📚 A-Level OCR Biology: Genetics Revision Guide | 遗传考点精讲
Genetics forms a fundamental pillar of the OCR A-Level Biology specification, explaining how traits are passed from one generation to the next and how variation arises within populations. This revision guide consolidates the key principles, from Mendelian laws to population genetics, providing clear explanations, essential terminology, and worked examples that match the style of exam questions. Whether you are tackling monohybrid crosses, sex linkage, or the Hardy–Weinberg equation, a solid grasp of these concepts will strengthen your analytical skills and boost your exam confidence.
遗传学是 OCR A-Level 生物学大纲的支柱内容，阐释性状如何在世代间传递以及变异如何在群体中产生。这份复习指南整合了从孟德尔定律到群体遗传学的核心原理，提供清晰的解释、必备术语以及与考题风格相符的例题解析。无论你面对的是单基因杂交、性连锁还是哈代–温伯格方程，扎实掌握这些概念都将强化你的分析能力，提升你的应试信心。
1. Key Genetic Terms | 关键遗传术语
A gene is a sequence of DNA that codes for a functional polypeptide or RNA molecule. Alleles are alternative forms of the same gene occupying the same locus on homologous chromosomes. The genotype of an organism is its complete set of alleles, while the phenotype is the observable characteristics resulting from the interaction of the genotype with the environment.
基因是一段编码功能性多肽或 RNA 分子的 DNA 序列。等位基因是位于同源染色体相同基因座上的同一基因的不同形式。基因型是指生物体全套等位基因的组成，表现型则是基因型与环境相互作用后产生的可观察特征。
Alleles are described as dominant if they express their effect in the heterozygous state, and recessive if their effect is masked by a dominant allele. Codominant alleles both contribute to the phenotype in a heterozygote. An individual with two identical alleles for a gene is homozygous; an individual with two different alleles is heterozygous.
等位基因若在杂合状态下即可表达其效应则称为显性，若其效应被显性等位基因掩盖则称为隐性。共显性等位基因在杂合子中会共同表达。对于某个基因，拥有两个相同等位基因的个体为纯合子，拥有两个不同等位基因的个体为杂合子。
2. Monohybrid Inheritance and Mendel’s First Law | 单基因遗传与孟德尔第一定律
Monohybrid inheritance involves a single gene with two alleles. When true-breeding homozygous dominant and homozygous recessive parents are crossed, all F₁ offspring are heterozygous and show the dominant phenotype. Self-pollinating the F₁ generates an F₂ with a phenotypic ratio of 3:1 (dominant : recessive) and a genotypic ratio of 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.
单基因遗传涉及一个具有两个等位基因的基因。当纯合显性与纯合隐性亲本杂交，所有 F₁ 子代均为杂合子并表现显性性状。F₁ 自交产生的 F₂ 表现型比为 3:1（显性 : 隐性），基因型比为 1 纯合显性 : 2 杂合 : 1 纯合隐性。
Mendel’s First Law (the Law of Segregation) states that the two alleles for a trait separate during gamete formation, so each gamete carries only one allele. A test cross, in which an individual showing the dominant trait is crossed with a homozygous recessive, can determine the unknown genotype – if any offspring show the recessive trait, the tested individual must be heterozygous.
孟德尔第一定律（分离定律）指出，控制某一性状的一对等位基因在配子形成时彼此分离，每个配子只携带一个等位基因。测交是将表现显性性状的个体与纯合隐性个体杂交，若子代中出现隐性性状，则可判定该测试个体为杂合子。
3. Dihybrid Inheritance and Mendel’s Second Law | 双基因遗传与孟德尔第二定律
Dihybrid inheritance examines two genes located on different chromosomes. When pure-breeding parents differing in two traits are crossed, the F₁ are all heterozygous for both genes. Self-pollination of the F₁ produces a phenotypic ratio of 9:3:3:1 in the F₂, provided the genes assort independently.
双基因遗传考察位于不同染色体上的两个基因。当两个纯合且两对性状不同的亲本杂交，F₁ 均为双杂合子。F₁ 自交产生的 F₂ 代表现型比为 9:3:3:1，前提是这两个基因自由组合。
Mendel’s Second Law (the Law of Independent Assortment) states that alleles of different genes are distributed to gametes independently of one another. The four types of gametes formed by a dihybrid (e.g., RrYy) are produced in equal proportions (RY, Ry, rY, ry). A dihybrid test cross with a double recessive yields a 1:1:1:1 ratio.
孟德尔第二定律（自由组合定律）指出，不同基因的等位基因在配子形成时彼此独立分配。双杂合子（如 RrYy）产生的四种配子（RY, Ry, rY, ry）比例相等。用双隐性个体进行测交，子代比例为 1:1:1:1。
4. Codominance and Multiple Alleles | 共显性与复等位基因
Codominance occurs when both alleles in a heterozygote are fully expressed in the phenotype. A classic example is the human ABO blood group system, which is controlled by three alleles of the I gene: Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant; both are dominant to i. The possible genotypes and phenotypes are: IᴬIᴬ or Iᴬi → blood group A, IᴮIᴮ or Iᴮi → blood group B, IᴬIᴮ → blood group AB, ii → blood group O.
共显性是指杂合子中两个等位基因的表现型同时充分表达。经典例子是人类 ABO 血型系统，由 I 基因的三个等位基因控制：Iᴬ、Iᴮ 和 i。Iᴬ 和 Iᴮ 为共显性，且均对 i 为显性。可能的基因型与表现型对应如下：IᴬIᴬ 或 Iᴬi → A 型血，IᴮIᴮ 或 Iᴮi → B 型血，IᴬIᴮ → AB 型血，ii → O 型血。
Multiple alleles exist when more than two alleles control a trait within a population, although any individual still carries only two alleles. This increases phenotypic variation. The concept of multiple alleles is also seen in coat colour in rabbits and in the major histocompatibility complex (MHC) genes.
复等位基因是指在群体中存在两个以上等位基因控制同一性状，尽管每个个体仍然只携带其中两个。这增加了表现型的多样性。复等位基因的概念也见于兔毛颜色以及主要组织相容性复合体（MHC）基因中。
5. Sex Linkage | 性连锁遗传
Sex linkage refers to genes located on the sex chromosomes, most commonly the X chromosome. Because males (XY) have only one X chromosome, any recessive allele on the X will be expressed in the male phenotype. Females (XX) require two copies of the recessive allele to express the trait, making them carriers if heterozygous.
性连锁是指基因位于性染色体上，最常见于 X 染色体。由于男性（XY）只有一条 X 染色体，X 染色体上的任何隐性等位基因都会在男性表现型中表达。女性（XX）需要两个隐性等位基因才表现出该性状，若为杂合子则为携带者。
Examples of X-linked recessive disorders include red-green colour blindness and haemophilia. In a cross between a carrier female (XᴺXⁿ) and a normal male (XᴺY), half of the sons will be affected (XⁿY), while daughters will be either normal or carriers. This pattern is known as criss-cross inheritance, where a trait skips generations and is transmitted through carrier females to affected males.
X 连锁隐性遗传病的实例包括红绿色盲和血友病。携带者女性（XᴺXⁿ）与正常男性（XᴺY）婚配，一半的儿子将患病（XⁿY），女儿则正常或为携带者。这种模式称为交叉遗传，即性状隔代传递，通过女性携带者传递给患病的男性。
6. Autosomal Linkage and Recombination | 常染色体连锁与重组
Autosomal linkage occurs when two or more genes are located on the same autosome. Linked genes do not assort independently; instead, they tend to be inherited together. If no crossing over occurs, a dihybrid (AaBb) with linked genes in the cis configuration (AB/ab) produces only two types of gametes (AB and ab), altering the expected 9:3:3:1 ratio.
常染色体连锁指的是两个或以上基因位于同一条常染色体上。连锁基因不遵循自由组合定律，而是倾向于一同遗传。如果不发生交换，处于顺式排列（AB/ab）的双杂合子仅产生两种配子（AB 和 ab），从而改变预期的 9:3:3:1 比例。
Crossing over during prophase I of meiosis can separate linked alleles, producing recombinant gametes (Ab and aB). The recombination frequency is used to estimate the map distance between two genes on a chromosome: 1% recombination frequency equals 1 map unit (centimorgan). A recombination frequency less than 50% confirms linkage.
减数分裂前期 I 的交叉互换可将连锁的等位基因分开，产生重组型配子（Ab 和 aB）。重组频率可用于估算染色体上两个基因的图距：1% 重组频率等于 1 图距单位（厘摩）。重组频率低于 50% 即证实连锁存在。
7. Epistasis | 上位效应
Epistasis occurs when the expression of one gene is masked or modified by another gene at a different locus. It often changes the expected Mendelian phenotypic ratios. In recessive epistasis, a homozygous recessive allele at one locus prevents the expression of alleles at a second locus, resulting in a 9:3:4 ratio (e.g., coat colour in Labrador retrievers).
上位效应是指一个基因的表达被另一位点上的另一个基因所掩蔽或修饰。它常会改变预期的孟德尔表现型比例。在隐性上位中，一位点的纯合隐性等位基因阻止另一位点等位基因的表达，产生 9:3:4 的比例（例如拉布拉多犬的毛色）。
Dominant epistasis is where a dominant allele at one locus masks the effect of a second locus, giving a 12:3:1 ratio (e.g., fruit colour in summer squash). Other epistatic interactions can produce 13:3, 9:7, or 15:1 ratios, depending on whether the genes act in complementary or duplicate pathways. Recognizing these ratios in dihybrid crosses is a valuable exam skill.
显性上位是指一位点的显性等位基因掩蔽另一位点的效应，产生 12:3:1 的比例（例如夏季南瓜果色）。其他上位互作还可产生 13:3、9:7 或 15:1 的比例，取决于基因是互补作用还是重复作用。在双基因杂交中识别这些比例是一项重要的应试技能。
8. Chi-Squared Test in Genetics | 遗传学中的卡方检验
The chi-squared (χ²) test is a statistical method used to determine whether the difference between observed and expected phenotypic frequencies is due to chance or whether it is significant. The formula is χ² = Σ (O − E)² / E, where O = observed frequency and E = expected frequency based on a Mendelian ratio.
卡方（χ²）检验是一种统计方法，用于判断观测表现型频率与期望频率之间的差异是由偶然造成的还是显著的。公式为 χ² = Σ (O − E)² / E，其中 O 为观测值，E 为依据孟德尔比例计算的期望值。
After calculating χ², the degrees of freedom (d.f.) are determined – for a monohybrid cross d.f. = number of phenotypic classes − 1, while for a dihybrid cross it is usually 3. The calculated χ² value is then compared with a critical value at a chosen probability level (typically p = 0.05). If χ² exceeds the critical value, the null hypothesis (no significant difference) is rejected, suggesting that the observed ratio does not fit the expected ratio.
计算出 χ² 后，需确定自由度（d.f.）——单基因杂交的自由度为表现型类别数减 1，双基因杂交的自由度通常为 3。然后将计算出的 χ² 值与所选概率水平（通常为 p = 0.05）下的临界值比较。若 χ² 超过临界值，则拒绝零假设（无显著差异），表明观测比例不符合预期比例。
9. Gene Mutations and Variation | 基因突变与变异
Gene mutations are changes in the nucleotide sequence of DNA. Substitution mutations replace one base with another, which may cause a different amino acid (missense), a premature stop codon (nonsense), or no change (silent) due to the degenerate nature of the genetic code. Insertion or deletion of a nucleotide leads to a frameshift, altering the entire amino acid sequence downstream.
基因突变是 DNA 核苷酸序列的改变。碱基替换突变会将一个碱基替换为另一个，可能引起氨基酸改变（错义）、提前出现终止密码子（无义），或由于遗传密码的简并性而不造成任何变化（沉默突变）。插入或缺失一个核苷酸则会导致移码，改变下游所有的氨基酸序列。
Mutations are the ultimate source of genetic variation. During meiosis, random assortment, crossing over, and the random fusion of gametes further reshuffle alleles, creating extensive phenotypic diversity. Many mutations are neutral or harmful, but some can provide a selective advantage and drive evolution.
突变是遗传变异的根本来源。在减数分裂过程中，染色体的随机分配、交叉互换以及配子的随机融合进一步重组了等位基因，产生广泛的表型多样性。许多突变是中性的或有害的，但有些能提供选择优势并驱动进化。
10. Hardy–Weinberg Principle | 哈代–温伯格原理
The Hardy–Weinberg principle allows the calculation of allele and genotype frequencies in a large, randomly mating population that is not subject to mutation, selection, migration, or genetic drift. If p represents the frequency of the dominant allele and q represents the frequency of the recessive allele, then p + q = 1.
哈代–温伯格原理可用于计算一个大型且随机交配、不受突变、选择、迁移或遗传漂变影响的群体中的等位基因频率和基因型频率。若 p 代表显性等位基因的频率，q 代表隐性等位基因的频率，则 p + q = 1。
The genotype frequencies are given by the equation p² + 2pq + q² = 1, where p² is the frequency of homozygous dominant, 2pq the frequency of heterozygotes, and q² the frequency of homozygous recessive individuals. To calculate the percentage of carriers for a recessive condition, first determine q from the proportion of affected individuals (q²), then compute 2pq.
基因型频率由方程 p² + 2pq + q² = 1 给出，其中 p² 为纯合显性频率，2pq 为杂合子频率，q² 为纯合隐性频率。若要计算某种隐性疾病的携带者比例，可先从患病个体比例（q²）求出 q，再计算 2pq。
Published by TutorHao | Genetics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply