📚 Multiple Alleles: ABO Blood Groups and Beyond | 复等位基因：以ABO血型为例及拓展
Many A-level students first meet genetics through Mendel’s pea plants, where each gene has only two alternative forms. In real populations, however, a single gene can exist in more than two allelic variants. This concept is called multiple alleles, and the ABO blood group system is the most important example you need to master for Cambridge A-level Biology. Understanding multiple alleles also clarifies how dominance, codominance and genetic notation work beyond simple monohybrid crosses.
许多 A-level 学生最初通过孟德尔的豌豆实验接触遗传学，其中每个基因只有两种相对形式。然而在真实种群中，一个基因可能存在两种以上的等位基因变异。这一概念称为复等位基因，而 ABO 血型系统是剑桥 A-level 生物中必须掌握的最重要例子。理解复等位基因还有助于厘清显性、共显性以及遗传符号在简单单基因杂交之外如何运作。
1. What Are Multiple Alleles? | 什么是复等位基因
A gene occupies a specific locus on a chromosome. A diploid organism carries two copies of each gene, one on each homologous chromosome. When three or more alternative forms of the same gene exist in a population, they are called multiple alleles. Each allele differs from the others by one or more DNA base changes, but they all occupy the same locus.
基因位于染色体上的特定位点。二倍体生物每个基因有两个拷贝，分别位于两条同源染色体上。当同一个基因在一个种群中存在三种或更多种不同形式时，这些形式就称为复等位基因。每个等位基因与其他等位基因有一个或多个 DNA 碱基差异，但它们都占据同一位点。
Crucially, multiple alleles exist at the population level. An individual diploid organism can still carry only two of them, because it has only two homologous copies of the gene. The ABO blood group gene, for example, has three common alleles, but a person inherits only two — one from each parent.
关键点在于，复等位基因是在种群水平上存在的。一个二倍体个体仍然只能携带其中两个等位基因，因为它只有两个同源基因拷贝。例如 ABO 血型基因有三种常见等位基因，但一个人只能遗传两个——一个来自父亲，一个来自母亲。
2. Multiple Alleles vs. Mendelian Two-Allele Model | 复等位基因与孟德尔双等位基因模型的区别
Mendel studied characters such as pea height or flower colour, where each trait was controlled by one gene with two alleles, one dominant and one recessive. Multiple alleles do not break Mendel’s laws; they simply extend the number of possible alleles at one locus. Segregation and independent assortment still apply because each gamete receives only one allele of a gene.
孟德尔研究的是豌豆株高或花色等性状，每个性状由一个具有两个等位基因的基因控制，其中一个显性、一个隐性。复等位基因并不违反孟德尔定律；它们只是扩展了单个基因座上可能存在的等位基因数量。分离定律和自由组合定律仍然适用，因为每个配子只能获得一个基因的一个等位基因。
It is also important not to confuse multiple alleles with polygenic inheritance. Multiple alleles involve one gene locus with many allelic variants in the population. Polygenic inheritance involves several different gene loci acting together to control one characteristic, such as height or skin colour.
同样重要的是不要把复等位基因与多基因遗传混淆。复等位基因涉及一个基因座在种群中存在多种等位基因变异。多基因遗传则涉及多个不同基因座共同控制一个性状，例如身高或肤色。
3. The ABO Blood Group System: A Classic Example | ABO血型系统：经典实例
The ABO blood group system is controlled by a single gene on chromosome 9. This gene codes for an enzyme called glycosyltransferase, which modifies a carbohydrate known as the H antigen on the surface of red blood cells. Different alleles of this gene add different sugars to the H antigen, producing different blood types.
ABO 血型系统由 9 号染色体上的一个基因控制。这个基因编码一种称为糖基转移酶的酶，该酶修饰红细胞表面的 H 抗原。这个基因的不同等位基因向 H 抗原添加不同的糖，从而产生不同的血型。
The three most common alleles are Iᴬ, Iᴮ and i. Together they produce four possible blood group phenotypes: A, B, AB and O. This makes ABO an ideal case study for multiple alleles, codominance and dominance at a single locus.
最常见的三个等位基因是 Iᴬ、Iᴮ 和 i。它们共同产生四种可能的血型表现型：A 型、B 型、AB 型和 O 型。这使得 ABO 成为研究单基因座上复等位基因、共显性和显性关系的理想案例。
4. The Three ABO Alleles: Iᴬ, Iᴮ and i | ABO三个等位基因：Iᴬ、Iᴮ和i
Allele Iᴬ codes for an enzyme that adds N-acetylgalactosamine to the H antigen, producing antigen A. Allele Iᴮ codes for a slightly different enzyme that adds galactose to the H antigen, producing antigen B. Allele i produces a non-functional enzyme, so the H antigen remains unchanged; this gives neither A nor B antigen and corresponds to blood group O.
等位基因 Iᴬ 编码一种酶，将 N-乙酰半乳糖胺添加到 H 抗原上，产生 A 抗原。等位基因 Iᴮ 编码略有不同的酶，将半乳糖添加到 H 抗原上，产生 B 抗原。等位基因 i 产生无功能的酶，因此 H 抗原保持不变；这样既不产生 A 抗原也不产生 B 抗原，对应 O 型血。
In terms of dominance, Iᴬ and Iᴮ are both dominant over i. This means a person with genotype Iᴬi has type A blood, and a person with genotype Iᴮi has type B blood. The i allele is recessive and only shows its effect in the homozygous genotype ii.
就显性关系而言，Iᴬ 和 Iᴮ 对 i 均为显性。这意味着基因型为 Iᴬi 的人为 A 型血，基因型为 Iᴮi 的人为 B 型血。i 等位基因是隐性的，只有在纯合基因型 ii 中才表现其效应。
5. Genotypes and Phenotypes in ABO Blood Group | ABO血型的基因型与表现型
Because there are three alleles, there are six possible genotypes but only four phenotypes. The table below summarises the relationship between ABO genotypes and phenotypes.
由于存在三个等位基因，共有六种可能的基因型，但只有四种表现型。下表总结了 ABO 基因型与表现型之间的关系。
| Phenotype | 表现型 | Possible genotypes | 可能的基因型 | Antigen on red blood cells | 红细胞上的抗原 |
|---|---|---|
| A | IᴬIᴬ or Iᴬi | A antigen |
| B | IᴮIᴮ or Iᴮi | B antigen |
| AB | IᴬIᴮ | Both A and B antigens |
| O | ii | Neither A nor B antigen |
From the table, you can see that phenotype A can result from two different genotypes, IᴬIᴬ and Iᴬi. The same is true for phenotype B. In genetic diagrams, you must therefore show the possible genotypes, not just the phenotypes, when predicting offspring blood groups.
从表中可以看出，A 型表现型可以由 IᴬIᴬ 和 Iᴬi 两种基因型产生。B 型表现型也是如此。因此，在遗传图解中预测子代血型时，必须写出可能的基因型，而不仅仅是表现型。
6. Codominance in ABO Inheritance | ABO遗传中的共显性
Codominance occurs when both alleles in a heterozygous individual are fully expressed. In the genotype IᴬIᴮ, the Iᴬ allele produces A antigen and the Iᴮ allele produces B antigen. Both antigens appear on the red blood cell surface, giving the AB phenotype.
共显性是指杂合个体中的两个等位基因都得到充分表达。在基因型 IᴬIᴮ 中，Iᴬ 等位基因产生 A 抗原，Iᴮ 等位基因产生 B 抗原。两种抗原都出现在红细胞表面，形成 AB 表现型。
This is different from incomplete dominance, where the heterozygote shows an intermediate phenotype, and from complete dominance, where one allele masks the other. In ABO blood groups, neither Iᴬ nor Iᴮ is recessive to the other; they are equal in expression when paired.
这不同于不完全显性，即杂合子表现出中间表现型；也不同于完全显性，即一个等位基因掩盖另一个等位基因。在 ABO 血型中，Iᴬ 和 Iᴮ 彼此都不隐性；当它们配对时，表达是相等的。
7. Predicting ABO Blood Group Inheritance | 预测ABO血型遗传
Genetic crosses involving ABO multiple alleles follow normal Mendelian principles. Consider a cross between a heterozygous blood group A parent (Iᴬi) and a heterozygous blood group B parent (Iᴮi). The possible gametes are Iᴬ and i from the first parent, and Iᴮ and i from the second parent.
涉及 ABO 复等位基因的遗传杂交遵循正常的孟德尔原理。考虑 A 型杂合亲本（Iᴬi）与 B 型杂合亲本（Iᴮi）之间的杂交。第一个亲本可产生 Iᴬ 和 i 两种配子，第二个亲本可产生 Iᴮ 和 i 两种配子。
The Punnett square gives four equally likely offspring genotypes: IᴬIᴮ, Iᴬi, Iᴮi and ii. The corresponding phenotypes are AB, A, B and O, in a ratio of 1 : 1 : 1 : 1. This ratio is very different from the 3 : 1 ratio seen in a simple monohybrid cross with two alleles.
庞纳特方格给出四种等概率的子代基因型：IᴬIᴮ、Iᴬi、Iᴮi 和 ii。对应的表现型分别为 AB、A、B 和 O，比例为 1 : 1 : 1 : 1。这一比例与只有两个等位基因的简单单基因杂交中常见的 3 : 1 比例有很大不同。
Another common cross is Iᴬi × Iᴬi. Both parents produce gametes Iᴬ and i. The offspring genotypes are IᴬIᴬ, Iᴬi, Iᴬi and ii, giving a phenotypic ratio of 3 blood group A : 1 blood group O. Here the dominant Iᴬ allele masks the recessive i allele in the heterozygotes.
另一个常见杂交是 Iᴬi × Iᴬi。两个亲本都产生 Iᴬ 和 i 配子。子代基因型为 IᴬIᴬ、Iᴬi、Iᴬi 和 ii，表现型比例为 3 个 A 型血 : 1 个 O 型血。此处显性 Iᴬ 等位基因在杂合子中掩盖了隐性 i 等位基因。
8. Antigens and Antibodies: The Molecular Basis | 抗原与抗体：分子基础
The different ABO phenotypes are not just genetic labels; they have important immunological consequences. A person with blood group A has A antigens on red blood cells and produces anti-B antibodies in the plasma. A person with blood group B has B antigens and anti-A antibodies.
不同的 ABO 表现型不仅是遗传标签，还具有重要的免疫学意义。A 型血的人红细胞上有 A 抗原，血浆中产生抗 B 抗体。B 型血的人有 B 抗原和抗 A 抗体。
Blood group AB individuals have both A and B antigens, so they produce neither anti-A nor anti-B antibodies. Blood group O individuals have neither A nor B antigens, so they produce both anti-A and anti-B antibodies. These antibody patterns explain why ABO matching is essential for safe blood transfusion.
AB 型血的人同时有 A 和 B 抗原，因此既不产生抗 A 也不产生抗 B 抗体。O 型血的人既没有 A 抗原也没有 B 抗原，因此同时产生抗 A 和抗 B 抗体。这些抗体模式解释了为什么 ABO 配型对安全输血至关重要。
Because group O red blood cells lack A and B antigens, they can be given to patients of any ABO group; O is often called the universal donor. Because group AB plasma lacks anti-A and anti-B antibodies, AB individuals can receive red cells from any ABO group; AB is often called the universal recipient.
由于 O 型红细胞缺少 A 和 B 抗原，它们可以输给任何 ABO 血型的患者；O 型常被称为万能供血者。由于 AB 型血浆缺乏抗 A 和抗 B 抗体，AB 型个体可以接受任何 ABO 血型的红细胞；AB 型常被称为万能受血者。
9. Other Examples of Multiple Alleles | 复等位基因的其他实例
The ABO system is the most exam-relevant example, but multiple alleles occur in many organisms. In rabbits, a single coat-colour gene has multiple alleles that produce full colour, chinchilla, Himalayan and albino phenotypes. These alleles show a dominance hierarchy rather than simple recessive inheritance.
ABO 系统是最常考的实例，但复等位基因存在于许多生物中。在家兔中，一个毛色基因有多个等位基因，可产生全色、银灰色、喜马拉雅色和白化等表现型。这些等位基因表现出等级显性关系，而不是简单的隐性遗传。
In plants such as Brassica, self-incompatibility is controlled by many alleles at a single S locus. Pollen with an S allele that matches either S allele in the pistil is rejected, preventing self-fertilisation. This is another clear example of multiple alleles at one locus creating many possible mating types in a population.
在芸苔属等植物中，自交不亲和性由单个 S 基因座上的许多等位基因控制。如果花粉的 S 等位基因与雌蕊中任一 S 等位基因相同，该花粉就会被拒绝，从而防止自花受精。这是又一个清晰的例子，说明单基因座上的复等位基因可以在种群中产生许多交配类型。
10. Exam Tips and Common Misconceptions | 考试提示与常见误区
A common exam error is to state that an individual can have three or more alleles for one gene. In fact, a diploid individual has only two alleles at a locus. Multiple alleles means that many different alleles exist across the population, not within one person’s cells.
考试中一个常见错误是认为一个个体可以拥有一个基因的三个或更多等位基因。事实上，二倍体个体在一个基因座上只有两个等位基因。复等位基因是指种群中存在许多不同的等位基因，而不是一个人的细胞中有多个等位基因。
Another misconception is to confuse the allele i with the absence of a gene. The i allele is a real allele; it simply codes for a non-functional enzyme. Blood group O results from the genotype ii, not from missing the ABO gene.
另一个误区是把等位基因 i 与基因缺失混淆。i 等位基因是一个真实存在的等位基因，它只不过编码无功能的酶。O 型血来自基因型 ii，而不是因为缺少 ABO 基因。
When constructing genetic diagrams, always define your symbols, show the parental genotypes, state the possible gametes, and use a Punnett square where appropriate. For ABO crosses, write the allele symbols clearly and do not use A and B as if they were single letters; use Iᴬ, Iᴮ and i to avoid confusion.
在绘制遗传图解时，一定要定义符号，写出亲本基因型，列出可能的配子，并在适当之处使用庞纳特方格。对于 ABO 杂交，要清楚地书写等位基因符号，不要用单个字母 A 和 B，而要使用 Iᴬ、Iᴮ 和 i，以免混淆。
11. Summary | 总结
Multiple alleles arise when more than two allelic forms of a gene exist in a population, even though each diploid individual carries only two. The ABO blood group system shows how three alleles — Iᴬ, Iᴮ and i — produce four phenotypes through a combination of dominance and codominance. Mastering this topic will help you answer both straightforward inheritance questions and more applied questions on blood transfusion and genetic prediction.
当种群中存在同一基因的两种以上等位基因形式时，就出现了复等位基因，尽管每个二倍体个体只携带两个。ABO 血型系统展示了三个等位基因——Iᴬ、Iᴮ 和 i——如何通过显性与共显性的组合产生四种表现型。掌握这一主题将帮助你解答直接的遗传题以及涉及输血和遗传预测的应用题。
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