📚 IGCSE WJEC Science: Genetics – Key Revision Points | IGCSE WJEC 科学：遗传 考点精讲
Genetics is the branch of biology that studies how characteristics are passed from parents to offspring. In IGCSE WJEC Science, you need to understand the basic principles of inheritance, including the structure of DNA, genes and alleles, monohybrid crosses, and the causes of variation. This article provides a thorough revision of these key concepts to help you prepare for your exam.
遗传学是研究性状如何从父母传递给后代的生物学分支。在 IGCSE WJEC 科学中，你需要理解遗传的基本原理，包括 DNA 结构、基因和等位基因、单基因杂交以及变异的原因。本文将对这些关键概念进行透彻复习，助你备考。
1. DNA, Genes and Chromosomes | DNA、基因和染色体
The nucleus of most cells contains chromosomes, which are long strands of DNA tightly coiled around proteins. In humans, each body cell contains 23 pairs of chromosomes, giving a total of 46.
大多数细胞的细胞核含有染色体，它是 DNA 缠绕在蛋白质上形成的长链。人类每个体细胞含有 23 对染色体，共 46 条。
A gene is a short section of DNA that carries the code for making a specific protein. Genes are located at fixed positions (loci) on chromosomes.
基因是 DNA 上一小段携带制造特定蛋白质编码的片段。基因位于染色体上固定的位置（基因座）。
DNA (deoxyribonucleic acid) has a double helix structure and is made up of nucleotides containing sugar, phosphate and a base. The sequence of bases determines the amino acid sequence in a protein.
DNA（脱氧核糖核酸）具有双螺旋结构，由含有糖、磷酸和碱基的核苷酸组成。碱基序列决定了蛋白质中的氨基酸序列。
Chromosomes are passed from parents to offspring during reproduction, ensuring the continuity of genetic information across generations.
染色体在繁殖过程中从亲代传递给子代，保证了遗传信息在世代间的连续性。
2. Key Terminology: Alleles, Dominant and Recessive | 关键术语：等位基因、显性与隐性
An allele is an alternative form of a gene. For example, the gene for plant height can have a tall allele (T) and a short allele (t). Alleles occupy the same locus on homologous chromosomes.
等位基因是基因的另一种形式。例如，植物株高的基因可以有高茎等位基因 (T) 和矮茎等位基因 (t)。等位基因位于同源染色体同一基因座上。
A dominant allele is always expressed in the phenotype if present, represented by an uppercase letter (e.g. T). A recessive allele is only expressed when there is no dominant allele present, represented by a lowercase letter (e.g. t).
显性等位基因一旦存在即会在表型中表达，用大写字母表示（如 T）。隐性等位基因只有在没有显性等位基因存在时才表达，用小写字母表示（如 t）。
Homozygous means an individual has two identical alleles for a gene (e.g. TT or tt). Heterozygous means the individual has two different alleles for a gene (e.g. Tt).
纯合指个体在某基因上具有两个相同的等位基因（如 TT 或 tt）。杂合指个体具有两个不同的等位基因（如 Tt）。
The genotype is the genetic makeup of an organism (the combination of alleles), while the phenotype is the observable physical characteristic (e.g. tall or short).
基因型是生物体的遗传组成（等位基因组合），而表型是可观察的物理特征（如高或矮）。
3. Monohybrid Crosses and Mendel’s Work | 单基因杂交与孟德尔研究
Monohybrid inheritance involves the study of a single characteristic controlled by one gene. Gregor Mendel used pea plants to discover the basic laws of heredity through monohybrid crosses.
单基因遗传涉及研究由单个基因控制的一个性状。格里哥·孟德尔利用豌豆植物通过单基因杂交发现了遗传的基本定律。
In one experiment, Mendel crossed true-breeding tall plants (TT) with true-breeding short plants (tt). All F₁ offspring were tall (Tt), showing that the tall allele is dominant.
在一项实验中，孟德尔将纯种高茎植物 (TT) 与纯种矮茎植物 (tt) 杂交。所有 F₁ 子代都是高茎 (Tt)，表明高茎等位基因是显性。
When he self-pollinated the F₁ generation (Tt × Tt), the F₂ offspring showed a phenotypic ratio of approximately 3 tall : 1 short.
当他让 F₁ 世代自花传粉 (Tt × Tt) 时，F₂ 子代表现出大约 3 高茎 : 1 矮茎的表型比。
Mendel concluded that alleles segregate during gamete formation, so each gamete carries only one allele for each gene. This is the law of segregation.
孟德尔得出结论：在配子形成过程中等位基因会发生分离，因此每个配子只携带着每个基因的一个等位基因。这就是分离定律。
4. Using Punnett Squares | 使用庞纳特方格
A Punnett square is a diagram used to predict the possible genotypes of offspring from a genetic cross. The alleles from one parent are written across the top, and alleles from the other parent down the side.
庞纳特方格是一种用于预测遗传杂交后代表现型可能性的图解。将一方亲本的等位基因写在顶部，另一方亲本的等位基因写在左侧。
Each box inside the square is filled by combining the parental alleles, showing all possible outcomes. This helps calculate the probability of each genotype and phenotype.
方格内的每个格子通过组合亲本等位基因来填写，显示出所有可能的结果。这有助于计算每种基因型和表型的概率。
For a cross between two heterozygous individuals (Aa × Aa), the Punnett square gives the genotypic ratio 1 AA : 2 Aa : 1 aa.
对于两个杂合个体 (Aa × Aa) 之间的杂交，庞纳特方格产生的基因型比为 1 AA : 2 Aa : 1 aa。
Genotypic ratio = 1 AA : 2 Aa : 1 aa
表型比 = 3 显性 : 1 隐性
5. Predicting Ratios: Example Cross | 预测比例：杂交示例
Consider a heterozygous tall pea plant (Tt) crossed with another heterozygous tall (Tt). The Punnett square shows the genotypes TT, Tt, Tt, tt. This gives a 3/4 probability of tall offspring and 1/4 probability of short offspring.
考虑一株杂合高茎豌豆 (Tt) 与另一杂合高茎 (Tt) 杂交。庞纳特方格显示基因型为 TT、Tt、Tt、tt。高茎后代的概率为 3/4，矮茎为 1/4。
In a test cross, an organism with a dominant phenotype but unknown genotype is crossed with a homozygous recessive (tt). If any offspring show the recessive trait, the unknown parent must be heterozygous.
在测交中，将具有显性表型但基因型未知的个体与隐性纯合子 (tt) 杂交。如果任何后代出现隐性性状，则该未知亲本必然是杂合子。
For WJEC exams, you should be able to construct genetic diagrams, including Punnett squares, and interpret the phenotypic and genotypic ratios.
应对 WJEC 考试，你应能绘制遗传图解（包括庞纳特方格），并能解释表型比和基因型比。
6. Family Pedigrees | 家族系谱图
A pedigree chart (family tree) shows how a trait is inherited across generations. Squares represent males, circles represent females. Shaded symbols indicate an individual with the trait; unshaded symbols indicate unaffected individuals.
系谱图（家系树）展示某一性状在世系间的遗传方式。正方形代表男性，圆形代表女性。实心符号表示该个体有此性状；空心符号表示未受累个体。
Pedigrees allow us to determine whether a trait is dominant or recessive. If two unaffected parents have an affected child, the trait is usually recessive. If an affected parent always has an affected child, it suggests dominant inheritance.
系谱图能让我们判断一个性状是显性还是隐性。如果两个未受累的父母生出一个患病孩子，该性状通常是隐性的。如果患病的父母总是生出患病的孩子，则提示显性遗传。
You may be asked to deduce genotypes of individuals in a pedigree. Use the pattern of inheritance and assign alleles accordingly (e.g., D/d). Remember that individuals marrying into the family are often assumed to be homozygous unaffected unless evidence suggests otherwise.
你可能会被要求推断系谱图中个体的基因型。根据遗传模式并分配相应的等位基因（如 D/d）。记得嫁入家族的个体通常被假设为未受累纯合子，除非有其他证据。
7. Sex Determination | 性别决定
In humans, sex is determined by a pair of sex chromosomes: XY in males and XX in females. All other chromosomes (autosomes) are the same in both sexes.
在人类中，性别由一对性染色体决定：男性为 XY，女性为 XX。所有其他染色体（常染色体）在两性中相同。
A father produces two types of sperm: half carry an X chromosome and half carry a Y chromosome. A mother always produces eggs with an X chromosome. The sex of the offspring depends on which sperm fertilises the egg.
父亲产生两种类型的精子：一半携带 X 染色体，一半携带 Y 染色体。母亲总是产生携带 X 染色体的卵子。后代的性别取决于哪个精子使卵子受精。
A Punnett square for sex determination shows a 1:1 ratio of XX (female) to XY (male). Therefore, there is always a 50% chance of having a boy or a girl in each pregnancy.
性别决定的庞纳特方格显示 XX（女性）与 XY（男性）的比为 1:1。因此，每次怀孕生男生女的概率总是 50%。
Xᴹ Xᴹ × X Y → ½ XX (female) : ½ XY (male)
8. Variation: Continuous and Discontinuous | 变异：连续变异与不连续变异
Variation refers to differences between individuals of the same species. It can be caused by genetic factors, environmental factors, or a combination of both.
变异指同一物种个体间的差异。它可能由遗传因素、环境因素或两者共同导致。
Continuous variation produces a range of phenotypes without distinct categories, e.g. height, weight. It is usually controlled by many genes (polygenic) and influenced by the environment. Data often forms a normal distribution curve.
连续变异产生一系列没有明确类别的表型，如身高、体重。通常由多个基因（多基因）控制并受环境影响。数据常呈正态分布曲线。
Discontinuous variation results in distinct groups, e.g. blood type, ability to roll tongue. It is typically controlled by a single gene with limited environmental influence.
不连续变异产生截然不同的群体，如血型、卷舌能力。通常由单一基因控制，环境的影响有限。
Mutation and meiosis (crossing over and independent assortment) are major sources of genetic variation. Environmental variation includes diet, climate and lifestyle.
突变和减数分裂（交叉互换与独立分配）是遗传变异的主要来源。环境变异包括饮食、气候和生活方式。
9. Mutation and Genetic Diversity | 突变与遗传多样性
A mutation is a random change in the sequence of DNA bases. Mutations can occur spontaneously or be induced by mutagens such as radiation and certain chemicals.
突变是 DNA 碱基序列的随机变化。突变可自发发生，也可由辐射和某些化学物质等诱变剂诱导。
Mutations create new alleles, which increases genetic diversity in a population. Most mutations are neutral or harmful, but occasionally a mutation can give a survival advantage (e.g., antibiotic resistance in bacteria).
突变产生新的等位基因，增加种群内的遗传多样性。多数突变是中性的或有害的，但偶尔也会带来生存优势（如细菌的抗生素耐药性）。
An example of a harmful mutation is the allele causing sickle cell anaemia, where a single base substitution leads to abnormal haemoglobin and misshapen red blood cells.
有害突变的一个例子是导致镰刀型细胞贫血的等位基因，单个碱基替换导致异常血红蛋白和畸形红细胞。
In IGCSE WJEC Science, you need to understand that mutations add to the gene pool, and natural selection can then act on this variation, leading to evolution over time.
在 IGCSE WJEC 科学中，你需要理解突变增加了基因库的多样性，自然选择可作用于这种变异，使得物种随着时间不断进化。
10. Inherited Disorders | 遗传性疾病
Some genetic disorders are caused by recessive alleles. For a person to suffer from the disorder, they must inherit two copies of the recessive allele (e.g., cystic fibrosis). Carriers are heterozygous and do not show symptoms.
有些遗传性疾病是由隐性等位基因引起的。一个人必须遗传到两个隐性等位基因的拷贝才会患病（如囊性纤维化）。携带者为杂合子且不表现症状。
Cystic fibrosis is caused by a recessive allele on chromosome 7, leading to thick, sticky mucus that affects the lungs and digestive system. It exemplifies how a single gene mutation can have severe health effects.
囊性纤维化由 7 号染色体上的隐性等位基因引起，导致黏稠的黏液影响肺部和消化系统。它体现了单基因突变如何造成严重的健康影响。
Some disorders are dominant, such as Huntington’s disease, where only one copy of the faulty allele is needed. A person with a dominant disorder can pass it on even if the other parent is unaffected.
有些疾病是显性的，如亨廷顿病，只需要一个异常等位基因的拷贝就会发病。患有显性疾病的人即使另一方未受累，也能将此病遗传下去。
Genetic screening and counselling help families understand the risk of inherited disorders. Amniocentesis and chorionic villus sampling are techniques used to detect genetic abnormalities in unborn babies.
遗传筛查和咨询有助于家庭了解遗传性疾病的风险。羊膜腔穿刺术和绒毛膜取样技术被用于检测未出生婴儿的遗传异常。
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