📚 GCSE CIE Biology: Genetics Key Points Explained | GCSE CIE 生物：遗传学 考点精讲
Genetics is the branch of biology that studies heredity and variation in living organisms. In the CIE GCSE Biology syllabus, understanding how genetic information is passed from parents to offspring and how it determines characteristics is essential. This article breaks down key concepts, from DNA structure to single-gene inheritance patterns, genetic disorders, and exam strategies, helping you to revise effectively and tackle exam questions with confidence.
遗传学是生物学的重要分支，研究生物的遗传与变异。在 CIE GCSE 生物学考纲中，理解遗传信息如何由亲代传递给子代，以及如何决定性状，是核心考点之一。本文系统梳理了从 DNA 结构到单基因遗传模式、遗传病及应试技巧等关键内容，帮助你高效复习，从容应对考试。
1. DNA, Genes and Chromosomes | DNA、基因与染色体
DNA (deoxyribonucleic acid) is a long molecule organised into a double helix. It is made up of repeating units called nucleotides, each consisting of a sugar, a phosphate group, and a nitrogenous base (A, T, C, or G). The sequence of these bases stores the genetic code.
DNA（脱氧核糖核酸）是一种双螺旋结构的长链分子，由称为核苷酸的重复单元组成，每个核苷酸包含一个糖、一个磷酸基团和一个含氮碱基（A、T、C 或 G）。碱基的排列顺序储存了遗传密码。
A gene is a short section of DNA that codes for a specific protein. Proteins are responsible for most of an organism’s characteristics. For example, a gene may code for an enzyme that determines eye colour or for haemoglobin in red blood cells.
基因是 DNA 上的一段特定序列，负责编码某种蛋白质。蛋白质决定了生物体的大部分性状。例如，某个基因可以编码决定眼睛颜色的酶，或编码红细胞中的血红蛋白。
Chromosomes are thread-like structures found in the nucleus of most cells. They consist of DNA tightly coiled around proteins called histones. Human body cells contain 46 chromosomes arranged in 23 pairs; one chromosome in each pair comes from the mother and the other from the father.
染色体是存在于大多数细胞核中的线状结构，由 DNA 紧密缠绕在组蛋白上构成。人体细胞含有 46 条染色体，组成 23 对；每对染色体中一条来自母亲，另一条来自父亲。
2. Alleles and Inheritance | 等位基因与遗传
An allele is one of two or more alternative forms of a gene. For any given gene, an individual inherits one allele from each parent. Alleles occupy the same position (locus) on a pair of homologous chromosomes.
等位基因是一个基因的两种或多种可替换形式之一。对于任意一个基因，个体从每个亲本各继承一个等位基因。等位基因位于同源染色体上的相同位置（基因座）。
If the two alleles are identical, the individual is homozygous for that gene. If they are different, the individual is heterozygous. The combination of alleles an organism possesses is its genotype, and this underlying genetic make-up influences the visible characteristics, or phenotype.
如果两个等位基因相同，该个体在该基因上是纯合的；如果不同，则是杂合的。生物体拥有的等位基因组合称为基因型，这一潜在的遗传组成影响了可见的性状，即表型。
3. Dominant and Recessive Alleles | 显性与隐性等位基因
A dominant allele is one that is always expressed in the phenotype, even if only one copy is present. A recessive allele is only expressed when two copies are present (i.e., when the individual is homozygous recessive). In genetics diagrams, dominant alleles are often represented by an uppercase letter (e.g., R) and recessive alleles by a lowercase letter (e.g., r).
显性等位基因是指只要存在一个拷贝，其性状就会在表型中表现出来。隐性等位基因只有在存在两个拷贝时（即个体为隐性纯合子）才会表达。在遗传图解中，显性等位基因常用大写字母表示（如 R），隐性等位基因用小写字母表示（如 r）。
In heterozygous individuals, the dominant allele masks the recessive allele. For instance, if the allele for tall stems (T) is dominant to the allele for dwarf stems (t), a plant with genotype Tt will be tall.
在杂合个体中，显性等位基因掩盖了隐性等位基因。例如，若高茎等位基因 (T) 对矮茎等位基因 (t) 为显性，那么基因型为 Tt 的植株表现为高茎。
4. Genotype and Phenotype | 基因型与表型
The genotype is the genetic constitution of an organism, while the phenotype is the physical expression of this genotype, influenced by both genes and the environment. For example, two plants may have the same genotype for height, but one grown in poor soil may be shorter—demonstrating environmental interaction.
基因型是生物体的遗传组成，而表型是基因型在与环境相互作用下表现出的特征。例如，两株植物可能具有相同的高矮基因型，但生长在贫瘠土壤中的一株会更矮——这体现了环境的影响。
When solving genetic problems, clearly distinguish between genotype and phenotype ratios. A monohybrid cross between two heterozygous parents often produces a genotype ratio of 1 : 2 : 1 and a phenotype ratio of 3 : 1 when dominance is complete.
在解决遗传问题时，要清楚地区分基因型比例和表型比例。在完全显性条件下，两个杂合亲本的单基因杂交通常产生 1 : 2 : 1 的基因型比例和 3 : 1 的表型比例。
5. Monohybrid Inheritance and Punnett Squares | 单基因遗传与庞纳特方格
Monohybrid inheritance involves the inheritance of a single characteristic controlled by one gene with two alleles. A Punnett square is a grid used to predict the genotypes of offspring from a particular cross. All possible gametes from one parent are written along the top, and those from the other parent along the side.
单基因遗传指由一个基因的两个等位基因控制的单一性状的遗传。庞纳特方格是一种用于预测特定杂交后代基因型的表格。将一方亲本所有可能的配子写在顶行，另一方亲本的配子写在侧列。
For example, a cross between two heterozygous parents for a trait where B (brown eyes) is dominant to b (blue eyes):
例如，两个杂合亲本杂交，B（棕色眼）对 b（蓝色眼）为显性：
| B | b | |
| B | BB | Bb |
| b | Bb | bb |
The predicted phenotype ratio is 3 brown-eyed : 1 blue-eyed offspring. Always state both ratios where asked, and check whether the question requires a probability, a percentage, or a ratio.
预测表型比例为 3 棕眼 : 1 蓝眼。在回答问题时，如果要求，应同时给出比例，并留意题目要求的是概率、百分比还是比例。
6. Family Pedigrees | 家族系谱图
A family pedigree chart traces the inheritance of a trait through several generations. Standard symbols are used: squares represent males (□), circles represent females (○), and shaded symbols (■ or ●) indicate individuals expressing the trait. A horizontal line between two shapes represents mating, and vertical lines show offspring.
家族系谱图用于追踪某一性状在数代中的遗传情况。使用标准符号：方框代表男性 (□)，圆圈代表女性 (○)，实心符号 (■ 或 ●) 表示表现出该性状的个体。两符号之间的横线表示婚配，竖线指向后代。
- □ – unaffected male
- ○ – unaffected female
- ■ – affected male
- ● – affected female
中文对照：□ – 未患病男性；○ – 未患病女性；■ – 患病男性；● – 患病女性。
To determine whether a trait is dominant or recessive from a pedigree, look for affected offspring from unaffected parents. If two unaffected parents produce an affected child, the trait must be recessive, because the parents must be heterozygous carriers.
要从系谱图判断某一性状是显性还是隐性，注意观察未患病父母是否生出患病子女。若未患病的父母生出患病孩子，则该性状必定为隐性，因为父母必然是杂合携带者。
If an autosomal dominant trait is shown, every affected individual will have at least one affected parent (unless new mutation), and the trait does not skip generations.
若是常染色体显性性状，每个患病个体通常至少有一位患病亲本（除非新发突变），该性状不会隔代出现。
7. Sex Determination | 性别决定
In humans, sex is determined by a pair of sex chromosomes: females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The 22 remaining pairs are called autosomes and are the same in both sexes.
人类的性别由一对性染色体决定：女性有两条 X 染色体 (XX)，男性有一条 X 和一条 Y 染色体 (XY)。其余 22 对染色体为常染色体，男女相同。
During sperm production, a male produces two types of sperm in equal numbers: half carry an X chromosome (23,X) and half carry a Y chromosome (23,Y). All eggs from the mother carry one X chromosome (23,X). A Punnett square below illustrates the 1 : 1 sex ratio.
在精子形成过程中，男性产生两种数量相等的精子：一半携带 X 染色体 (23,X)，一半携带 Y 染色体 (23,Y)。母亲所有的卵细胞都携带一条 X 染色体 (23,X)。下面的庞纳特方格展示了 1 : 1 的性别比例。
| X (23,X) | Y (23,Y) | |
| X (23,X) | XX (female) | XY (male) |
Therefore, the probability of having a boy or a girl is 50% each, and the father’s sperm determines the sex of the child.
因此，生男生女的概率各为 50%，父亲的精子决定了孩子的性别。
8. Co-dominance and Blood Groups | 共显性与血型
Co-dominance occurs when both alleles in a heterozygous individual are fully expressed in the phenotype, without blending. The ABO blood group system is a classic example. The gene responsible has three alleles: Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are co-dominant, while i is recessive to both.
共显性是指杂合个体中两个等位基因都能在表型中完全表现出来，而非混合。ABO 血型系统是一个经典例子。控制血型的基因有三种等位基因：Iᴬ、Iᴮ 和 i。Iᴬ 与 Iᴮ 为共显性，i 对两者均为隐性。
The possible genotypes and resulting phenotypes are:
可能的基因型及相应的表型如下：
| Genotype | Phenotype (Blood Group) |
| Iᴬ Iᴬ or Iᴬ i | Group A |
| Iᴮ Iᴮ or Iᴮ i | Group B |
| Iᴬ Iᴮ | Group AB (both A and B antigens) |
| ii | Group O |
In co-dominance, the phenotype ratios from heterozygous crosses may differ from the typical 3 : 1 ratio. For example, a cross between Iᴬ i (group A) and Iᴮ i (group B) can produce all four blood types in the offspring.
在共显性遗传中，杂合子杂交的表型比例可能与典型的 3 : 1 不同。例如，Iᴬ i（A 型）与 Iᴮ i（B 型）杂交，后代可能出现全部四种血型。
9. Mutations | 基因突变
A mutation is a permanent change in the DNA sequence of a gene or chromosome. Gene mutations involve changes in a single gene, such as base substitution, insertion, or deletion, which can alter the protein produced. Mutations may arise spontaneously during DNA replication or be induced by mutagens such as ionising radiation and certain chemicals.
突变是指基因或染色体中 DNA 序列的永久性改变。基因突变涉及单个基因的变化，如碱基替换、插入或缺失，这些变化可能改变所编码的蛋白质。突变可在 DNA 复制时自然发生，也可由电离辐射和某些化学物质等诱变剂诱导产生。
Most mutations are neutral or harmful, but some can produce new alleles that increase variation in a population. A well-known example is the sickle cell allele (HbS), a base substitution in the gene for the β-globin chain of haemoglobin. This single nucleotide change causes valine to replace glutamic acid, leading to distorted red blood cells.
大多数突变是中性的或有弊的，但有些能产生新等位基因，增加种群的变异。一个著名的例子是镰刀型细胞贫血症等位基因 (HbS)，即血红蛋白 β-珠蛋白基因中的一个碱基替换。这一单个核苷酸的改变导致谷氨酸被缬氨酸取代，引起红细胞变形。
10. Genetic Disorders – Cystic Fibrosis and Sickle Cell Anaemia | 遗传病——囊性纤维化与镰刀型细胞贫血症
Cystic fibrosis (CF) is an autosomal recessive disorder caused by a defective allele of the CFTR gene. The mutated allele produces a faulty protein that disrupts chloride ion transport, leading to thick, sticky mucus in the lungs, pancreas, and other organs.
囊性纤维化 (CF) 是一种常染色体隐性遗传病，由 CFTR 基因的缺陷等位基因引起。突变等位基因产生异常的蛋白质，干扰氯离子转运，导致肺、胰腺等器官积聚黏稠的黏液。
If both parents are heterozygous carriers (genotype Ff), the Punnett square shows:
如果父母双方均为杂合携带者（基因型 Ff），庞纳特方格展示如下：
| F | f | |
| F | FF (normal) | 更多咨询请联系16621398022（同微信）
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