OCR GCSE生物学 等位基因 单基因杂交 概率
OCR Gateway Biology A 的 B5 单元叫 Genes, inheritance and selection，考纲把它拆成 Inheritance 与 Natural selection and evolution 两部分。遗传题在卷面上很少靠复杂计算区分学生，丢分集中在术语用错和读题太快。下面按 B5.1 的学习目标逐条过一遍，每个知识点配一个常见的错法。
Topic B5 of OCR Gateway Biology A is titled Genes, inheritance and selection, and the specification splits it into Inheritance and Natural selection and evolution. Genetics questions rarely separate candidates by hard arithmetic; most lost marks come from terminology and from reading the question too quickly.
基因、染色体与基因组是三层包含关系
先澄清三个词。染色体是 DNA 缠在蛋白质上形成的一条结构，人的体细胞里有 46 条，两两配对。基因是染色体上一小段 DNA，通常决定一种蛋白质的结构，进而影响某个性状。基因组是这套遗传物质的全部，包含所有基因和不编码蛋白质的片段。学生常把三个词倒着用，写出「基因组成染色体」这种句子，逻辑一崩后面全错。作答前先在心里排好顺序：基因组最大，染色体在中间，基因最小，配子只带其中一套。
Start with the three words. A chromosome is a single DNA molecule wound around proteins, and a human body cell holds 46 of them in matching pairs. A gene is a short stretch of that DNA which usually determines the structure of one protein and therefore influences a characteristic. The genome is everything carried by the organism, all the genes plus the stretches that code for nothing; candidates who reverse the order collapse the rest of the answer.
显性等位基因不等于压制隐性等位基因
等位基因是同一个基因的不同版本，一个人每个基因有两个拷贝，一个来自母亲一个来自父亲。显性等位基因只要有一个就表现出对应的性状，隐性等位基因要两个都是隐性才表现。OCR 的考纲点出了一个常见误解，显性并不会把隐性压住，隐性也不是显性的缺失。真实情形是这对等位基因各自编码的蛋白质在细胞里怎样起作用，表现型只是被观察到的结果。答题时写显性等位基因编码的蛋白质有功能、隐性等位基因编码的没有功能，比写「A 压制 a」更贴近得分点。
Alleles are different versions of the same gene, and each person carries two copies, one from each parent. A dominant allele produces its characteristic when only one copy is present, while a recessive allele needs two copies to show. The OCR specification flags a specific misunderstanding here, namely that dominance is an observed pattern in phenotypes rather than one allele silencing another; describing what each allele makes is far safer than describing dominance as suppression.
基因型与表现型：字母怎么写才不丢分
基因型是等位基因的组合，表现型是看得见的性状。写基因型时同一个基因用同一个字母，显性用大写、隐性用小写，纯合是 BB 或 bb，杂合是 Bb。字母要挑大小写形状差得远的，A/a、B/b、D/d 都行，C/c、S/s、O/o、X/x 手写容易糊成一片。表现型由基因型和环境共同决定，同一基因型在不同环境下未必长得一样，这一点写在考纲的 B5.1c 里。常见的错误是把基因型直接当成表现型，题目问表现型比例，答案却写成 1 BB 比 2 Bb 比 1 bb，而题目要的是 3 比 1。
The genotype is the combination of alleles and the phenotype is what you can actually see. Use one letter per gene, capital for the dominant version and lower case for the recessive one, so homozygotes read BB or bb and heterozygotes read Bb. Pick letters whose two cases look clearly different, because C and c or S and s blur together in handwriting. The specification in B5.1c adds that phenotype depends on the genome interacting with the environment, so identical genotypes need not look identical.
单基因杂交与 Punnett 方格：3 比 1 从哪里来
单基因杂交只跟踪一个基因。把父母双方的配子写在方格两侧，格子里的组合就是子代可能的基因型，这种格子叫 Punnett 方格。两个杂合个体相交，Rr 乘 Rr，四格得到 RR、Rr、Rr、rr，基因型比例是 1 比 2 比 1，表现型比例是 3 比 1。这个 3 比 1 只对显性完全的情况成立，题干一旦说明是不完全显性或者共显性，比例就变了，得按方格重新数。画方格时先把配子拆开写在边框上，再逐个填格子，最后照题意回答比例或者概率。
A monohybrid cross tracks a single gene. Write each parent's gametes along the edges of a grid, and every cell inside shows one possible genotype for the offspring. Crossing two heterozygotes gives RR, Rr, Rr and rr, so the genotype ratio is 1 to 2 to 1 while the phenotype ratio is 3 to 1. That 3 to 1 holds only for complete dominance; once a question mentions incomplete dominance or codominance, count the grid again rather than repeating a memorised ratio.
从方格到概率和预期个体数
概率问题是把方格翻译成数字。四格里有一格是隐性纯合，概率就是 1/4，也就是 25%，两百个后代里预期五十个，这是数学期望而不是保证。每一次受精都是独立事件，前面连续出现三个隐性个体，下一胎的概率仍然是 1/4。很多学生以为「前面出多了就该轮到显性」，这类答案一分不给。题干如果说实际观察到 40 个隐性、期望是 50 个，就该讨论样本大小与随机波动，而不是断言实验失败。
Probability questions translate the grid into numbers. One recessive homozygote in four cells means a probability of one in four, so 200 offspring give an expectation of 50 rather than a promise. Each fertilisation is independent, which means three recessive offspring in a row leave the next probability unchanged at one in four. Candidates who argue that a dominant offspring is now due earn nothing for that line, however confident the reasoning sounds.
单倍体与二倍体：配子为什么要先减半
二倍体细胞的染色体成对，写成 2n，单倍体细胞只有一套，写成 n。人的体细胞是 46 条，配子是 23 条。精子与卵细胞结合时两个单倍体合成一个二倍体，染色体数回到 46，每一代都稳定在这个数目上。如果配子不减半，受精之后染色体数会一代一代翻倍，这个推论是理解减数分裂的入口。答题时看到配子、合子这类词，先把 n 与 2n 标在草稿上，后面算染色体数就不容易乱。
Diploid cells carry chromosomes in pairs and are written 2n, while haploid cells carry a single set and are written n. A human body cell has 46 chromosomes and a gamete has 23. Fusion of a sperm and an egg returns the diploid number, which keeps the count stable from one generation to the next; without the halving step it would double at every fertilisation.
减数分裂产生的配子为什么互不相同
减数分裂连续分裂两次，染色体只复制一次，结果是四个单倍体细胞。第一次分裂时同源染色体分开，每一对里的两条走向哪一极是随机的，人的 23 对染色体因此能组合出极多的配子类型。同源染色体之间还会发生片段交换，把等位基因重新搭配，这是第二个变异来源。有丝分裂不同，它保留染色体数目，产生的两个细胞与母细胞一致。题目问为什么兄弟姐妹长得不一样，答案要落在配子的随机组合与交换上，只写「基因不同」拿不到分。
Meiosis runs two divisions with only one round of chromosome replication, ending in four haploid cells. In the first division the homologous partners separate, and which one travels to which pole is a matter of chance, so 23 pairs give an enormous number of possible gametes; crossing over between partners reshuffles the alleles further. Mitosis, by contrast, keeps the chromosome number and produces cells identical to the parent cell, which is why it cannot generate this variety.
性别决定：XX 与 XY 的方格
人的第 23 对染色体是性染色体，女性是 XX，男性是 XY。母亲的卵细胞都带 X，父亲的精子一半带 X 一半带 Y。方格填出来是两格 XX、两格 XY，所以每次生育男女各占一半，孩子的性别由父亲的精子决定。这个结论常被题干绕成「母亲决定性别」来设陷阱，看清题目问的是谁提供哪条染色体。Y 染色体很短，携带的基因少，X 上的基因在男性身上只有一个拷贝，伴性遗传的题后面要用到这一点。写方格时把 X 与 Y 当作等位基因处理，格式与单基因杂交完全一样。
The twenty-third pair of human chromosomes carries the sex chromosomes: females have XX and males have XY. Every egg carries an X, whereas half the sperm carry X and half carry Y, so a filled grid gives two XX cells and two XY cells. The father therefore determines the sex of the child; the short Y chromosome carries far fewer genes than the X, which matters once sex-linked conditions appear in a question.
突变、变异与编码区非编码区
所有变异最终都来自突变，突变是 DNA 序列的改变，可能在复制时发生，也可能由辐射或某些化学物质引起。多数突变不改变表现型，因为遗传密码有简并性，或者改变落在不编码的区域；一部分会改变表现型；极少数直接决定表现型。高阶卷 B5.1e 要求区分两种情形，编码区的变异改变蛋白质的结构，可能落在酶的活性位点，非编码区的变异影响基因表达，例如让转录停下来。答题时先判断变异落在哪一类 DNA，再谈蛋白质结构还是表达量，顺序反了容易自相矛盾。突变本身没有方向，说它为了适应环境才发生是错的。
Every variant ultimately comes from a mutation, meaning a change in the DNA sequence that may arise during replication or follow exposure to radiation or certain chemicals. Most mutations leave the phenotype untouched, some alter it, and very few determine it outright. At higher tier, B5.1e asks you to separate two routes: a variant in coding DNA changes protein structure, possibly at an active site, while one in non-coding DNA alters expression by affecting transcription.
连续变异、不连续变异与多基因性状
不连续变异只有几个清楚的类别，比如血型、豌豆的圆粒与皱粒，画成柱状图是几根独立的柱子。连续变异在一个范围内分布，比如身高、体重，画成直方图是接近钟形的曲线。连续变异的性状通常由多个基因共同决定，环境也有影响，所以同一对父母的孩子身高各不相同。OCR 的 B5.1l 明确要求知道大多数性状是多基因的，不是单基因。学生最常把身高当成单基因性状，然后拿 3 比 1 去算比例，方向从一开始就错了。
Discontinuous variation falls into a few clear categories such as blood group, and a bar chart of it shows separate columns. Continuous variation spreads across a range, as height and weight do, and its histogram looks roughly bell shaped. Traits of the second kind are usually controlled by many genes with the environment adding its own contribution, which is why siblings differ; B5.1l expects you to know that most characteristics are polygenic rather than single gene.
有性生殖与无性生殖各自的代价
无性生殖不需要配偶，速度快，一次能产生大量后代，代价是后代与亲代几乎一样，环境一变可能整群消失。有性生殖要花时间找配偶、产生配子、完成受精，后代数量少，好处是每一代都产生新的等位基因组合，种群在环境压力下更容易留下适应的个体。题目给一个变化的环境，问哪种生殖方式更有优势，答案要跟变异挂钩，而不是笼统说有性生殖更好。马铃薯用块茎繁殖属于无性生殖，这一点常与种子繁殖混淆。
Asexual reproduction needs no mate, works fast and can produce many offspring, but the offspring are near copies of the parent, so a shift in conditions can wipe out the whole group. Sexual reproduction costs time and gametes and yields fewer offspring, yet it reshuffles alleles every generation, which leaves a population better able to persist under environmental pressure. Read the question for the kind of environment described before choosing a side, because a stable habitat rewards the asexual route.
孟德尔、遗传学史与 B5 在卷面上的位置
孟德尔用豌豆做杂交实验，统计子代各类性状的数目，发现 3 比 1 这类比例，由此提出性状由成对的遗传因子控制。他的论文在 1866 年发表，当时没有引起重视，直到二十世纪初染色体与基因的概念建立起来，人们才回头读他的工作。今天我们用等位基因、基因型这些词解释他当年看到的现象。考试层面，OCR Gateway Biology A（J247，2016 年起施教）把 B5 放在 Paper 2 与 Paper 4 里考，基础卷与高阶卷都是 90 分、1 小时 45 分钟、各占 50%，官方 spec 标注这两份卷含综合题，并假定你已经掌握 B1 至 B3 的内容。复习时先把 B5.1 的术语过一遍，再回来做方格题，比直接刷题省时间。
Mendel crossed pea plants, counted the offspring of each type and found ratios such as 3 to 1, which led him to propose that characteristics are controlled by paired inherited factors. His paper appeared in 1866 and attracted little notice until chromosomes and genes were established in the early twentieth century. For assessment, the OCR Gateway Biology A specification (J247, taught from 2016) places B5 in Paper 2 and Paper 4, each worth 90 marks in 1 hour 45 minutes and 50 per cent of the qualification, with synoptic questions flagged on those papers and assumed knowledge of B1 to B3.
牛津AQA国际GCSE生物?OxfordAQA International GCSE Biology (9201): Onl?进口原版¥399.0📱 去京东购买京东精选
需要一对一辅导？Need One-on-One Tutoring?
A-Level / GCSE / IB / KS2-KS3 全科辅导，按考纲逐条对照
16621398022 同微信
Visit aleveler.com
或截图后用「扫一扫 → 相册」选择本图