📚 GCSE AQA Science: Inheritance Revision Highlights | GCSE AQA 科学：遗传 考点精讲
Inheritance is a core topic in GCSE AQA Science, covering how genetic information is passed from parents to offspring. This guide breaks down the essential ideas you need for the exam: DNA structure, alleles, monohybrid crosses, inherited disorders, sex determination and more.
遗传是 GCSE AQA 科学的核心主题，涉及遗传信息如何从亲代传递给子代。本文为你梳理考试必须掌握的核心概念：DNA 结构、等位基因、单基因杂交、遗传病、性别决定等。
1. DNA, Genes and Chromosomes | DNA、基因与染色体
DNA is a double‑stranded polymer that stores the genetic code. A gene is a short section of DNA that codes for a particular protein, and each gene is located at a specific position on a chromosome. In the nucleus of a typical human body cell, there are 46 chromosomes arranged in 23 pairs.
DNA 是储存遗传密码的双链聚合物。基因是 DNA 上编码特定蛋白质的一小段，每个基因位于染色体上的特定位置。在普通人体细胞的细胞核中，有 46 条染色体，排列成 23 对。
Chromosomes are long, coiled molecules of DNA. One chromosome from each pair is inherited from the mother and the other from the father, so offspring carry a mix of parental characteristics.
染色体是长链卷曲的 DNA 分子。每对染色体一条来自母亲，另一条来自父亲，因此后代携带父母的混合特征。
2. Alleles, Genotype and Phenotype | 等位基因、基因型与表现型
An allele is a different version of the same gene. For any gene, you have two alleles – one on each matching chromosome. The genotype is the combination of alleles an organism has (e.g. BB, Bb or bb), while the phenotype is the observable characteristic produced (e.g. brown eyes or blue eyes).
等位基因是同一基因的不同版本。对于任何基因，你都有两个等位基因——分别位于两条同源染色体上。基因型是个体所拥有的等位基因组合（如 BB、Bb 或 bb），而表现型是观察到的特征（如棕色眼睛或蓝色眼睛）。
If both alleles are the same, the individual is homozygous for that trait. If the alleles are different, the individual is heterozygous.
如果两个等位基因相同，该个体对该性状是纯合的；如果等位基因不同，则是杂合的。
3. Monohybrid Crosses and Dominant/Recessive Alleles | 单基因杂交与显隐性等位基因
A monohybrid cross looks at the inheritance of a single characteristic controlled by one gene. In these crosses, a dominant allele (represented by a capital letter) masks the effect of a recessive allele (lowercase letter) when they are together. The recessive phenotype only appears when an individual has two recessive alleles.
单基因杂交考察由单个基因控制的性状的遗传。在这些杂交中，显性等位基因（大写字母表示）会掩盖隐性等位基因（小写字母）的作用。隐性表现型只有当个体拥有两个隐性等位基因时才会出现。
For example, in pea plants the allele for tall stems (T) is dominant over the allele for dwarf stems (t). A plant with genotype Tt will be tall because the dominant allele is expressed.
例如，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 的植株会表现为高茎，因为显性等位基因得以表达。
4. Using Punnett Squares | 使用庞纳特方格
A Punnett square is a grid used to predict the genotypes and phenotypes of offspring from two parents. You place one parent’s gamete alleles along the top and the other parent’s down the side, then fill in the combinations.
庞纳特方格是用来预测子代基因型和表现型的网格。将一个亲本的配子等位基因写在顶部，另一个亲本的写在侧边，然后填充组合。
Example cross between two heterozygous tall pea plants (Tt × Tt):
两个杂合高茎豌豆 (Tt × Tt) 的杂交示例：
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
This gives an expected phenotypic ratio of 3 tall : 1 dwarf. The genotypic ratio is 1 TT : 2 Tt : 1 tt. Exam questions often ask you to calculate probabilities as percentages or fractions.
预期的表现型比例为 3 高 : 1 矮。基因型比例为 1 TT : 2 Tt : 1 tt。考题常要求以百分比或分数计算概率。
5. Family Pedigree Charts | 家族遗传系谱图
A pedigree chart shows the inheritance of a trait across several generations. Squares represent males, circles represent females, and shading usually indicates individuals who have the trait being studied. You can use the chart to deduce genotypes and whether the allele is dominant or recessive.
系谱图显示一个性状在几代人中的遗传情况。方框代表男性，圆圈代表女性，阴影通常表示拥有所研究性状的个体。你可以利用系谱图推断基因型以及等位基因是显性还是隐性。
If the trait appears in every generation, it is likely caused by a dominant allele. If it skips generations and appears in children of unaffected parents, the allele is probably recessive and the parents are carriers.
如果性状在每一代中都出现，很可能是由显性等位基因引起的。如果性状隔代出现，且未患病的父母却生出患病孩子，那么该等位基因很可能是隐性的，父母是携带者。
6. Inherited Disorders: Cystic Fibrosis | 遗传病：囊性纤维化
Cystic fibrosis (CF) is an inherited disorder caused by a recessive allele (f). The disorder affects the production of mucus, making it thick and sticky – this can block airways and pancreatic ducts. People with CF need daily physiotherapy and medication.
囊性纤维化 (CF) 是一种由隐性等位基因 (f) 引起的遗传病。该病影响黏液生成，使其变得浓稠黏腻，堵塞气道和胰腺导管。CF 患者需要每日理疗和药物治疗。
To have CF, a person must inherit two copies of the recessive allele (ff). If two carriers (Ff) have a child, there is a 1 in 4 chance the child will have CF (ff), a 1 in 2 chance the child will be a carrier (Ff), and a 1 in 4 chance the child will be homozygous dominant (FF) and unaffected.
要患该病，必须遗传两个隐性等位基因 (ff)。如果两个携带者 (Ff) 生育子女，孩子患囊性纤维化 (ff) 的几率为 1/4，成为携带者 (Ff) 的几率为 1/2，纯合显性正常 (FF) 的几率为 1/4。
7. Inherited Disorders: Polydactyly | 遗传病：多指症
Polydactyly is a condition where a person has extra fingers or toes. Unlike cystic fibrosis, it is caused by a dominant allele (D). This means that having just one copy of the dominant allele (DD or Dd) results in the disorder.
多指症是指患者拥有额外手指或脚趾的一种疾病。与囊性纤维化不同，它由显性等位基因 (D) 引起。这意味着只要有一个显性等位基因 (DD 或 Dd) 就会患病。
A person with polydactyly who is heterozygous (Dd) can pass the allele to half of their children if the other parent is homozygous recessive (dd). This leads to a 50% probability that each child inherits the condition.
如果多指症患者为杂合子 (Dd)，且另一方为纯合隐性正常 (dd)，那么每个孩子有 50% 的概率继承多指症。
8. Sex Determination | 性别决定
In humans, sex is determined by a pair of sex chromosomes: XX for females and XY for males. All egg cells carry an X chromosome. Sperm cells can carry either an X or a Y chromosome, so the father’s sperm determines the baby’s sex.
人类的性别由一对性染色体决定：XX 为女性，XY 为男性。所有卵细胞都携带一条 X 染色体。精子细胞可携带 X 或 Y 染色体，因此父亲的精子决定了婴儿的性别。
Using a genetic cross:
用遗传杂交图表示：
| X | X | |
| X | XX | XX |
| Y | XY | XY |
There is a 50% chance of a female (XX) and a 50% chance of a male (XY) at each pregnancy.
每次怀孕生女孩 (XX) 的几率为 50%，生男孩 (XY) 的几率也为 50%。
9. Gregor Mendel’s Experiments | 孟德尔的实验
Gregor Mendel was an Austrian monk who carried out breeding experiments on pea plants in the mid‑19th century. He carefully counted offspring and noticed that traits were inherited in predictable ratios, leading him to propose the idea of ‘units of inheritance’ – now known as genes.
格雷戈尔·孟德尔是 19 世纪中叶进行豌豆杂交实验的奥地利修道士。他仔细统计后代数量，发现性状按照可预测的比例遗传，进而提出了“遗传单位”的概念——即今天的基因。
Mendel’s work was not widely accepted during his lifetime, but later experiments confirmed his findings. Three key conclusions were: characteristics are determined by alleles passed from parents; some alleles are dominant and others recessive; and each gamete carries only one allele for each gene.
孟德尔的发现在当时未被广泛接受，但后来的实验证实了他的结论。三个关键结论是：性状由亲代传递的等位基因决定；有些等位基因是显性的，有些是隐性的；每个配子只携带每个基因的一个等位基因。
10. Understanding Genetic Variation and Mutations | 理解遗传变异与突变
Variation between individuals of the same species arises from differences in their genes (genetic variation) and from the environment. Mutations are random changes in the DNA base sequence that can create new alleles, increasing genetic diversity.
同一物种个体间的变异来源于基因差异（遗传变异）和环境因素。突变是 DNA 碱基序列的随机改变，可产生新的等位基因，从而增加遗传多样性。
Most mutations have no effect, but some can change the shape of a protein so it no longer works correctly. A very small number may give an organism a survival advantage, driving evolution by natural selection.
大多数突变没有影响，但有些会改变蛋白质的形状使其无法正常工作。极少数突变可能赋予生物体生存优势，通过自然选择推动进化。
11. Key Definitions and Common Misconceptions | 关键定义与常见误区
Students often mix up the terms gene and allele. A gene is a section of DNA coding for a trait, while an allele is a variant of that gene. Also, dominant does not mean ‘more common’ or ‘better’ – it simply describes the allele that is expressed in the heterozygous condition.
学生们经常混淆“基因”和“等位基因”。基因是编码性状的一段 DNA，而等位基因是该基因的变体。同样，“显性”并不代表“更常见”或“更好”——它仅描述在杂合状态下得以表达的等位基因。
Another common error is thinking that a 3:1 ratio in a Punnett square guarantees exactly three out of four offspring will show the dominant trait. In reality, this is a probability for each independent fertilization event, and small sample sizes can deviate significantly.
另一个常见错误是认为庞纳特方格中的 3:1 比例意味着四个后代中一定有三个表现显性性状。实际上，这是每次独立受精的概率，小样本可能明显偏离该比例。
Always define your symbols clearly (e.g. A = normal, a = cystic fibrosis) and show working in genetic diagram questions. Use the terminology homozygous, heterozygous, genotype and phenotype accurately in your exam answers.
在遗传图解题中一定要明确符号含义（如 A = 正常，a = 囊性纤维化），并展示步骤。在考试答案中准确使用纯合、杂合、基因型和表现型等术语。
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