📚 Genetic Inheritance: IB OCR Science Revision | IB OCR 科学：遗传 考点精讲
Genetics is the branch of biology that explains how traits are passed from parents to offspring. Understanding the principles of inheritance is essential for mastering both the IB Biology and OCR Gateway Science specifications. This article breaks down the key concepts you need to know, from Mendel’s classic experiments to sex-linked disorders, using clear explanations and worked examples.
遗传学是生物学中解释性状如何从亲代传递给子代的分支。掌握遗传规律对于精通 IB 生物和 OCR Gateway 科学课程至关重要。本文将从孟德尔的经典实验到伴性遗传疾病，用清晰的解释和实例逐一剖析你需要掌握的核心概念。
1. Introduction to Genetics | 遗传学导论
Genetics focuses on genes, the basic units of heredity. Genes are segments of DNA that code for specific proteins, which ultimately determine an organism’s characteristics. The combination of alleles an individual inherits creates a unique genotype, while the observable traits are referred to as the phenotype.
遗传学关注的是基因，即遗传的基本单位。基因是编码特定蛋白质的 DNA 片段，这些蛋白质最终决定了生物体的特征。个体继承的等位基因组合构成了独特的基因型，而可观察到的性状则称为表现型。
In both IB and OCR courses, you will need to distinguish between homozygous (two identical alleles) and heterozygous (two different alleles) conditions. A dominant allele expresses its trait even when only one copy is present, while a recessive allele requires two copies to be expressed.
在 IB 和 OCR 课程中，你需要区分纯合子（两个相同等位基因）和杂合子（两个不同等位基因）这两种情况。显性等位基因只要存在一份就能表达其性状，而隐性等位基因需要两份才能表达。
2. Mendelian Genetics: The Foundation | 孟德尔遗传学：奠基
Gregor Mendel’s work on pea plants established the fundamental laws of inheritance. He identified that traits are determined by discrete ‘factors’ (now called genes) that occur in pairs. His Law of Segregation states that allele pairs separate during gamete formation, so each gamete carries only one allele for each trait.
格雷戈尔·孟德尔在豌豆植株上的研究奠定了遗传的基本定律。他发现性状是由成对存在的离散“因子”（现称为基因）决定的。他的分离定律指出，等位基因对在配子形成过程中分离，因此每个配子只携带每个性状的一个等位基因。
Mendel also formulated the Law of Independent Assortment, which states that alleles for different traits are distributed to gametes independently of one another. This law applies when genes are located on different chromosomes or are far apart on the same chromosome.
孟德尔还提出了自由组合定律，即不同性状的等位基因独立地分配至配子中。这一定律适用于位于不同染色体上或在同一染色体上相距较远的基因。
3. Alleles and Genetic Terminology | 等位基因与遗传学术语
An allele is a variant form of a gene. For example, the gene for pea plant height has a tall allele (T) and a short allele (t). The dominant allele is conventionally written with a capital letter, while the recessive allele uses the corresponding lowercase letter.
等位基因是基因的变异形式。例如，豌豆株高基因有高茎等位基因 (T) 和矮茎等位基因 (t)。显性等位基因通常用大写字母表示，而隐性等位基因使用对应的小写字母。
A monohybrid cross examines the inheritance of a single trait. When you cross two heterozygous individuals (Tt × Tt), the genotypic ratio in the offspring is 1 TT : 2 Tt : 1 tt, and the phenotypic ratio is 3 tall : 1 short. This 3:1 ratio is a classic hallmark of Mendelian inheritance.
单杂交考察的是单一性状的遗传。当两个杂合子个体 (Tt × Tt) 杂交时，子代的基因型比例为 1 TT : 2 Tt : 1 tt，表现型比例为 3 高茎 : 1 矮茎。这个 3:1 的比例是孟德尔遗传的经典标志。
4. Monohybrid Crosses and Punnett Squares | 单杂交与庞纳特方格
Punnett squares are used to predict the genotypes of offspring from a genetic cross. For a monohybrid cross between two heterozygous parents (Tt), the square shows the four possible combinations: TT, Tt, tT, and tt.
庞纳特方格用于预测遗传杂交后代的基因型。对于两个杂合亲本 (Tt) 之间的单杂交，方格显示出四种可能的组合：TT、Tt、tT 和 tt。
Tt × Tt Punnett Square
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
The predicted genotypic ratio is 1:2:1, but because T is dominant, TT and Tt both produce the tall phenotype. Hence the observed phenotypic ratio is 3:1. Remember that Punnett squares show probabilities, not certainties, especially for small sample sizes.
预测的基因型比例为 1:2:1，但由于 T 是显性的，TT 和 Tt 都表现出高茎表现型，因此观察到的表现型比例为 3:1。请记住庞纳特方格显示的是概率，而不是必然结果，尤其是在小样本量时。
5. Dihybrid Crosses and Independent Assortment | 双因子杂交与自由组合
A dihybrid cross tracks two traits simultaneously, such as seed shape (round R, wrinkled r) and seed colour (yellow Y, green y). When two double heterozygotes (RrYy) are crossed, the gametes can be RY, Ry, rY, and ry in equal proportions.
双因子杂交同时追踪两个性状，例如种子形状（圆形 R，皱缩 r）和种子颜色（黄色 Y，绿色 y）。当两个双杂合子 (RrYy) 杂交时，配子可以是 RY、Ry、rY 和 ry，且比例相等。
The expected phenotypic ratio in the offspring is 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. This 9:3:3:1 ratio is typical for dihybrid crosses when both genes show complete dominance and are unlinked.
子代预期的表现型比例为 9 圆形黄色 : 3 圆形绿色 : 3 皱缩黄色 : 1 皱缩绿色。当两个基因都表现出完全显性且不连锁时，这个 9:3:3:1 的比例是双因子杂交的典型比例。
If genes are linked on the same chromosome, they do not assort independently, and the observed ratios deviate significantly from 9:3:3:1. Linked genes tend to be inherited together unless crossing over separates them during meiosis.
如果基因位于同一染色体上并连锁，它们就不会自由组合，观察到的比例会显著偏离 9:3:3:1。连锁基因倾向于一起遗传，除非在减数分裂过程中发生交叉互换使其分离。
6. Codominance and Incomplete Dominance | 共显性与不完全显性
In some cases, dominance is not absolute. Incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, crossing red snapdragons (RR) with white ones (WW) produces pink offspring (RW).
在某些情况下，显性并非绝对。不完全显性发生于杂合子表现型是两个纯合子表现型的中间混合体时。例如，将红色金鱼草 (RR) 与白色金鱼草 (WW) 杂交会产生粉色后代 (RW)。
Codominance occurs when both alleles in a heterozygote are fully expressed simultaneously. The classic IB and OCR example is the human ABO blood group system, where alleles Iᴬ and Iᴮ are codominant. An individual with genotype IᴬIᴮ expresses both A and B antigens on red blood cells.
共显性发生于杂合子中的两个等位基因同时完全表达时。经典的 IB 和 OCR 例子是人类 ABO 血型系统，其中等位基因 Iᴬ 和 Iᴮ 是共显性的。基因型为 IᴬIᴮ 的个体在红细胞上同时表达 A 和 B 抗原。
Both incomplete dominance and codominance produce phenotypic ratios that are different from the classic 3:1 Mendelian pattern. For codominance, a cross between two IᴬIᴮ individuals yields a 1 IᴬIᴬ : 2 IᴬIᴮ : 1 IᴮIᴮ phenotypic ratio, corresponding to blood groups A, AB, and B respectively.
不完全显性和共显性都会产生与经典 3:1 孟德尔模式不同的表现型比例。对于共显性，两个 IᴬIᴮ 个体之间的杂交产生 1 IᴬIᴬ : 2 IᴬIᴮ : 1 IᴮIᴮ 的表现型比例，分别对应 A 型、AB 型和 B 型血。
7. Sex-linked Inheritance | 伴性遗传
Sex-linked traits are controlled by genes located on the sex chromosomes, most commonly the X chromosome. In humans and many other organisms, females have two X chromosomes (XX) and males have one X and one Y (XY). Because males have only one X chromosome, they are more likely to express recessive X-linked traits.
伴性遗传性状是由位于性染色体（最常见的是 X 染色体）上的基因控制的。在人类和许多其他生物中，女性有两条 X 染色体 (XX)，男性有一条 X 和一条 Y (XY)。由于男性只有一条 X 染色体，他们更可能表达隐性 X 连锁性状。
Colour blindness and haemophilia are well-known X-linked recessive disorders. The allele for normal vision (Xᴺ) is dominant over the allele for red-green colour blindness (Xⁿ). A carrier female with genotype XᴺXⁿ has normal vision, but a male with XⁿY will be colour blind.
色盲和血友病是众所周知的 X 连锁隐性遗传疾病。正常视力等位基因 (Xᴺ) 对红绿色盲等位基因 (Xⁿ) 为显性。基因型为 XᴺXⁿ 的女性携带者视力正常，而基因型为 XⁿY 的男性则会是色盲。
In a cross between a carrier female (XᴺXⁿ) and a normal male (XᴺY), the predicted offspring ratios are: 25% normal female, 25% carrier female, 25% normal male, and 25% colour-blind male. Notice that daughters cannot be colour blind in this cross, while half the sons are affected.
在女性携带者 (XᴺXⁿ) 与正常男性 (XᴺY) 的杂交中，预测的子代比例为：25% 正常女性，25% 女性携带者，25% 正常男性，25% 色盲男性。请注意在这种杂交中女儿不会是色盲，而一半的儿子会受影响。
8. Multiple Alleles and Blood Groups | 复等位基因与血型
The ABO blood group system is a prime example of multiple alleles. A single gene, designated I, has three common alleles: Iᴬ, Iᴮ, and i. Iᴬ codes for the A antigen, Iᴮ for the B antigen, and i produces no functional antigen. The i allele is recessive to both Iᴬ and Iᴮ.
ABO 血型系统是复等位基因的一个典型例子。一个被命名为 I 的基因有三种常见的等位基因：Iᴬ、Iᴮ 和 i。Iᴬ 编码 A 抗原，Iᴮ 编码 B 抗原，而 i 不产生功能性抗原。i 等位基因对 Iᴬ 和 Iᴮ 均为隐性。
The six possible genotypes produce four phenotypes: A (IᴬIᴬ or Iᴬi), B (IᴮIᴮ or Iᴮi), AB (IᴬIᴮ), and O (ii). Blood group O individuals are known as universal donors for red blood cells, while AB individuals are universal recipients, a concept often tested in both IB and OCR contexts.
六种可能的基因型产生四种表现型：A 型 (IᴬIᴬ 或 Iᴬi)、B 型 (IᴮIᴮ 或 Iᴮi)、AB 型 (IᴬIᴮ) 和 O 型 (ii)。O 型血个体被称为红细胞万能供血者，而 AB 型个体是万能受血者，这一概念在 IB 和 OCR 考试中常被考到。
The Rh factor is another blood group system involving a dominant allele (Rh⁺) and a recessive allele (Rh⁻). Rh incompatibility between mother and fetus can cause serious complications, illustrating the medical importance of genetics.
Rh 因子是另一个血型系统，涉及显性等位基因 (Rh⁺) 和隐性等位基因 (Rh⁻)。母亲与胎儿之间的 Rh 不相容可导致严重并发症，这说明了遗传学的医学重要性。
9. Pedigree Analysis | 系谱分析
Pedigree charts are used to trace the inheritance of traits through generations. Squares represent males, circles represent females, and shaded symbols indicate affected individuals. By analysing patterns, you can determine whether a trait is dominant, recessive, autosomal, or sex-linked.
系谱图用于追踪性状在代际间的遗传情况。方块代表男性，圆圈代表女性，阴影符号表示受影响的个体。通过分析模式，你可以判断某个性状是显性还是隐性、常染色体遗传还是伴性遗传。
An autosomal dominant pedigree typically shows the trait in every generation, with affected individuals having at least one affected parent. In contrast, an autosomal recessive pedigree often skips generations, and affected children may have unaffected parents who are carriers.
常染色体显性系谱通常显示性状出现在每一代中，且患病个体至少有一方亲本患病。相比之下，常染色体隐性系谱常会隔代出现，患病的子女可能拥有未患病但为携带者的父母。
For X-linked recessive traits, more males are affected, and there is no male-to-male transmission. All daughters of an affected male are carriers, while none of his sons inherit the condition. Being able to interpret these patterns is a vital exam skill.
对于 X 连锁隐性性状，受影响的男性更多，且不存在男性到男性的传递。患病男性的所有女儿都是携带者，而他的儿子们都不会遗传该疾病。能够解读这些模式是一项关键的考试技能。
10. Genetic Disorders and Mutations | 遗传疾病与突变
Mutations are changes in the DNA sequence that can lead to genetic disorders. Cystic fibrosis is an autosomal recessive disorder caused by a mutation in the CFTR gene. Individuals with two copies of the mutated allele (cc) produce thick mucus that affects the lungs and digestive system.
突变是 DNA 序列的改变，可能导致遗传疾病。囊性纤维化是一种常染色体隐性遗传病，由 CFTR 基因突变引起。拥有两份突变等位基因 (cc) 的个体会产生浓厚的黏液，影响肺部和消化系统。
Huntington’s disease is an autosomal dominant disorder. A person with just one copy of the faulty allele (H) will develop symptoms, typically in middle age. Because its onset is later in life, the allele can be unknowingly passed to offspring.
亨廷顿病是一种常染色体显性遗传病。只要携带一份缺陷等位基因 (H)，人就会患病，症状一般在中年出现。由于发病较晚，等位基因可能在不知情的情况下传递给后代。
Sickle cell anaemia is a codominant disorder where heterozygotes (HbᴬHbˢ) have some sickle-shaped red blood cells but are generally healthy, and they have a survival advantage in malaria-endemic regions. This illustrates the concept of heterozygote advantage, which is a key evolutionary idea covered in both specifications.
镰状细胞贫血是一种共显性疾病，杂合子 (HbᴬHbˢ) 有一定数量的镰状红细胞，但总体上身体健康，并且在疟疾流行地区拥有生存优势。这说明了杂合子优势的概念，这是两种课程大纲中都会涉及的关键进化思想。
11. Variation: Continuous and Discontinuous | 变异：连续变异与不连续变异
Variation within a population can be classified as discontinuous or continuous. Discontinuous variation is controlled by a single gene and produces distinct categories, such as blood groups or the ability to roll one’s tongue. This type of variation is usually unaffected by the environment.
种群内的变异可分为不连续变异和连续变异。不连续变异由单个基因控制，产生截然不同的类别，如血型或卷舌能力。这种变异通常不受环境影响。
Continuous variation is influenced by multiple genes (polygenic inheritance) and environmental factors. Examples include height, skin colour, and body mass. When plotted on a graph, continuous traits show a bell-shaped normal distribution curve, a pattern frequently tested in OCR data-analysis questions.
连续变异受多个基因（多基因遗传）和环境因素的影响。例子包括身高、肤色和体重。在图表上绘制时，连续性状呈现钟形正态分布曲线，这是 OCR 数据分析题中常考的模式。
Meiosis generates variation through independent assortment and crossing over, while random fertilisation further increases genetic diversity. In addition, mutation introduces new alleles, providing the raw material upon which natural selection can act.
减数分裂通过自由组合和交叉互换产生变异，而随机受精进一步增加了遗传多样性。此外，突变引入新的等位基因，为自然选择提供了可作用的原材料。
12. Exam Tips for Genetics Questions | 遗传学题目应试技巧
Always define your symbols for alleles at the start of a cross. Use a key: e.g., ‘Let T = tall allele, t = short allele.’ This is essential to gain full marks in both IB and OCR assessments. When constructing Punnett squares, circle or write down the gametes clearly to avoid errors.
在解答杂交问题时，务必首先定义等位基因符号。使用图例说明，例如“设 T = 高茎等位基因，t = 矮茎等位基因”。这对于在 IB 和 OCR 评估中获得满分至关重要。绘制庞纳特方格时，清晰圈出或写出配子，以避免错误。
For sex-linked problems, always include the sex chromosomes in the genotypes (e.g., XᴺXⁿ, XᴺY). Never ignore the Y chromosome in males. If a question asks for a phenotypic ratio, make sure the ratio is reduced to its simplest whole numbers and correctly labelled.
对于伴性遗传问题，务必在基因型中包含性染色体（例如 XᴺXⁿ、XᴺY）。切勿忽略男性中的 Y 染色体。如果问题要求表现型比例，确保比例已化简为最简整数，且标签正确。
Finally, practise interpreting pedigree diagrams and calculating probabilities. Many IB and OCR students lose marks by forgetting that each childbirth is an independent event, so the probability resets for each new offspring. Confidence with these skills comes from repeated practice with past paper questions.
最后，练习系谱图解读和概率计算。许多 IB 和 OCR 学生因忘记每次生育都是独立事件而失分，即每个新子代概率都重新计算。通过反复练习往年真题，可以熟练地掌握这些技能。
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