📚 Essential Genetic Terminology for IGCSE Biology | IGCSE生物学必会遗传术语
In IGCSE Biology, especially for the Edexcel specification, a solid grasp of genetic terms is crucial for explaining how characteristics are inherited. This article explains the meanings of dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance, using clear examples and Punnett squares to build your confidence.
在IGCSE生物学（特别是Edexcel考试局）中，牢固掌握遗传学术语对于解释性状如何遗传至关重要。本文将借助清晰的例子和旁氏表阐释显性、隐性、纯合、杂合、表现型、基因型以及共显性的含义，帮助你建立信心。
1. Genetics and Heredity | 遗传与遗传学
Genetics is the branch of biology that studies how traits are passed from parents to offspring. Heredity refers to the actual transmission of genetic information. In the mid-19th century, Gregor Mendel's experiments with pea plants laid the foundation for our understanding of inheritance. He discovered that organisms carry pairs of 'factors' (now called genes) that determine their characteristics.
遗传学是研究性状如何从亲代传递给子代的生物学分支。遗传指的是遗传信息的实际传递过程。19 世纪中期，格雷戈尔·孟德尔的豌豆实验为理解遗传奠定了基础。他发现生物携带成对的“因子”（现称基因），这些因子决定了生物的特征。
Modern genetics reveals that genes are made of DNA and are located on chromosomes inside the cell nucleus. Diploid organisms, including humans and pea plants, have two copies of each chromosome — one from each parent — so each gene is present in two copies (alleles). These alleles may be identical or different, leading to the variation we see in populations.
现代遗传学表明，基因由 DNA 构成，位于细胞核内的染色体上。包括人类和豌豆在内的二倍体生物，每种染色体都有两套拷贝——一套来自父方，一套来自母方——因此每个基因都存在两个拷贝（等位基因）。这些等位基因可能相同也可能不同，从而造就了种群中可见的变异。
2. Genes, Alleles and Chromosomes | 基因、等位基因与染色体
A gene is a section of DNA that contains the coded instructions for making a specific protein. Different versions of the same gene are called alleles. For example, the gene that influences pea flower colour can have a 'purple' allele and a 'white' allele. Since diploid organisms inherit one allele from each parent, they can carry two copies of the same allele or two different alleles for a given gene.
基因是一段 DNA，含有制造特定蛋白质的编码指令。同一基因的不同版本称为等位基因。例如，影响豌豆花色的基因可以有一个“紫色”等位基因和一个“白色”等位基因。由于二倍体生物从亲代各继承一个等位基因，针对某一基因，它们可能携带两个相同的等位基因拷贝，也可能携带两个不同的等位基因。
Alleles occupy the same position (locus) on homologous chromosomes. When the two alleles are identical, the organism is said to be homozygous for that trait. When they are different, the organism is heterozygous. The combination of alleles an individual possesses is its genotype, while the observable physical or biochemical expression of those alleles is its phenotype.
等位基因位于同源染色体上的相同位置（基因座）。当两个等位基因相同时，称为纯合；当它们不同时，称为杂合。个体拥有的等位基因组合就是其基因型，而这些等位基因所表达出的可观察的物理或生化特征则是其表现型。
3. Dominant and Recessive Alleles | 显性与隐性等位基因
In a heterozygous individual, one allele may mask the effect of another. The allele that is expressed in the phenotype even when only one copy is present is called dominant. The allele whose effect is only seen when two copies are present is termed recessive. For instance, in pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). A plant with genotype Pp will have purple flowers because the dominant P allele masks the presence of the recessive p.
在杂合子个体中，一个等位基因可能掩盖另一个的作用。在仅有一个拷贝时就能在表现型中表达的等位基因称为显性。只有在两个拷贝都存在时才能看到其效应的等位基因称为隐性。例如，在豌豆植株中，紫花等位基因（P）对白花等位基因（p）呈显性。基因型为 Pp 的植株将开紫花，因为显性的 P 掩盖了隐性 p 的存在。
Dominant alleles are usually represented by an uppercase letter (e.g. P), while recessive alleles are given the corresponding lowercase letter (e.g. p). It is important to remember that 'dominant' does not mean 'more common' — in some populations, a recessive trait can be more frequent than the dominant one.
显性等位基因通常用大写字母表示（如 P），隐性等位基因则用对应的小写字母表示（如 p）。需要记住的是，“显性”并不意味着“更常见”——在某些种群中，隐性性状可能比显性性状更为普遍。
4. Homozygous and Heterozygous Genotypes | 纯合与杂合基因型
A genotype is homozygous when an individual has two identical alleles for a gene. It can be homozygous dominant (e.g. PP) or homozygous recessive (e.g. pp). In contrast, a heterozygous genotype contains two different alleles (e.g. Pp). The heterozygous condition often produces a phenotype identical to the homozygous dominant, because the dominant allele is expressed.
当个体某个基因的两个等位基因相同时，该基因型为纯合。它可以是纯合显性（如 PP）或纯合隐性（如 pp）。相反，杂合基因型含有两个不同的等位基因（如 Pp）。杂合状态往往产生与纯合显性相同的表现型，因为显性等位基因得到了表达。
Homozygous organisms are sometimes called 'true-breeding' because they produce gametes carrying only one type of allele. A heterozygous organism will produce two types of gametes in roughly equal proportions, each carrying a different allele. This distinction is fundamental when predicting the outcomes of genetic crosses.
纯合生物有时被称为“纯种”，因为它们产生的配子只携带一种类型的等位基因。杂合生物则将产生大致相等比例的两种配子，每种配子携带不同的等位基因。这一区别在预测遗传杂交结果时至关重要。
5. Genotype and Phenotype | 基因型与表现型
The genotype is the genetic makeup of an organism — the specific combination of alleles it carries. The phenotype is the observable characteristic resulting from the interaction of the genotype with the environment. For example, a pea plant with genotype PP or Pp has a purple-flower phenotype, while pp results in white flowers. However, environmental factors such as soil pH can also influence flower colour intensity, demonstrating that phenotype is not solely determined by genotype.
基因型是生物的遗传组成，即它所携带的特定等位基因组合。表现型则是基因型与环境相互作用后产生的可观察特征。例如，具有 PP 或 Pp 基因型的豌豆植株表现出紫花表现型，而 pp 则表现为白花。然而，土壤酸碱度等环境因素也会影响花色深浅，这表明表现型并非完全由基因型决定。
In the context of genetic crosses, we often use the terms 'phenotypic ratio' and 'genotypic ratio' elds a genotypic ratio of 1 PP : 2 Pp : 1 pp and a phenotypic ratio of 3 purple : 1 white.
在遗传杂交中，我们常用“表现型比率”和“基因型比率”来描述子代中不同性状或不同等位基因组合出现的比例。两个杂合子（Pp × Pp）的经典单基因杂交会产生 1 PP : 2 Pp : 1 pp 的基因型比率，以及 3 紫 : 1 白的表现型比率。
Genotypic ratio: 1 PP : 2 Pp : 1 pp
基因型比率：1 PP : 2 Pp : 1 pp
Phenotypic ratio: 3 purple : 1 white
表现型比率：3 紫 : 1 白
6. Understanding Codominance | 理解共显性
Codominance occurs when both alleles in a heterozygous organism are fully expressed, so the phenotype is a combination of both traits, rather than one masking the other. This differs from simple dominance, where only the dominant allele's effect is visible. A classic example of codominance in Edexcel IGCSE Biology is the ABO blood group system in humans.
共显性出现在杂合子个体的两个等位基因都完全表达时，此时表现型是两种性状的组合，而非一个掩盖另一个。这与简单显性不同，在简单显性中只有显性等位基因的效应可见。Edexcel IGCSE 生物学中典型的共显性例子是人类的 ABO 血型系统。
In ABO blood groups, there are three main alleles: IA, IB and i. The IA and IB alleles are codominant to each other, meaning that a person with the genotype IAIB will express both A and B antigens on their red blood cells, resulting in blood group AB. Both IA and IB are dominant over the i allele, which produces no functional antigen and results in blood group O when homozygous (ii).
在 ABO 血型系统中，主要有三种等位基因：IA、IB 和 i。IA 和 IB 彼此呈共显性，这意味着基因型为 IAIB 的人会在红细胞上同时表达 A 和 B 两种抗原，从而形成 AB 血型。IA 和 IB 对 i 等位基因都是显性的，i 等位基因不产生功能性抗原，在纯合（ii）时导致 O 血型。
Another example of codominance is seen in some cattle breeds where a red-haired bull (CRCR) crossed with a white-haired cow (CWCW) produces offspring with a mixture of red and white hairs (roan, CRCW). Both hair colours are expressed, creating a distinct roan pattern.
另一个共显性的例子见于一些牛品种：红毛公牛（CRCR）与白毛母牛（CWCW）交配，产生的后代兼有红毛和白毛（沙毛，CRCW）。两种毛色同时表达，形成独特的沙色斑纹。
7. Monohybrid Crosses with Punnett Squares | 单基因杂交与旁氏表
A monohybrid cross examines the inheritance of a single gene. A Punnett square is a grid used to predict the possible genotypes of offspring from a given pair of parents. To build a Punnett square, write the possible gametes from one parent along the top and the possible gametes from the other parent down the side. Then fill in the boxes by combining the alleles.
单基因杂交研究的是单个基因的遗传。旁氏表是一种用来预测特定亲本组合可能产生的子代基因型的网格。构建旁氏表时，将一个亲本可能产生的配子写在顶部，将另一亲本的配子写在侧边，然后通过组合等位基因填充各个格子。
es are P and p from each parent. The Punnett square below shows all possible combinations.
考虑两个杂合紫花豌豆植株（Pp × Pp）之间的杂交。每个亲本的配子为 P 和 p。下方的旁氏表展示了所有可能的组合。
| P | p | |
| P | PP | Pp |
| p | Pp | pp |
The completed square shows that there is a 1 in 4 (25%) chance of producing a homozygous dominant offspring (PP), a 2 in 4 (50%) chance of a heterozygous offspring (Pp) and a 1 in 4 (25%) chance of a homozygous recessive offspring (pp). In terms of phenotype, 3 out of 4 are expected to have purple flowers while 1 out of 4 will have white flowers.
完成的表格显示：产生纯合显性子代（PP）的概率为 1/4（25%），杂合子代（Pp）的概率为 2/4（50%），纯合隐性子代（pp）的概率为 1/4（25%）。就表现型而言，预期 3/4 的植株开紫花，1/4 开白花。
8. Worked Example: ABO Blood Groups | 实例剖析：ABO血型遗传
Let us apply the concepts of codominance, genotype and phenotype to human blood groups. A man with blood group A (genotype IAi) and a woman with blood group B (genotype IBi) wish to know the possible blood groups of their children. Since the man produces gametes IA and i, and the woman produces IB and i, a Punnett square can be used.
让我们将共显性、基因型和表现型等概念应用于人类血型。一位 A 型血（基因型 IAi）的男性和一位 B 型血（基因型 IBi）的女性希望了解他们孩子可能出现的血型。由于男性产生 IA 和 i 两种配子，女性产生 IB 和 i 两种配子，因此可以使用旁氏表进行预测。
| IB | i | |
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