📚 Genes and Alleles: The Basis of Inherited Variation | 基因与等位基因：遗传变异的基础
Variation is one of the most visible features of life. From eye colour to blood type, the differences between individuals of the same species arise from the combination of genes inherited from parents. Understanding how genes, alleles and chromosomes work together is essential for explaining heredity, predicting traits and appreciating the genetic basis of evolution. In IGCSE Biology, students explore the mechanisms by which genetic information is passed down and expressed, building a foundation for more advanced genetics.
变异是生命最显著的特征之一。从眼睛颜色到血型，同一物种个体之间的差异源自从父母那里继承的基因组合。理解基因、等位基因和染色体如何协同工作，对于解释遗传、预测性状以及认识进化的遗传基础至关重要。在IGCSE生物学中，学生将探索遗传信息传递和表达的机制，为更高级的遗传学学习打下基础。
1. What Is a Gene? | 什么是基因？
A gene is a specific section of DNA that contains the coded instructions for making a particular protein. Genes are located on chromosomes, which are thread‑like structures found in the nucleus of almost every cell. Each gene occupies a fixed position, called a locus, on a chromosome.
基因是DNA上的一段特定序列，含有制造某种蛋白质的编码指令。基因位于染色体上，染色体是几乎所有细胞核内存在的线状结构。每个基因在染色体上都有一个固定的位置，称为基因座。
The sequence of bases (A, T, C, G) in a gene determines the sequence of amino acids in a protein. This, in turn, dictates the protein’s shape and function. For many characteristics, a single gene controls the trait, but others are influenced by multiple genes and the environment.
基因中碱基（A、T、C、G）的排列顺序决定了蛋白质中氨基酸的序列，进而决定了蛋白质的形状和功能。许多性状由单个基因控制，但也有一些性状受多个基因和环境的共同影响。
2. The Concept of Alleles | 等位基因的概念
Although a gene defines a characteristic, there can be different versions of the same gene. These alternative forms are called alleles. For example, the gene that determines flower colour in pea plants has an allele for purple flowers and another allele for white flowers. Diploid organisms, such as humans, possess two copies of each gene—one inherited from each parent. These two copies can be the same allele or different alleles.
虽然基因决定了一个性状，但同一基因可能存在不同的版本。这些不同的形式称为等位基因。例如，豌豆植株中决定花色的基因，存在一个紫色花等位基因和一个白色花等位基因。二倍体生物（例如人类）每个基因有两个拷贝——一个来自父亲，一个来自母亲。这两个拷贝可以是相同的等位基因，也可以是不同的等位基因。
Alleles arise through mutations, which are changes in the DNA sequence. Over generations, mutations create genetic variation within a population. The combination of alleles an organism has makes up its genotype, influencing its observable characteristics.
等位基因起源于突变，即DNA序列的改变。经过世代积累，突变在种群中产生遗传变异。一个生物体所拥有的等位基因组合构成其基因型，影响着它的可观察特征。
3. Dominant and Recessive Alleles | 显性与隐性等位基因
Alleles interact in different ways. If one allele masks the expression of another when both are present, it is described as dominant. The allele whose effect is hidden is called recessive. In a heterozygous individual (one with two different alleles for a gene), only the dominant trait is observed.
等位基因之间以不同方式相互作用。如果某个等位基因在两者共存时掩盖了另一个的表达，则称为显性等位基因；其效应被掩盖的等位基因称为隐性等位基因。在一个杂合子个体（某个基因拥有两个不同等位基因）中，只能观察到显性性状。
For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A plant with the genotype Tt will be tall. The recessive allele is only expressed when an individual has two copies (tt).
例如，在豌豆中，高茎等位基因（T）对矮茎等位基因（t）为显性。基因型为Tt的植株表现为高茎。隐性等位基因只有在个体拥有两个拷贝（tt）时才会表达。
4. Genotype and Phenotype | 基因型与表型
The genotype is the complete set of alleles an organism carries for a given trait. It is the genetic constitution. The phenotype is the physical expression of that genotype—the actual observable characteristic. The phenotype depends on both the genotype and environmental factors.
基因型指的是一个生物体针对某一性状所拥有的全部等位基因，是遗传组成。表型则是该基因型的物理表现，即可实际观察到的特征。表型既取决于基因型，也受环境因素的影响。
For instance, two plants with the same genotype for height might grow to different heights if one receives more light and nutrients. However, some traits, like blood type, are almost entirely determined by genotype.
例如，两株拥有相同高度基因型的植物如果获得的光照和养分不同，可能长到不同的高度。但有些性状，如血型，几乎完全由基因型决定。
5. Homozygous and Heterozygous | 纯合子与杂合子
An individual with two identical alleles for a gene is said to be homozygous (e.g. TT or tt). A person with two different alleles is heterozygous (e.g. Tt). Being homozygous or heterozygous affects whether a recessive trait can appear.
如果个体的某个基因具有两个相同的等位基因，则称该个体为纯合子（例如TT或tt）。拥有两个不同等位基因的个体称为杂合子（例如Tt）。纯合或杂合影响隐性性状能否表现。
Recessive traits only show in the phenotype when the organism is homozygous recessive. A heterozygous carrier does not show the trait but can pass the recessive allele to offspring.
隐性性状只有在生物体为隐性纯合子时才会在表型中表现出来。杂合子携带者并不表现该性状，但可将隐性等位基因传递给后代。
6. Monohybrid Cross and Mendel’s Laws | 单基因杂交与孟德尔定律
A monohybrid cross involves studying the inheritance of a single gene. Gregor Mendel’s experiments with pea plants established the basic principles of heredity. He discovered the law of segregation: the two alleles for each gene separate during gamete formation, so each gamete carries only one allele for each gene.
单基因杂交涉及对单一基因遗传的研究。格雷戈尔·孟德尔通过豌豆实验确立了遗传的基本原理。他发现了分离定律：每个基因的两个等位基因在配子形成过程中分离，因此每个配子只携带每个基因的一个等位基因。
Mendel also observed the law of independent assortment, which states that alleles of different genes are distributed to gametes independently of one another. These laws form the basis for predicting genetic crosses.
孟德尔还发现了自由组合定律，即不同基因的等位基因独立地分配到配子中。这些定律构成了预测遗传杂交结果的基础。
7. Punnett Squares and Predicting Inheritance | 庞纳特方格与遗传预测
A Punnett square is a diagram used to predict the possible genotypes of offspring from a cross. The alleles from one parent are written across the top, and those from the other parent along the side. The combinations inside the square show the expected genotype frequencies.
庞纳特方格是一种用于预测杂交后代可能基因型的图表。将一个亲本的等位基因写在上方，另一个亲本的等位基因写在左侧。方格内的组合表示预期的基因型频率。
plants (Tt × Tt). The Punnett square below shows a 3:1 phenotypic ratio of tall to short.
考虑两个杂合高茎豌豆植株（Tt × Tt）的杂交。下面的庞纳特方格显示出3:1的高茎与矮茎表型比。
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
The genotypic ratio is 1 TT : 2 Tt : 1 tt. Because T is dominant, both TT and Tt give tall plants, leaving only tt to produce short plants.
基因型比例为1 TT : 2 Tt : 1 tt。由于T为显性，TT和Tt均表现为高茎，只有tt产生矮茎。
8. Codominance and Multiple Alleles | 共显性与复等位基因
Not all allele interactions follow simple dominance. In codominance, both alleles are expressed equally in the phenotype. A classic example is the human ABO blood group system. Three alleles exist: IA, IB and i. IA and IB are codominant, while i is recessive to both.
并非所有等位基因的相互作用都遵循简单的显隐性关系。在共显性中，两个等位基因在表型中平等表达。典型的例子是人的ABO血型系统。存在三个等位基因：IA、IB和i。IA与IB为共显性，i则对两者均为隐性。
Thus, a person with genotype IA IB has blood type AB, expressing both A and B antigens. Blood type O only occurs with the ii genotype.
因此，基因型为IA IB的人血型为AB型，同时表达A和B抗原。O型血仅在基因型为ii时出现。
| Genotype | Blood type (phenotype) |
|---|---|
| IA IA or IA i | A |
| IB IB or IB i | B |
| IA IB | AB |
| ii | O |
Multiple alleles increase the possible number of genotypes and phenotypes in a population, contributing to genetic diversity.
复等位基因增加了种群中可能的基因型和表型数量，促进了遗传多样性。
9. Sex Chromosomes and Sex Determination | 性染色体与性别决定
In humans, sex is determined by a pair of sex chromosomes: XX in females and XY in males. The other 22 pairs are autosomes. Females produce eggs that each carry an X chromosome. Males produce sperm, half carrying an X chromosome and half carrying a Y chromosome.
在人类中，性别由一对性染色体决定：女性为XX，男性为XY。其余22对为常染色体。女性产生的卵子均携带一条X染色体；男性产生的精子，一半携带X染色体，一半携带Y染色体。
The sex of the offspring depends on which sperm fertilises the egg. An X‑bearing sperm produces a female (XX); a Y‑bearing sperm produces a male (XY). The father’s sperm therefore determines the sex of the child.
后代的性别取决于哪一种精子使卵子受精。带X染色体的精子产生女性（XX），带Y染色体的精子产生男性（XY）。因此，父亲的精子决定了孩子的性别。
10. Mutations as a Source of New Alleles | 突变是新等位基因的来源
Mutations are permanent changes in the DNA sequence. They can arise spontaneously during DNA replication or be induced by mutagens such as radiation and certain chemicals. When a mutation occurs within a gene, it can create a new allele.
突变是DNA序列的永久性改变。它们可能在DNA复制过程中自发产生，也可能由辐射和某些化学物质等诱变剂诱发。当突变发生在基因内部时，就可能产生新的等位基因。
Most mutations are neutral or harmful, but occasionally a new allele provides a survival advantage. Such beneficial mutations are the raw material for natural selection and evolution. For example, the allele for sickle‑cell trait provides resistance to malaria in heterozygous individuals.
大多数突变是中性的或有害的，但偶尔也会有新的等位基因带来生存优势。这些有益突变为自然选择和进化提供了原材料。例如，镰状细胞性状的等位基因在杂合子个体中可提供对疟疾的抵抗力。
11. Pedigree Diagrams | 系谱图
A pedigree is a chart that shows the inheritance of a trait across several generations of a family. Squares represent males, circles represent females. Shaded symbols show individuals who express the trait. Pedigrees are useful for determining whether an allele is dominant, recessive or sex‑linked.
系谱图是显示一个性状在家族数代中传递情况的图表。方块代表男性，圆圈代表女性；涂色的符号表示表现出该性状的个体。系谱图有助于判断等位基因是显性、隐性还是伴性遗传的。
In a pedigree of a recessive trait, affected individuals may have unaffected parents (both heterozygous carriers). For a dominant trait, every affected individual usually has an affected parent. Analysis of pedigrees allows geneticists to advise on the probability of inheriting genetic disorders.
在隐性性状的系谱图中，患病的个体可能来自未患病（均为杂合携带者）的父母。对于显性性状，通常每个患者都有一个患病的亲本。通过系谱分析，遗传学家能够就遗传病的遗传概率提出建议。
12. Summary and Key Takeaways | 总结与关键要点
Genes are segments of DNA that code for proteins. Alternative forms of a gene, called alleles, arise by mutation and exist at the same locus on homologous chromosomes. Dominant alleles mask recessive ones, but codominant alleles can both be expressed. The genotype determines the potential phenotype, which may also be influenced by the environment. Monohybrid crosses and Punnett squares allow us to predict the inheritance of single‑gene traits. In humans, sex is determined by the XX/XY system. Multiple alleles and mutations enrich genetic variation, providing the foundation for diversity and evolution.
基因是编码蛋白质的DNA片段。同一基因的不同形式称为等位基因，通过突变产生并位于同源染色体的同一基因座上。显性等位基因掩盖隐性等位基因，但共显性等位基因可以同时表达。基因型决定了潜在的表型，而表型也可能受环境影响。单基因杂交和庞纳特方格使我们能够预测单基因性状的遗传。人类的性别由XX/XY系统决定。复等位基因和突变丰富了遗传变异，为多样性和进化奠定了基础。
Understanding these principles is central to the IGCSE Edexcel Biology specification and provides an essential toolkit for exploring more complex genetic topics, including evolution, genetic engineering and the treatment of inherited diseases.
理解这些原理是IGCSE Edexcel生物学课程的核心，也为探索更复杂的遗传学课题（包括进化、基因工程和遗传病治疗）提供了基本工具。
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