Genetics and Inheritance | 遗传与基因

📚 Genetics and Inheritance | 遗传与基因

In this revision article, we explore the fundamental principles of genetics and inheritance, a core topic of the Edexcel IGCSE Biology syllabus. We will examine how genetic information is stored in DNA, how it is passed from generation to generation, and how it produces the observable characteristics of all living organisms.

本篇复习文章探讨遗传学与基因传递的基础原理，这是 Edexcel IGCSE 生物考纲中的一个核心主题。我们将研究遗传信息如何在 DNA 中储存、如何逐代传递，以及如何产生所有生物体的可观测性状。

1. Chromosomes, Genes and DNA | 染色体、基因与 DNA

Deoxyribonucleic acid (DNA) is the molecule of inheritance. It is found in the nucleus of eukaryotic cells, tightly coiled around proteins to form structures called chromosomes. Most human body cells contain 46 chromosomes arranged in 23 pairs.

脱氧核糖核酸（DNA）是遗传物质分子。它存在于真核细胞的细胞核中，紧密盘绕在蛋白质周围，形成称为染色体的结构。大多数人体体细胞含有 46 条染色体，排列成 23 对。

DNA has the structure of a double helix, resembling a twisted ladder. The ‘rungs’ of the ladder are made of complementary base pairs: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). The sequence of these bases along the DNA molecule carries the genetic code.

DNA 具有双螺旋结构，如同一条扭曲的梯子。梯子的“横档”由互补碱基对构成：腺嘌呤（A）始终与胸腺嘧啶（T）配对，鸟嘌呤（G）始终与胞嘧啶（C）配对。这些碱基沿 DNA 分子的排列顺序携带遗传密码。

A gene is a section of DNA that codes for a particular protein or polypeptide. The order of bases in a gene determines the order of amino acids in the protein, which in turn determines the protein’s structure and function. A chromosome is a long, coiled DNA molecule that contains many genes arranged in a linear sequence.

基因是 DNA 上决定特定蛋白质或多肽的片段。基因中碱基的排列顺序决定蛋白质中氨基酸的顺序，进而决定蛋白质的结构与功能。染色体是一条长而盘绕的 DNA 分子，其上线性排列着许多基因。

2. Alleles: Dominant and Recessive | 等位基因：显性与隐性

An allele is an alternative version of a gene. In diploid organisms, chromosomes come in pairs (homologous chromosomes), and each pair carries the same gene at the same locus (position). However, the alleles at that locus may differ.

等位基因是基因的不同版本。在二倍体生物中，染色体成对存在（同源染色体），每对染色体在同一基因座（位置）上携带同一基因，但该位置的等位基因可能不同。

If an organism inherits two identical alleles for a gene, it is said to be homozygous for that gene. If the two alleles are different, the organism is heterozygous. A dominant allele is one that is expressed in the phenotype even when only one copy is present. A recessive allele is only expressed when two copies are present, i.e. in the homozygous state.

如果生物体遗传了两个相同的等位基因，称为该基因的纯合子；如果两个等位基因不同，则称为杂合子。显性等位基因即使只有一份也能在表现型中表达；隐性等位基因只有在两份都存在（即纯合状态）时才能表达。

By convention, dominant alleles are represented by a capital letter (e.g. T for tall) and recessive alleles by the corresponding lowercase letter (e.g. t for short). A heterozygous individual (Tt) will show the dominant trait, but is also described as a carrier if the recessive allele is harmful or disease-causing.

按惯例，显性等位基因用大写字母表示（如 T 代表高茎），隐性等位基因用对应的小写字母表示（如 t 代表矮茎）。杂合个体（Tt）会表现显性性状，但如果隐性等位基因是有害的或致病的，该个体也被称为携带者。

3. Genotype and Phenotype | 基因型与表现型

Two key terms are used to distinguish between the genetic makeup of an organism and its observable traits. The genotype is the full set of alleles that an organism possesses for a particular gene. The phenotype is the observable physical or biochemical characteristic produced by the genotype, influenced where relevant by the environment.

两个关键术语用于区分生物的遗传组成与其可观测性状。基因型是生物体对某一特定基因所拥有的全部等位基因组合。表现型是基因型所产生的可观测的物理或生化特征，并在相关情况下受环境的影响。

For example, in pea plants, the genotype TT produces tall plants, tt produces short plants, and Tt also produces tall plants because T is dominant. Therefore, two different genotypes (TT and Tt) can produce the same phenotype (tall). This distinction is essential for solving genetics problems correctly.

例如，在豌豆植株中，基因型 TT 产生高茎植株，tt 产生矮茎植株，而 Tt 也产生高茎植株，因为 T 是显性的。因此，两个不同的基因型（TT 和 Tt）可能产生相同的表现型（高茎）。这一区分对于正确解答遗传题至关重要。

Genotype = genetic constitution (e.g. Tt) | 基因型 = 遗传组成（如 Tt）
Phenotype = observable feature (e.g. tall) | 表现型 = 可观测特征（如高茎）
Homozygous = two identical alleles (TT or tt) | 纯合子 = 两个相同等位基因（TT 或 tt）
Heterozygous = two different alleles (Tt) | 杂合子 = 两个不同等位基因（Tt）

4. Monohybrid Inheritance and Punnett Squares | 单基因杂交遗传与庞尼特方格

Monohybrid inheritance concerns the inheritance of a single characteristic controlled by one gene with two alleles. A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

单基因杂交遗传涉及由一对等位基因控制的单一性状的遗传。庞尼特方格（Punnett square）是一种用于预测遗传杂交后代基因型和表现型的图表工具。

Consider a cross between two heterozygous pea plants for stem height (Tt × Tt). Each parent produces two types of gametes: T and t, in roughly equal proportions. The Punnett square below shows all possible combinations.

以两株杂合豌豆茎高杂交为例（Tt × Tt）。每个亲本产生两种配子：T 和 t，比例大致相等。下面的庞尼特方格显示了所有可能的组合。

	T (sperm)	t (sperm)
T (egg)	TT (tall)	Tt (tall)
t (egg)	Tt (tall)	tt (short)

From this cross, the expected phenotypic ratio is 3 tall : 1 short, and the genotypic ratio is 1 TT : 2 Tt : 1 tt. When solving problems, always state the parental genotypes, determine the gametes, complete the Punnett square, then present the ratios clearly.

由此杂交可得，预期的表现型比例为 3 高茎 : 1 矮茎，基因型比例为 1 TT : 2 Tt : 1 tt。解题时，一定要先写出亲本基因型、确定配子、完成庞尼特方格，然后清晰写出比例。

5. Pedigree Diagrams | 家族系谱图

A pedigree diagram (family tree) uses symbols to trace the inheritance of a characteristic through several generations. Males are represented by squares, females by circles; a horizontal line between a square and a circle indicates a mating, and vertical lines lead to their offspring.

家族系谱图使用符号追踪某个性状在多代人中的遗传。男性用正方形表示，女性用圆形表示；正方形与圆形之间的横线表示婚配，竖线连接至其子女。

Shaded symbols indicate individuals expressing the trait. By analysing whether a trait appears in every generation (suggesting dominance) or skips generations (suggesting recessiveness), you can deduce the pattern of inheritance and often the genotypes of specific individuals.

填充的符号表示表现该性状的个体。通过分析性状是出现在每一代（提示显性遗传）还是隔代出现（提示隐性遗传），可以推断遗传模式，并常常能确定特定个体的基因型。

Square = male | 正方形 = 男性
Circle = female | 圆形 = 女性
Shaded = affected | 填充 = 患此性状者
Unshaded = unaffected | 未填充 = 未患此性状者
Roman numbers = generation; Arabic numbers = individual | 罗马数字 = 世代；阿拉伯数字 = 个体

6. Sex Determination | 性别决定

In humans, sex is determined by the sex chromosomes, X and Y. Females have two X chromosomes (XX) while males have one X and one Y chromosome (XY). The Y chromosome carries the SRY gene that triggers male development.

在人类中，性别由性染色体 X 和 Y 决定。女性有两条 X 染色体（XX），男性有一条 X 和一条 Y 染色体（XY）。Y 染色体上携带触发男性发育的 SRY 基因。

During meiosis, a female produces eggs that all contain one X chromosome. A male produces two types of sperm: half contain an X chromosome and half contain a Y chromosome. This produces the following cross:

在减数分裂过程中，女性产生的卵子全部含有一条 X 染色体；男性产生两类精子：一半含 X 染色体，一半含 Y 染色体。由此产生以下杂交组合：

	X (sperm)	Y (sperm)
X (egg)	XX (female)	XY (male)
X (egg)	XX (female)	XY (male)

The expected sex ratio is therefore 1 : 1, or approximately 50% female and 50% male in each pregnancy. In exam questions, always note that the father’s gametes determine the sex of the offspring.

因此，预期的性别比例是 1 : 1，即每次妊娠中大约 50% 为女性、50% 为男性。在考试题目中，务必注意：父亲的配子决定后代的性别。

7. Codominance and ABO Blood Groups | 共显性与 ABO 血型

Codominance occurs when two alleles are both expressed in the phenotype of a heterozygous individual. In the ABO blood group system, the alleles Iᴬ and Iᴮ are codominant, while allele i is recessive.

共显性是指杂合个体中两个等位基因均在表现型中得到表达。在 ABO 血型系统中，等位基因 Iᴬ 和 Iᴮ 为共显性，而等位基因 i 为隐性。

Blood type depends on which antigens are present on the surface of red blood cells. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. The possible genotypes are as follows:

血型取决于红细胞表面存在何种抗原。A 型血具有 A 抗原，B 型血具有 B 抗原，AB 型血同时具有两种抗原，而 O 型血两种都没有。可能的基因型如下：

Type A: IᴬIᴬ or Iᴬi | A 型：IᴬIᴬ 或 Iᴬi
Type B: IᴮIᴮ or Iᴮi | B 型：IᴮIᴮ 或 Iᴮi
Type AB: IᴬIᴮ | AB 型：IᴬIᴮ
Type O: ii | O 型：ii

Because Iᴬ and Iᴮ are codominant, an individual with genotype IᴬIᴮ shows both A and B antigens. This is different from simple dominance and is a common exam question. Remember that a cross between Iᴬi and Iᴮi parents can produce offspring of all four blood types.

由于 Iᴬ 与 Iᴮ 是共显性的，基因型为 IᴬIᴮ 的个体同时表现 A 和 B 抗原。这与简单显性不同，是常见的考试题目。请记住，Iᴬi 与 Iᴮi 亲本杂交的后代可以出现全部四种血型。

8. Variation and Mutation | 变异与突变

Variation is the difference in characteristics between individuals of the same species. Variation can be caused by genetic factors, environmental factors, or a combination of both. Genetic variation arises from the mixing of alleles during sexual reproduction and from mutation.

变异是同种个体之间性状的差异。变异可由遗传因素、环境因素或两者共同造成。遗传变异来源于有性生殖过程中的等位基因重新组合以及突变。

Sources of genetic variation include the independent assortment of chromosomes during meiosis, crossing over (exchange of chromosome segments) during prophase I, and the random fusion of gametes at fertilisation. These processes ensure that every individual is genetically unique.

遗传变异的来源包括：减数分裂中染色体的自由组合、前期 I 中的交叉互换（染色体片段交换），以及受精时配子的随机结合。这些过程确保每个个体在遗传上都是独一无二的。

A mutation is a spontaneous change in the base sequence of DNA. Mutations that occur in body cells are not inherited, but mutations in gametes can be passed to offspring. A single gene mutation can change the structure of a protein, often with harmful effects; occasionally it is beneficial or has no effect at all.

突变是 DNA 碱基序列的自发性改变。发生在体细胞中的突变不会被遗传，但发生在配子中的突变可以传递给后代。单个基因突变可能改变蛋白质的结构，通常产生有害影响；偶尔也可能是有益的或完全无影响的。

9. Natural Selection and Evolution | 自然选择与进化

Charles Darwin’s theory of natural selection explains how evolution occurs. Within any population, there is variation; individuals are in competition for limited resources such as food, space and mates. Those with beneficial characteristics are more likely to survive, reproduce and pass on their alleles.

查尔斯·达尔文的自然选择理论解释了进化如何发生。在任何种群中都存在变异；个体之间为了食物、空间和配偶等有限资源而竞争。具有有利性状的个体更有可能生存、繁殖，并将它们的等位基因传递下去。

This process, often described as ‘survival of the fittest’, leads to a change in the allele frequency of a population over many generations. Speciation can occur if populations become separated by geographical barriers and experience different selection pressures, eventually becoming unable to interbreed.

这一过程常被描述为“适者生存”。经过许多世代，它导致种群中等位基因频率的改变。如果种群因地理隔离而分开并遭受不同的选择压力，最终不能进行交配，就可能形成新物种。

Examiners often ask for the sequence: variation → competition → selection pressure → survival of the fittest → reproduction → advantageous alleles become more common. Always use the term ‘advantageous alleles’ rather than ‘good traits’ in your answers.

考官常要求按以下顺序作答：变异 → 竞争 → 选择压力 → 适者生存 → 繁殖 → 有利等位基因变得更加普遍。请务必使用“有利等位基因”这一术语，而不是“好的性状”。

10. Selective Breeding and Genetic Engineering | 选择性育种与基因工程

Selective breeding (artificial selection) is the deliberate breeding of organisms with desirable characteristics. Humans choose parents with the best traits, breed them, and then continue selecting the best offspring over many generations. This has produced high-yielding crops, dairy cattle with high milk yields, and dogs with specific temperaments.

选择性育种（人工选择）是人为地繁育具有理想性状的生物体。人类选择具有最佳性状的亲本进行繁殖，然后在许多世代中继续筛选最优良的后代。这种方法培育出了高产作物、高乳产量的奶牛以及具有特定气质的犬类。

Genetic engineering involves directly transferring a gene from one organism into another, usually of a different species. For example, human insulin gene has been inserted into bacteria, enabling them to produce human insulin for medical use. Genetically modified crops may carry genes for herbicide resistance or improved nutritional content.

基因工程涉及将一个生物体的基因直接转入另一个生物体，通常是不同物种。例如，人类胰岛素基因已被插入细菌中，使细菌能够生产用于医疗的人胰岛素。转基因作物可能携带抗除草剂基因或改善营养含量的基因。

The main differences between these two methods are: selective breeding takes many generations and is limited to the same species, while genetic engineering can transfer genes between species in one generation. Ethical concerns about genetic engineering include its effect on biodiversity, food safety, and the potential for unforeseen long-term consequences.

这两种方法的主要区别是：选择性育种需要许多世代，且仅限于同一物种；而基因工程可以在一个世代内实现跨物种基因转移。关于基因工程的伦理问题包括其对生物多样性的影响、食品安全，以及可能产生的不可预见的长期后果。

When tackling inheritance questions in the exam, always follow these steps: identify the dominant and recessive alleles, write down the parental genotypes, determine the possible gametes, construct the Punnett square, and state both the genotypic and phenotypic ratios. Practising these routines will greatly improve your accuracy and speed.

在考试中解答遗传题时，请始终遵循以下步骤：确认显性和隐性等位基因，写出亲本基因型，确定可能的配子，构建庞尼特方格，然后写出基因型比例和表现型比例。反复练习这些步骤将大大提高准确性和解题速度。

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Genetics and Inheritance | 遗传与基因