📚 Alleles: Alternative Forms of Genes and Inherited Variation | 等位基因：基因的替代形式与遗传变异
Have you ever wondered why some people have blue eyes while others have brown, or why certain families carry a particular disease? The answer lies in tiny variations within our DNA called alleles. For IGCSE Biology students, understanding that genes exist in alternative forms – alleles – and that these alleles give rise to differences in inherited characteristics is a fundamental concept that underpins genetics, inheritance and evolution.
你是否想过为什么有些人有蓝眼睛而有些人是棕色的，为什么某些家族会携带某种特定疾病？答案就在我们 DNA 中微小的变异里，这些变异称为等位基因。对于 IGCSE 生物学的学生来说，理解基因以替代形式存在（即等位基因），并且这些等位基因导致了遗传性状的差异，是支撑遗传学、遗传与进化的基础概念。
1. Introduction: What Are Alleles? | 什么是等位基因？
An allele is simply an alternative version of a gene. Genes are segments of DNA that carry the instructions for making proteins, and for any given gene there may be two or more slightly different versions. These different versions are called alleles, and they arise from mutations in the DNA base sequence. Each individual inherits two alleles for every gene – one from each parent – and the combination of these alleles determines their unique set of inherited characteristics.
等位基因就是基因的替代版本。基因是携带制造蛋白质指令的 DNA 片段，而对于任何一个特定的基因，都可能存在两个或多个略有不同的版本。这些不同的版本就称为等位基因，它们源自 DNA 碱基序列的突变。每个个体从父母双方各继承一个等位基因，因此每一个基因都有两个等位基因，它们的组合决定了个体独特的遗传特征。
2. The Molecular Basis: Genes, DNA, and Chromosomes | 分子基础：基因、DNA与染色体
To fully grasp what an allele is, it helps to recall the hierarchy of genetic material. DNA is a long molecule that stores genetic information. A complete DNA molecule is organised into structures called chromosomes. Along each chromosome, there are thousands of specific sequences of DNA that code for proteins or functional RNA molecules; these sequences are called genes. Because chromosomes occur in homologous pairs (one from each parent), any given gene is present twice in a diploid cell, and the two copies may be identical alleles or different alleles.
为了完全领会等位基因是什么，先回顾遗传物质的层次结构会很有帮助。DNA 是储存遗传信息的长链分子。一条完整的 DNA 分子被组织成称为染色体的结构。沿着每条染色体，有成千上万个编码蛋白质或功能性RNA分子的特定DNA序列，这些序列就是基因。由于染色体成对出现（同源染色体，一条来自父方，一条来自母方），每一个基因在二倍体细胞中都会出现两次，这两个拷贝可能是相同的等位基因，也可能是不同的等位基因。
3. Gene Loci and Homologous Pairs | 基因座与同源染色体对
The precise physical location of a gene on a chromosome is called its locus (plural: loci). On a pair of homologous chromosomes, the same gene occupies the same locus on each chromosome. However, the DNA sequence at that locus may not be identical in both chromosomes – one may carry a normal allele while the other carries a variant allele. This is the basis of heterozygosity and explains how different alleles can be inherited separately.
基因在染色体上的精确物理位置称为基因座。在一对同源染色体上，相同的基因占据着每一染色体的同一基因座。然而，该基因座上的DNA序列可能并不完全相同——一条染色体可能携带正常等位基因，而另一条携带一个变异的等位基因。这就是杂合性的基础，也解释了不同的等位基因如何能够单独被遗传。
| Chromosome from mother | Gene A allele 1 |
| Chromosome from father | Gene A allele 2 |
Same gene locus, possibly different alleles
4. How Alleles Arise: Mutation | 等位基因的来源：突变
New alleles are created by mutation – a permanent change in the DNA sequence of a gene. Mutations can be as small as a single base substitution (point mutation) or as large as deletions or insertions of many bases. Most mutations are neutral and have no effect on the organism’s characteristics, but some create a new allele that leads to a different phenotype. Over generations, mutations build up the variety of alleles in a population’s gene pool.
新的等位基因是由突变产生的，即基因 DNA 序列上的永久性改变。突变可以小到一个碱基的替换（点突变），也可以大到多个碱基的缺失或插入。大多数突变是中性的，对生物体的性状没有影响，但有一些会产生导致不同表型的新等位基因。经过数代的积累，突变建立起一个种群基因库中等位基因的多样性。
5. Dominant and Recessive Alleles | 显性和隐性等位基因
Not all alleles have equal influence on the phenotype. When an organism is heterozygous for a gene (carrying two different alleles), one allele may mask the effect of the other. The allele that is expressed in the phenotype is called dominant, while the allele whose effect is masked is called recessive. A recessive allele only produces its characteristic phenotype when the organism carries two copies of it (homozygous recessive). This concept is fundamental to predicting the outcomes of genetic crosses.
并非所有等位基因对表型的影响都相同。当生物体在某一基因上是杂合的（携带两个不同的等位基因）时，一个等位基因可能掩盖另一个基因的效果。在表型中表达出来的等位基因称为显性，而被掩盖效果的等位基因称为隐性。隐性等位基因只有在生物体携带两个拷贝（纯合隐性）时才会表现出其特征表型。这一概念是预测遗传杂交结果的基础。
6. How Alleles Produce Different Characteristics: Protein Synthesis | 等位基因如何产生不同的性状：蛋白质合成
The key to understanding how alleles give rise to different inherited characteristics lies in protein synthesis. A gene provides the code for a specific polypeptide chain. Different alleles of a gene differ in their base sequence, so they may code for slightly different versions of the protein. This altered protein may have a different shape and function – or be non-functional – which in turn affects the organism’s phenotype. For example, a single nucleotide change in an allele can replace one amino acid in a protein, altering its biological activity.
理解等位基因如何导致不同的遗传性状，关键在于蛋白质合成。一个基因提供合成一条特定多肽链的密码。基因的不同等位基因在碱基序列上存在差异，因此可能编码出略有不同的蛋白质版本。这种改变的蛋白质可能具有不同的形状和功能——或者干脆失去功能——从而影响生物体的表型。例如，等位基因中单个核苷酸的变化就能替换蛋白质中的一个氨基酸，改变其生物活性。
DNA base sequence → mRNA codon → amino acid sequence → protein shape → phenotype
DNA碱基序列 → mRNA密码子 → 氨基酸序列 → 蛋白质形状 → 表型
7. Worked Example: Cystic Fibrosis and the CFTR Gene | 实例：囊性纤维化与CFTR基因
A classic IGCSE example that illustrates how alleles cause different characteristics is cystic fibrosis (CF). The gene involved is the CFTR gene, which codes for a chloride ion channel protein in cell membranes. The normal allele (F) produces a functional channel, while the recessive disease allele (f) carries a deletion mutation that results in a misfolded protein that cannot reach the cell surface. Individuals with genotype FF or Ff are healthy, but those homozygous ff have thick, sticky mucus in their lungs and digestive system – a striking difference in phenotype caused entirely by a single allele.
一个说明等位基因如何导致不同性状的经典IGCSE例子是囊性纤维化（CF）。涉及该病的基因是CFTR基因，它编码细胞膜上的氯离子通道蛋白。正常的等位基因（F）产生功能正常的通道，而隐性的致病等位基因（f）携带一个缺失突变，导致蛋白质折叠错误，无法到达细胞表面。基因型为FF或Ff的个体是健康的，但纯合子ff个体的肺部和消化系统会出现黏稠的黏液——这是由一个等位基因完全导致的显著表型差异。
8. Multiple Alleles and Codominance: ABO Blood Groups | 复等位基因与共显性：ABO血型
Although any diploid individual has only two alleles for a gene, a population can harbour more than two alleles – this is called multiple allelism. The human ABO blood group system is a perfect example. The gene (I) has three common alleles: IA, IB and i. Allele IA codes for the A antigen on red blood cells, IB codes for the B antigen, and i produces no functional antigen. IA and IB are codominant, meaning that in the heterozygous genotype IAIB, both antigens are expressed equally, giving rise to blood group AB. This beautifully demonstrates how different alleles can produce clearly distinguishable phenotypes in the population.
尽管任何二倍体个体只拥有一个基因的两个等位基因，但一个种群可以保有超过两个等位基因——这称为复等位现象。人类ABO血型系统就是一个完美的例子。该基因（I）有三个常见的等位基因：IA、IB和i。等位基因IA编码红细胞上的A抗原，IB编码B抗原，而i不产生功能性抗原。IA和IB是共显性的，意味着在杂合基因型IAIB中，两种抗原均等表达，产生AB血型。这优美地展示了不同的等位基因如何在种群中产生清晰可辨的表型。
9. Incomplete Dominance: Blending of Traits | 不完全显性：性状的融合
In some cases, neither allele is completely dominant. This is called incomplete dominance. For example, in snapdragon flowers, one allele codes for red pigment (CR) and another for white (CW). Heterozygous plants (CRCW) produce pink flowers because the single functional allele produces only half the amount of pigment. Here, the phenotype is intermediate between the two homozygous forms, directly showing that allele dosage can influence the expression of characteristics.
在某些情况下，没有哪个等位基因是完全占主导的。这称为不完全显性。例如，在金鱼草花中，一个等位基因编码红色色素（CR），另一个编码白色（CW）。杂合植株（CRCW）开粉红色的花，因为唯一有功能的等位基因只产生一半的色素量。在这里，表型介于两种纯合形式之间，直接表明等位基因的剂量可以影响性状的表达。
10. Alleles and Genetic Disorders | 等位基因与遗传疾病
Many inherited diseases are caused by recessive alleles that produce a non-functional or harmful protein. Carriers (heterozygotes) do not show symptoms because the normal dominant allele produces enough functional protein to keep cells working properly. This is why two healthy parents can have a child with a genetic disorder if both are carriers. Punnett squares can be used to calculate the probability of an offspring inheriting two copies of the disease allele. Understanding the concept of disease alleles helps explain the persistence of certain genetic conditions in families and populations.
许多遗传疾病是由产生无功能或有害蛋白质的隐性等位基因引起的。携带者（杂合子）不会表现出症状，因为正常的显性等位基因能够产生足够的功能蛋白，维持细胞的正常工作。这就是为什么两个健康的父母如果都是携带者，却可能生下一个患有遗传疾病的孩子的原因。可以用旁氏表计算后代继承两个致病等位基因的概率。理解致病等位基因的概念有助于解释某些遗传病在家族和种群中持续存在的原因。
11. The Role of Alleles in Evolution and Variation | 等位基因在进化和变异中的作用
Allelic variation is the raw material for evolution by natural selection. In any population, different alleles confer slight differences in characteristics. Some alleles may give individuals a survival advantage in a particular environment, making them more likely to reproduce and pass on those advantageous alleles. Over many generations, the frequency of beneficial alleles increases, while harmful alleles may decrease. This process, driven by allele differences, leads to adaptation and ultimately to the formation of new species.
等位基因的变异是自然选择进化的原始材料。在任何种群中，不同的等位基因带来性状上的细微差异。某些等位基因也许赋予个体在特定环境中的生存优势，使它们更可能繁殖并将这些有利等位基因传递给后代。经过许多世代，有利等位基因的频率上升，而有害等位基因的频率下降。这种由等位基因差异驱动的过程导致了适应，最终导致新物种的形成。
12. Summary: Key Points for Edexcel IGCSE Biology | 总结：IGCSE生物学关键点
To recap, genes are lengths of DNA that code for a protein, and they exist in alternative forms called alleles. These alleles occupy the same gene locus on homologous chromosomes and arise through mutation. The combination of alleles an individual possesses determines its inherited characteristics, with dominant alleles masking recessive ones and codominant alleles both being expressed. Whether it is eye colour, blood group or the risk of a genetic disorder, phenotypic differences among organisms are rooted in the different alleles they carry. For Edexcel IGCSE Biology, being able to explain how alleles give rise to variation is essential, as it links directly to monohybrid inheritance, Punnett squares and the understanding of genetic disorders.
重述一下，基因是编码蛋白质的一段DNA，它们以称为等位基因的替代形式存在。这些等位基因占据同源染色体上同一基因座，并通过突变产生。个体拥有的等位基因组合决定了它的遗传性状，显性等位基因会掩盖隐性等位基因，共显性等位基因则同时表达。无论是眼睛颜色、血型还是遗传疾病的风险，生物体之间的表型差异都根植于它们携带的不同等位基因。对于Edexcel IGCSE生物学来说，能够解释等位基因如何导致变异至关重要，因为它直接联系着单基因遗传、旁氏表以及对遗传疾病的理解。
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