 1:2:1 and phenotype ratio 3:1, the most tested cross of all. Third, heterozygous × homozygous recessive, such as Tt × tt, called a test cross: half the offspring are Tt and half are tt, giving a 1:1 phenotype ratio.
测交有现实用途:育种者无法从表型区分 TT 与 Tt,把未知个体与隐性纯合子 tt 杂交,只要后代中出现任何一株矮茎个体,就能证明被测亲本携带隐性等位基因,基因型必为 Tt。医学上最常用的遗传例子是囊性纤维化:致病等位基因隐性,一对均为携带者 Ff 的父母,生出患病孩子 ff 的概率正是 1/4,这在遗传咨询题中反复出现。
The test cross has a practical role: a breeder cannot tell TT from Tt by appearance, so crossing the unknown individual with a homozygous recessive tt partner reveals the genotype, because any dwarf offspring proves the tested parent carries the recessive allele and must be Tt. The most common medical example in exams is cystic fibrosis: the faulty allele is recessive, so two carrier parents, both Ff, have a 1 in 4 chance of producing an affected child with genotype ff, a result that reappears constantly in genetic counselling questions.
4. 伴性遗传: X 连锁隐性的色盲与血友病
人类有 23 对染色体,最后一对是性染色体。女性为 XX,男性为 XY。红绿色盲和血友病的致病基因位于 X 染色体上,且为隐性,这就是 X 连锁隐性遗传。男性只有一条 X 染色体,只要带上致病等位基因就会患病;女性需要两条 X 都带致病等位基因才患病,因此男性患者远多于女性。书写时用 X^B 表示正常等位基因,X^b 表示色盲等位基因,Y 染色体上不携带该基因。
Humans have 23 pairs of chromosomes, and the last pair is the sex chromosomes: females are XX and males are XY. Red-green colour blindness and haemophilia are caused by recessive alleles carried on the X chromosome, a pattern called X-linked recessive inheritance. A male has only one X chromosome, so a single faulty allele makes him affected; a female needs two faulty copies, which is why affected males vastly outnumber affected females. Write the alleles as X^B for normal and X^b for colour blind, and remember the Y chromosome carries no version of this gene.
携带者母亲 X^B X^b × 正常父亲 X^B Y
女儿: X^B X^B 或 X^B X^b,均不患病,但有一半是携带者
儿子: X^B Y 或 X^b Y,患色盲概率为 1/2
血友病同理:携带者母亲 X^H X^h 与正常男性 X^H Y 的后代中,儿子患病概率 1/2,女儿均正常但半数携带。反向推理也常考:一个患病男孩的母亲必为携带者或患者,因为男孩的 X 染色体一定来自母亲。做这类题最容易漏写 Y 配子,父亲产生的配子一半带 X 一半带 Y,两条都要写进方格,否则儿子的格子会缺行;还要看清题目问的是所有孩子还是只问儿子。
Haemophilia follows the same pattern: a carrier mother X^H X^h and a normal father X^H Y have sons with a 1 in 2 chance of being affected, while daughters are unaffected but half are carriers. The reverse deduction is also examined: an affected boy must have a carrier or affected mother, because his X chromosome always comes from her. The classic mistake in these grids is forgetting the Y gamete, since a father produces X-bearing and Y-bearing gametes in equal numbers and both must appear in the grid. Always read whether the question asks about all children or only the sons.
5. 共显性与 ABO 血型遗传
不是所有等位基因都分显隐。共显性(codominance)指两个等位基因同时表达,例如纯合红色花与纯合白色花杂交,F1 出现红白斑驳的花。ABO 血型更复杂一点:它由 I^A、I^B 和 i 三个等位基因决定,其中 I^A 与 I^B 对 i 都显性,但 I^A 与 I^B 彼此共显性。I^A I^A 或 I^A i 为 A 型血,I^B I^B 或 I^B i 为 B 型血,I^A I^B 为 AB 型血,ii 为 O 型血。
Not every pair of alleles is simply dominant or recessive. Codominance means both alleles are expressed at the same time, such as red and white flowers producing blotched offspring. ABO blood groups go one step further: the trait is controlled by three alleles, I^A, I^B and i. Both I^A and I^B are dominant over i, but I^A and I^B are codominant with each other. Genotypes I^A I^A or I^A i give blood group A, I^B I^B or I^B i give group B, I^A I^B gives group AB, and ii gives group O.
经典设问是:O 型血母亲与 AB 型血父亲的孩子可能是什么血型?母亲只能提供 i 配子,父亲提供 I^A 或 I^B,所以孩子只能是 A 型(I^A i)或 B 型(I^B i),永远不可能出现 O 型或 AB 型。这类三等位基因题,关键是把亲本所有可能的配子列全,再逐格合并。输血常识也常被串进来考:传统上 O 型被称为万能供血者,AB 型为万能受血者,但现代临床一律主张同型输注。
A favourite exam question asks what blood groups are possible when an O group mother has children with an AB group father. The mother can only pass on i, while the father passes on I^A or I^B, so every child is either group A (I^A i) or group B (I^B i); group O and group AB are impossible. For three-allele questions the key is listing every possible gamete from each parent, then combining them box by box. Transfusion facts often appear alongside: traditionally group O is called the universal donor and group AB the universal recipient, although modern practice always uses matching groups.
6. 谱系图与遗传题的高频失分点
谱系图(pedigree)用方块代表男性、圆圈代表女性,是 AQA 的常考呈现方式。读图先判断显隐:双亲正常却生出患病孩子,致病等位基因必为隐性;双亲患病却生出正常孩子,则必为显性。判断染色体位置看性别分布:正常父母生出患病女儿,可排除 X 连锁隐性,因为女孩的两条 X 一条来自父亲,父亲正常就不可能传下致病等位基因,所以这种情况必为常染色体隐性,且父母都是杂合子。
A pedigree uses squares for males and circles for females, and AQA questions love this format. Start by judging dominance: affected children born to unaffected parents mean a recessive condition, while unaffected children born to affected parents mean a dominant allele. Then locate the gene using the sex pattern: an affected daughter born to two unaffected parents rules out X-linked recessive inheritance, because a girl receives one X from her father, and an unaffected father cannot pass on the faulty allele. The condition must therefore be autosomal recessive with two heterozygous parents.
最后盘点五类高频失分点。第一,配子数量写错:每个配子只含一对等位基因中的一个,TT 的配子只能写 T,不能写 TT。第二,把基因型比和表型比混淆:3 : 1 是表型比,1 : 2 : 1 是基因型比,两者不能互换。第三,分数与百分比换算出错:3 : 1 中隐性个体占 1/4,换算成 25% 而非 75%。第四,答非所问:题目用 predict(预测)、explain(解释)还是 state(写出)决定得分方式,要求 explain 时必须给出配子或方格作为依据,不能只写结论。第五,伴性遗传忽略性别限定,问儿子时不要用全部后代的概率作答。
Finally, five classic traps. First, gamete errors: each gamete carries one allele from the pair, so a TT parent produces only T gametes, never TT. Second, do not swap the two ratios: 3:1 is the phenotype ratio and 1:2:1 is the genotype ratio. Third, convert fractions carefully, since the recessive individual in a 3:1 cross is 1/4, which is 25%, not 75%. Fourth, match the command word: predict, explain and state are marked differently, and an explain question requires the gametes or the grid as evidence, not a bare conclusion. Fifth, respect the sex filter in X-linked questions, and do not use the probability for all children when only sons are asked about.
作答时一定要把亲本基因型、配子和方格全部写在答题纸上。GCSE 阅卷按步骤给分,即使最终比例算错,画对亲本与配子也能拿到过程分。把本文三个方格实例和谱系图判断各练两遍,遗传题就从易错题变成送分题。
Always write the parental genotypes, the gametes and the full grid on your answer paper. GCSE mark schemes award method marks, so even a wrong final ratio still earns credit for a correct cross and correct gametes. Practise the three grid examples and the pedigree logic in this guide twice each, and inheritance questions will move from your error list to your gift list.
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