📚 Understanding Genetic Terms: Alleles, Dominant, Recessive, Genotype and Phenotype | 理解遗传学术语：等位基因、显性、隐性、基因型与表型
Genetics is the study of how traits are passed from parents to offspring. In Edexcel IGCSE Biology, a clear understanding of the terms ‘gene’, ‘allele’, ‘genotype’ and ‘phenotype’ is essential for answering inheritance questions.
遗传学是研究亲代如何将性状传递给子代的科学。在Edexcel IGCSE生物中，清晰理解“基因”、“等位基因”、“基因型”和“表型”这些术语，对于回答遗传类题目至关重要。
This article explains each key term step by step, using examples and a genetic cross to help you build confidence.
本文将逐步解释每个关键术语，并借助实例和遗传杂交图解，帮助你建立信心。
1. Introduction to Genetics | 遗传学导论
Genetics is the study of heredity and variation. Heredity is the passing of characteristics from one generation to the next, while variation is the differences between individuals.
遗传学是研究遗传与变异的科学。遗传是指特征从一代传递到下一代，而变异则是个体之间的差异。
Every living organism has a set of instructions that determines its features. In IGCSE Biology, you need to know how these instructions are stored and transmitted.
每种生物都有一套决定其特征的指令。在IGCSE生物中，你需要了解这些指令如何储存和传递。
The key building blocks are DNA, chromosomes, genes and alleles.
关键的构成要素包括DNA、染色体、基因和等位基因。
2. What Is a Gene? | 什么是基因？
A gene is a short section of DNA that codes for a specific protein. This protein may control a particular characteristic, such as eye colour or blood type.
基因是一小段DNA，它编码特定的蛋白质。这种蛋白质可能控制某种特定特征，例如眼睛颜色或血型。
In the Edexcel IGCSE specification, you must understand that a gene is a sequence of DNA bases that codes for a protein.
在Edexcel IGCSE大纲中，你必须理解基因是一段DNA碱基序列，编码一种蛋白质。
Each gene occupies a specific position on a chromosome called a ‘locus’.
每个基因在染色体上占据一个特定的位置，称为“基因座”。
3. Chromosomes and DNA | 染色体与DNA
Chromosomes are long, coiled molecules of DNA found in the nucleus. They carry genes in a linear order.
染色体是细胞核中长的、螺旋状的DNA分子。它们按线性顺序承载基因。
In humans, most body cells contain 23 pairs of chromosomes, making 46 in total. One pair, the sex chromosomes, determines the sex of the individual.
在人类中，大多数体细胞含有23对染色体，共46条。其中一对性染色体决定个体的性别。
During gamete formation, chromosomes are halved so that each gamete contains only one set, i.e. 23 chromosomes in humans.
在配子形成过程中，染色体数量减半，因此每个配子只含有一套染色体，人类即23条。
This halving is essential for maintaining a constant chromosome number from generation to generation.
这种减半对于维持世代间染色体数目恒定至关重要。
4. What Is an Allele? | 什么是等位基因？
An allele is a different version of the same gene. For a given characteristic, a person may inherit two alleles, one from each parent.
等位基因是同一基因的不同版本。对于某一特征，个体可能从父母双方各继承一个等位基因。
For example, the gene for eye colour has a ‘brown eye’ allele and a ‘blue eye’ allele. These are alternative forms of the same gene.
例如，眼睛颜色基因有“棕色眼睛”等位基因和“蓝色眼睛”等位基因。它们是同一基因的不同形式。
Alleles are located at the same position, or locus, on homologous chromosomes.
等位基因位于同源染色体的相同位置，即基因座。
You must be able to recognise that different alleles may lead to different versions of a characteristic.
你必须能够识别出不同的等位基因可能导致特征的不同表现。
5. Genotype | 基因型
The genotype is the genetic makeup of an organism for a particular trait. It refers to the alleles an individual possesses.
基因型是有机体在某一特定性状上的遗传组成。它指个体所拥有的等位基因。
For example, a person may have a genotype that contains two brown-eye alleles, written as ‘BB’.
例如，一个人的基因型可能包含两个棕色眼睛等位基因，写作“BB”。
The genotype is usually represented by letters: capital letters for dominant alleles and lower-case letters for recessive alleles.
基因型通常用字母表示：大写字母代表显性等位基因，小写字母代表隐性等位基因。
You should also know that genotype is hidden; you cannot see it directly. It must be inferred from the phenotype and family history.
你还应知道基因型是隐藏的，不能直接看到。必须通过表型和家族史来推断。
6. Phenotype | 表型
The phenotype is the observable characteristic or physical appearance of an organism. It is the result of the genotype interacting with the environment.
表型是有机体可观察到的特征或外貌。它是基因型与环境相互作用的结果。
For example, ‘brown hair’, ‘blue eyes’ and ‘tall height’ are phenotypes.
例如，“棕色头发”、“蓝色眼睛”和“高个子”都是表型。
Two individuals with the same genotype do not always have exactly the same phenotype, because environmental factors can affect the trait.
基因型相同的两个个体不一定具有完全相同的表型，因为环境因素可能影响该性状。
In IGCSE exam questions, you may be asked to identify the phenotype from a genetic diagram.
在IGCSE考试题中，你可能会被要求从遗传图解中识别表型。
7. Dominant and Recessive Alleles | 显性与隐性等位基因
A dominant allele is an allele that is always expressed in the phenotype, even if only one copy is present.
显性等位基因是一种即使在只有一个拷贝时也总是会在表型中表现出来的等位基因。
A recessive allele is only expressed in the phenotype when two copies are present, i.e. when there is no dominant allele.
隐性等位基因只有在存在两个拷贝时才会在表型中表现出来，即没有显性等位基因时。
In genetic notation, a dominant allele is written with a capital letter, such as ‘B’. A recessive allele is written with the same letter in lower-case, such as ‘b’.
在遗传学记法中，显性等位基因用大写字母表示，如“B”。隐性等位基因用相同字母的小写形式表示，如“b”。
- If the genotype is ‘BB’ or ‘Bb’, the dominant trait appears.
- 如果基因型是“BB”或“Bb”，显性性状就会表现。
- If the genotype is ‘bb’, the recessive trait appears.
- 如果基因型是“bb”，隐性性状就会表现。
8. Homozygous and Heterozygous | 纯合子与杂合子
A homozygous individual has two identical alleles for a gene. This can be homozygous dominant (‘BB’) or homozygous recessive (‘bb’).
纯合子个体拥有一个基因的两个相同等位基因。这可以是显性纯合子（“BB”）或隐性纯合子（“bb”）。
A heterozygous individual has two different alleles for a gene, such as ‘Bb’.
杂合子个体拥有一个基因的两个不同等位基因，如“Bb”。
Heterozygous individuals are also called ‘carriers’ when they carry a recessive allele that is not expressed.
当杂合子携带未表达的隐性等位基因时，也称其为“携带者”。
For a recessive genetic disorder, a heterozygous person will not show the disorder but can pass the recessive allele to their offspring.
对于隐性遗传病，杂合子不会表现该疾病，但可能把隐性等位基因传递给后代。
9. Constructing a Genetic Cross | 构建遗传杂交
A genetic cross is a diagram that shows how alleles are passed from parents to offspring. It helps predict the probability of each genotype and phenotype in the next generation.
遗传杂交图解是一种显示等位基因如何从亲代传递给子代的图。它有助于预测下一代中每种基因型和表型的概率。
Follow these steps for a monohybrid cross:
单基因杂交应遵循以下步骤：
- Write down the parent genotypes.
- 写下亲本的基因型。
- Separate the alleles into gametes.
- 将等位基因分开到配子中。
- Combine the gametes in a Punnett square.
- 在庞尼特方格中组合配子。
- State the genotype and phenotype ratios.
- 写出基因型和表型的比例。
A simple cross between a homozygous dominant parent (‘BB’) and a homozygous recessive parent (‘bb’) is shown below.
以下是显性纯合亲本（“BB”）与隐性纯合亲本（“bb”）之间的简单杂交。
| B | B | |
| b | Bb | Bb |
| b | Bb | Bb |
All offspring are heterozygous (‘Bb’) and show the dominant phenotype.
所有后代都是杂合子（“Bb”），并表现显性表型。
10. Worked Example: Inheriting a Trait | 实例：性状的遗传
Consider a cross between two heterozygous parents (‘Bb’ × ‘Bb’), where ‘B’ gives brown eyes and ‘b’ gives blue eyes.
考虑两个杂合亲本（“Bb” × “Bb”）的杂交，其中“B”决定棕色眼睛，“b”决定蓝色眼睛。
| B | b | |
| B | BB | Bb |
| b | Bb | bb |
The genotype ratio of the offspring is 1 BB : 2 Bb : 1 bb.
后代的基因型比例为1 BB : 2 Bb : 1 bb。
The phenotype ratio is 3 brown eyes : 1 blue eye.
表型比例为3棕色眼睛 : 1蓝色眼睛。
Phenotypic ratio = 3 : 1 (dominant : recessive)
表型比例 = 3 : 1（显性 : 隐性）
11. Common Misconceptions | 常见误解
One common mistake is thinking that a dominant allele is more common in a population. Dominance and allele frequency are not the same thing.
一个常见错误是认为显性等位基因在群体中更常见。显性性和等位基因频率不是一回事。
Another mistake is confusing ‘homozygous dominant’ with ‘heterozygous’. Both show the dominant phenotype, but they have different genotypes.
另一个错误是混淆“显性纯合子”和“杂合子”。两者都表现显性表型，但基因型不同。
Students often forget that the genotype ratio and phenotype ratio are different. Always state both clearly.
学生经常忘记基因型比例和表型比例是不同的。务必清楚写明两者。
Also remember that recessive traits can skip a generation because they are hidden in heterozygous carriers.
还要记住，隐性性状可能会在杂合携带者中隐藏，从而隔代遗传。
12. Exam Tips and Summary | 考试提示与总结
In the Edexcel IGCSE exam, definitions of these terms are frequently tested. Learn them exactly and use the correct vocabulary.
在Edexcel IGCSE考试中，这些术语的定义经常被测试。请准确记忆并使用正确的词汇。
When answering questions, always write the allele symbols clearly and show your working for genetic crosses.
回答问题时，始终清晰写出等位基因符号，并展示遗传杂交的计算过程。
Use capital letters for dominant alleles and lower-case letters for recessive alleles, even if you use a different letter in your answer.
使用大写字母表示显性等位基因，小写字母表示隐性等位基因，即使你使用不同的字母也没有关系。
Finally, remember that the phenotype is not always fully controlled by the genotype; environmental factors can also play a role.
最后，请记住，表型并非完全由基因型控制；环境因素也可能起作用。
By mastering these terms and practising Punnett squares, you will be well prepared for this section of your IGCSE Biology exam.
通过掌握这些术语并练习庞尼特方格，你将能够从容应对IGCSE生物考试中的这一部分。
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