📚 Inheritance | 遗传
Welcome to this comprehensive revision guide on inheritance, a fundamental topic in the Edexcel IGCSE Biology specification. Inheritance explains how traits are passed from parents to offspring through genes, which are sections of DNA. Understanding key concepts such as alleles, dominant and recessive traits, monohybrid crosses, and family pedigrees is essential for success in your exams. This article breaks down the entire topic into clear, bite‑sized sections, with paired English and Chinese explanations to support bilingual learners.
欢迎阅读这份关于遗传的综合复习指南，它是爱德思IGCSE生物课程中的核心主题。遗传揭示了性状如何通过DNA上的基因由亲代传递给子代。掌握等位基因、显性与隐性性状、单基因杂交及家族系谱图等关键概念对于考试至关重要。本文将这些内容拆分为清晰的小节，每小节均提供配对的中英文解释，帮助双语学习者牢固掌握。
1. The Genome and Genes | 基因组与基因
The genome is the entire set of genetic material in an organism, composed of DNA. In humans, the genome is organised into 23 pairs of chromosomes found inside the nucleus of most cells. A gene is a short section of DNA that codes for a specific protein. Proteins determine the characteristics of an organism, from eye colour to enzyme function. Each gene has a fixed position on a chromosome, known as its locus.
基因组是指生物体内全部遗传物质的总和，由DNA组成。在人类中，基因组被组织成23对染色体，位于大多数细胞的细胞核内。基因是DNA上编码特定蛋白质的一小段序列。蛋白质决定生物体的特征，从眼睛颜色到酶的功能。每个基因在染色体上都有固定的位置，称为基因座。
DNA itself has a double helix structure made of two strands held together by complementary base pairs – adenine (A) with thymine (T), and cytosine (C) with guanine (G). The sequence of these bases forms the genetic code. When a gene is expressed, the DNA sequence is transcribed into mRNA and then translated into a chain of amino acids, which folds into a functional protein.
DNA本身具有双螺旋结构，两条链通过互补碱基配对连接——腺嘌呤（A）与胸腺嘧啶（T），胞嘧啶（C）与鸟嘌呤（G）。这些碱基的序列构成了遗传密码。当基因表达时，DNA序列被转录为信使RNA，然后翻译为氨基酸链，最终折叠成有功能的蛋白质。
2. Chromosomes and DNA Packaging | 染色体与DNA的包装
In the nucleus, DNA is tightly coiled around histone proteins to form chromosomes. This packaging allows the very long DNA molecules to fit inside the tiny nuclear space. Humans have 46 chromosomes in each body cell (23 pairs), with one set inherited from the mother and one from the father. The 23rd pair determines biological sex – XX in females and XY in males.
在细胞核中，DNA紧密缠绕在组蛋白上形成染色体。这种包装使极长的DNA分子能够容纳在微小的细胞核内。人体每个体细胞含有46条染色体（23对），一套来自母亲，一套来自父亲。第23对染色体决定生物性别——女性为XX，男性为XY。
During cell division, chromosomes replicate and become visible under a light microscope. A duplicated chromosome consists of two identical chromatids joined at the centromere. The number of chromosomes is characteristic of each species; for example, fruit flies have 8 chromosomes, while garden peas have 14. Any change in chromosome number can lead to genetic disorders, such as Down syndrome, which is caused by an extra copy of chromosome 21.
在细胞分裂过程中，染色体会复制并在光学显微镜下可见。一条复制后的染色体由两条相同的染色单体构成，它们在着丝粒处相连。染色体数目是物种的特征性标志；例如，果蝇有8条染色体，而豌豆有14条。染色体数目的任何改变都可能导致遗传病，如唐氏综合征就是由第21号染色体多出一条引起的。
3. Alleles: Dominant and Recessive Forms | 等位基因：显性与隐性
Genes can exist in different versions called alleles. For any given gene, an individual inherits one allele from each parent. Alleles may be dominant or recessive. A dominant allele is always expressed in the phenotype even if only one copy is present. A recessive allele is only expressed when two copies are inherited (one from each parent) and there is no dominant allele present.
基因可以有不同的版本，称为等位基因。对于任何一个基因，个体从父母双方各继承一个等位基因。等位基因可以是显性或隐性。显性等位基因即使只存在一个拷贝也会在表型中表达。隐性等位基因只有在从父母各继承一个拷贝且没有显性等位基因存在时才会表达。
We often use letters to represent alleles, with capital letters for dominant alleles and lowercase letters for recessive alleles. For example, in pea plants, the allele for tall height (T) is dominant over the allele for dwarf height (t). A plant with genotype TT or Tt will be tall, while only the homozygous tt plant will be dwarf.
我们通常用字母表示等位基因，大写字母代表显性等位基因，小写字母代表隐性等位基因。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或Tt的植株表现为高茎，只有纯合的tt植株是矮茎。
4. Key Genetic Terms: Homozygous, Heterozygous, Genotype, Phenotype | 关键遗传学术语：纯合子、杂合子、基因型、表型
To solve genetics problems, you must master four terms. Homozygous means having two identical alleles for a gene (e.g., TT or tt). Heterozygous means having two different alleles (e.g., Tt). The genotype is the combination of alleles an organism possesses, while the phenotype is the observable characteristic that results from the genotype and environmental influences.
要解决遗传学问题，你需要掌握四个术语。纯合子 指一个基因的两个等位基因完全相同（如TT或tt）。杂合子 指两个等位基因不同（如Tt）。基因型 是生物体拥有的等位基因组合，而表型 是由基因型与环境共同作用形成的可观察特征。
It is important to realise that the phenotype is not always a direct read‑out of the genotype. For instance, identical twins have the same genotype but may show slightly different phenotypes due to environmental factors. In exam questions, you must be able to deduce possible genotypes from given phenotypes and use them to predict offspring traits.
你需要认识到，表型并不总是基因型的直接反映。例如，同卵双胞胎具有相同的基因型，但由于环境因素可能表现出细微的表型差异。在考试中，你必须能够从给定表型推断可能的基因型，并用它们预测后代的性状。
5. Monohybrid Inheritance and Punnett Squares | 单基因遗传与庞纳特方格
two heterozygous individuals (Tt × Tt), the expected genotype ratio is 1 TT : 2 Tt : 1 tt.
单基因遗传研究的是一个基因的两个等位基因。通过庞纳特方格可以将遗传杂交直观地展示出来，它显示了亲本配子的所有可能组合，帮助预测后代的基因型比例和表型比例。对于两个杂合个体（Tt × Tt）的杂交，预期的基因型比例为1 TT : 2 Tt : 1 tt。
re placed along the top, and those from the other parent along the side. Combining each gamete gives the offspring genotypes.
下面是Tt × Tt杂交的庞纳特方格。一方的配子列在顶部，另一方的配子列在左侧，结合每一对配子即可得到后代的基因型。
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Phenotypic ratio 3 : 1 (tall : dwarf)
The phenotypic ratio is 3 tall to 1 dwarf because both TT and Tt result in the tall phenotype. This classic 3:1 ratio appears whenever two heterozygous parents are crossed for a trait showing complete dominance. In exam diagrams, always label the gametes and write the offspring genotypes clearly.
表型比例为3高 : 1矮，因为TT和Tt均表现为高茎。只要两个杂合亲本杂交，且性状呈完全显性，就会出现这种经典的3:1比例。在考试作图中，一定要标明配子并清晰写出后代的基因型。
6. Interpreting Family Pedigrees | 解读家族系谱图
A family pedigree is a chart that shows the inheritance of a trait across several generations. Squares represent males and circles represent females. Shaded symbols indicate individuals who express the trait, while unshaded symbols indicate those who do not. By analysing the pattern of shaded and unshaded symbols, you can determine whether the trait is dominant or recessive.
家族系谱图是显示某一性状在多个世代中传递情况的图表。方形代表男性，圆形代表女性。实心符号表示表现该性状的个体，空心符号表示不表现该性状的个体。通过分析实心和空心的分布模式，你可以判断该性状是显性还是隐性。
If the trait is dominant, every affected individual will have at least one affected parent. If the trait is recessive, affected individuals can have parents who are both unaffected carriers. In the pedigree, you can also infer genotypes of individuals: for a recessive trait, an affected person must be homozygous recessive, while an unaffected person could be homozygous dominant or heterozygous.
如果性状是显性的，每个患病的个体至少有一个患病的亲本。如果性状是隐性的，患病个体的父母可能都是未患病的携带者。在系谱图中，你还可以推断个体的基因型：对于隐性性状，患者必然是隐性纯合子，而未患病者可能是显性纯合子或杂合子。
7. Sex Determination in Humans | 人类的性别决定
Sex in humans is determined by the 23rd pair of chromosomes, known as the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During gamete formation, eggs always carry a single X chromosome, whereas sperm carry either an X or a Y chromosome. Therefore, the sex of the baby is determined by the sperm that fertilises the egg.
人类的性别由第23对染色体——性染色体决定。女性有两条X染色体（XX），男性有一条X染色体和一条Y染色体（XY）。在配子形成过程中，卵细胞总是携带一条X染色体，而精子携带一条X染色体或一条Y染色体。因此，婴儿的性别由使卵细胞受精的精子决定。
The Punnett square for sex inheritance shows a 1:1 ratio of male to female offspring. Because the Y chromosome carries the SRY gene, which triggers male development, the presence of a Y chromosome leads to a male phenotype. This simple chromosomal mechanism means that, in each pregnancy, there is a 50% chance of having a boy and a 50% chance of having a girl.
性别决定的庞纳特方格显示出男女后代1:1的比例。由于Y染色体携带触发男性发育的SRY基因，Y染色体的存在导致男性表型。这一简单的染色体机制意味着，每次怀孕生男孩和生女孩的概率各为50%。
8. Sources of Variation | 变异的来源
Variation refers to the differences between individuals of the same species. It can be caused by genetic factors, environmental factors, or a combination of both. Genetic variation arises from mutations, the random fusion of gametes during fertilisation, and the independent assortment and crossing over of chromosomes during meiosis.
变异是指同一物种个体之间的差异。它可以由遗传因素、环境因素或二者共同引起。遗传变异来源于突变、受精时配子的随机融合以及减数分裂过程中染色体的独立分配和交叉互换。
Environmental factors such as diet, climate, and exercise also influence an organism's phenotype. For example, a plant grown in abundant sunlight may grow taller than a genetically identical plant grown in shade. Only genetic variation can be passed on to the next generation, because it is encoded in the DNA of gametes.
环境因素如饮食、气候和锻炼也会影响生物体的表型。例如，在充足阳光下生长的植物可能比遗传上相同的遮荫植物长得更高。只有遗传变异能够传递给下一代，因为它编码在配子的DNA中。
9. Mutations and Their Effects | 突变及其影响
A mutation is a random change in the sequence of DNA bases, which may create a new allele. Mutations can occur spontaneously during DNA replication or be induced by mutagenic agents such as ionising radiation and certain chemicals. Most mutations are neutral or harmful, but occasionally a mutation provides an advantage – for example, enabling bacteria to survive in the presence of an antibiotic.
突变是DNA碱基序列的随机改变，可能产生新的等位基因。突变可以在DNA复制过程中自发发生，也可以由电离辐射和某些化学物质等诱变剂诱导。大多数突变是中性的或有害的，但偶尔也会提供有利条件——例如，使细菌能够在抗生素存在下存活。
A change in just one base (a point mutation) can alter a protein's shape and function, as seen in sickle‑cell anaemia. Mutations in gametes can be inherited, while mutations in body cells only affect the individual. The rate of mutation is increased by exposure to mutagens, which is why wearing sunscreen and avoiding tobacco smoke help protect DNA.
仅一个碱基的改变（点突变）就可能改变蛋白质的形状和功能，镰刀型细胞贫血症就是一个例子。发生在配子中的突变可以遗传，而体细胞突变仅影响个体本身。突变率会因接触诱变剂而升高，这就是为什么涂防晒霜和避免烟草烟雾有助于保护DNA。
10. Summary and Exam Tips | 总结与考试技巧
Inheritance questions in Edexcel IGCSE Biology often require you to construct genetic diagrams, interpret pedigrees, and calculate offspring ratios. Always define the alleles first, show the parental genotypes, list the gametes, and then complete a clear Punnett square. Remember to state the phenotypic ratio to avoid losing marks for incomplete answers.
爱德思IGCSE生物考试中的遗传题经常要求你构建遗传图解、解读系谱图并计算后代比例。一定要先定义等位基因，写出亲本基因型，列出配子，然后完成清晰的庞纳特方格。务必写出表型比例，以免因答案不完整而丢分。
Practical investigations, such as modelling monohybrid crosses using beads or cards, can help solidify understanding. Also, practise linking the concepts of meiosis, fertilisation, and mutations to explain how variation arises. By mastering the vocabulary and practising a wide range of genetic crosses, you will be well prepared for any inheritance question on your exam paper.
动手探究，如用珠子或卡牌模拟单基因杂交，有助于巩固理解。此外，练习将减数分裂、受精和突变等概念联系起来解释变异是如何产生的。通过掌握相关术语并广泛练习各种遗传杂交，你将为试卷上任何一道遗传题做好充分准备。
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