📚 A-Level Biology: Genetics Core Concepts | A-Level 生物：遗传学核心概念精讲
Genetics is the branch of biology that studies heredity and variation, explaining how traits are passed from parents to offspring. In A-Level biology, a solid grasp of Mendelian principles, genetic terminology, and the molecular mechanisms behind inheritance is essential for tackling exam questions on monohybrid and dihybrid crosses, sex linkage, epistasis, and population genetics. This article distills the key points into concise, bilingual explanations to help you master the topic efficiently.
遗传学是研究遗传与变异的生物学分支，解释性状如何从亲代传递给子代。在 A-Level 生物学中，牢固掌握孟德尔原理、遗传学术语及其背后的分子机制，对于解答单基因杂交、双基因杂交、性连锁、上位性以及群体遗传学等问题至关重要。本文提炼核心考点，以简洁的双语解释帮助你高效掌握这一主题。
1. Mendelian Laws of Inheritance | 孟德尔遗传定律
Gregor Mendel established two fundamental laws based on his pea plant experiments. The Law of Segregation states that each organism carries two alleles for a trait, and these alleles separate during gamete formation so that each gamete receives only one allele. The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes.
孟德尔通过豌豆实验确立了两条基本定律。分离定律指出，每个生物体携带一对控制某一性状的等位基因，在配子形成过程中这对等位基因彼此分离，每个配子只含有一个等位基因。自由组合定律指出，不同基因的等位基因在配子形成过程中独立分配，前提是这些基因位于不同的染色体上。
These laws form the basis of Mendelian inheritance patterns, which can be clearly observed in monohybrid and dihybrid crosses with complete dominance.
这两条定律构成了孟德尔遗传模式的基础，在完全显性的单基因和双基因杂交中可被清晰地观察到。
2. Genetic Terminology | 遗传学术语
Key terms you must know:
必须掌握的关键术语：
- Gene – a length of DNA that codes for a polypeptide. | 基因 – 编码多肽的一段DNA。
- Allele – a variant form of a gene. | 等位基因 – 基因的不同形式。
- Genotype – the genetic makeup of an organism. | 基因型 – 生物体的遗传组成。
- Phenotype – the observable characteristics resulting from the genotype and environment. | 表现型 – 由基因型和环境共同导致的可观察特征。
- Dominant allele – an allele that is expressed in the phenotype when present. | 显性等位基因 – 存在时即在表现型中表达的等位基因。
- Recessive allele – an allele that is only expressed when two copies are present (homozygous). | 隐性等位基因 – 只有在两个拷贝都存在时（纯合）才表达的等位基因。
- Homozygous – having two identical alleles for a gene. | 纯合子 – 某一基因具有两个相同等位基因。
- Heterozygous – having two different alleles for a gene. | 杂合子 – 某一基因具有两个不同的等位基因。
- Locus – the specific position of a gene on a chromosome. | 基因座 – 基因在染色体上的特定位置。
3. Monohybrid Crosses and Test Crosses | 单基因杂交与测交
A monohybrid cross investigates the inheritance of one gene. For a cross between two heterozygous individuals (e.g., Aa × Aa), the phenotypic ratio in the offspring is 3 dominant : 1 recessive under complete dominance. The genotypic ratio is 1 AA : 2 Aa : 1 aa.
单基因杂交研究一对基因的遗传。当两个杂合子（如 Aa × Aa）杂交时，在完全显性条件下子代表现型比例为 3 显性 : 1 隐性。基因型比例为 1 AA : 2 Aa : 1 aa。
To determine the genotype of an individual exhibiting the dominant phenotype, a test cross is performed with a homozygous recessive individual. If any offspring show the recessive trait, the unknown parent is heterozygous.
为了确定具有显性性状个体的基因型，可用隐性纯合子进行测交。若子代中出现隐性性状，则未知亲本为杂合子。
4. Codominance and Incomplete Dominance | 共显性与不完全显性
In complete dominance, the heterozygote shows the dominant phenotype. However, not all alleles follow this pattern. In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes (e.g., snapdragon flower colour: red × white → pink).
在完全显性中，杂合子表现出显性性状。但并非所有等位基因都遵循此模式。在不完全显性中，杂合子的表现型是两个纯合子表现型的中间混合（例如金鱼草花色：红 × 白 → 粉）。
In codominance, both alleles are expressed equally in the heterozygote, resulting in a phenotype that shows both traits simultaneously (e.g., human blood type AB, where both IA and IB alleles are expressed, producing both A and B antigens on red blood cells).
在共显性中，杂合子中两个等位基因均等地表达，导致同时表现出两种性状的表现型（例如人类 AB 血型，IA 和 IB 等位基因同时表达，在红细胞表面产生 A 和 B 抗原）。
5. Multiple Alleles and ABO Blood Groups | 多等位基因与 ABO 血型系统
Many genes have more than two allelic forms in a population, though an individual can still possess only two. The human ABO blood group system is a classic example, governed by three alleles: IA, IB, and i. IA and IB are codominant to each other, and both are dominant over i.
许多基因在群体中存在两种以上的等位形式，但每个个体仍只能拥有其中两个。人类 ABO 血型系统是一个典型例子，由三个等位基因控制：IA、IB 和 i。IA 和 IB 彼此为共显性，且均对 i 呈显性。
| Genotype | Blood type (phenotype) | 基因型 | 血型（表现型） |
|---|---|---|---|
| IAIA or IAi | A | IAIA 或 IAi | A 型 |
| IBIB or IBi | B | IBIB 或 IBi | B 型 |
| IAIB | AB | IAIB | AB 型 |
| ii | O | ii | O 型 |
6. Sex Linkage | 性连锁遗传
Sex linkage refers to genes located on sex chromosomes (generally the X chromosome in mammals). Because males are hemizygous for X-linked genes (having only one X), recessive X-linked alleles are always expressed in males. Common examples include red-green colour blindness and haemophilia.
性连锁是指位于性染色体（哺乳动物中通常为 X 染色体）上的基因。由于雄性对 X 连锁基因是半合子（只有一条 X 染色体），隐性 X 连锁等位基因在雄性中总会表达。常见的例子包括红绿色盲和血友病。
In crosses, a carrier female (XNXn) and a normal male (XNY) can produce affected sons (XnY) with a 50% probability. An affected male (XnY) will pass the allele to all his daughters, making them carriers if the mother is normal.
在杂交中，一个携带者雌性（XNXn）与正常雄性（XNY）有 50% 的概率产生患病的儿子（XnY）。患病的雄性（XnY）会将致病等位基因传递给所有女儿，如果母亲正常，则女儿会成为携带者。
7. Dihybrid Inheritance and Epistasis | 双基因遗传与上位性
A dihybrid cross follows two genes simultaneously. Under independent assortment, a cross between two double heterozygotes (AaBb × AaBb) yields the classic phenotypic ratio 9 : 3 : 3 : 1. This ratio changes when gene interaction occurs.
双基因杂交同时追踪两个基因。在自由组合条件下，两个双杂合子（AaBb × AaBb）的杂交产生经典表现型比例 9 : 3 : 3 : 1。当发生基因相互作用时，此比例会改变。
Epistasis is when the expression of one gene is modified by another. In recessive epistasis, the homozygous recessive genotype at one locus masks the expression of the other gene (e.g., coat colour in Labrador retrievers: 9 : 3 : 4 ratio). In dominant epistasis, a dominant allele at one locus masks the other gene (e.g., 12 : 3 : 1 ratio).
上位性是指一个基因的表达受另一个基因的修饰。在隐性上位性中，一个基因座的隐性纯合基因型会掩盖另一个基因的表达（例如拉布拉多犬的毛色：9 : 3 : 4 比例）。在显性上位性中，一个基因座的显性等位基因掩盖另一个基因（例如 12 : 3 : 1 比例）。
8. Linkage and Recombination | 连锁与重组
Genes located on the same chromosome are linked and tend to be inherited together, violating the law of independent assortment. However, crossing over during prophase I of meiosis can separate linked alleles, producing recombinant gametes.
位于同一染色体上的基因是连锁的，倾向于一同遗传，这违背了自由组合定律。然而，减数分裂前期 I 的交叉互换可以分开连锁的等位基因，产生重组配子。
The frequency of recombination between two linked genes is used to calculate their relative distance (1% recombination = 1 map unit or centimorgan). Closely linked genes show low recombination frequencies, while loosely linked genes show higher frequencies.
两个连锁基因之间的重组频率用于计算其相对距离（1% 重组率 = 1 个图距单位或厘摩）。紧密连锁的基因重组频率低，松散连锁的基因重组频率高。
9. Chromosomal Mutations | 染色体突变
Changes in chromosome structure or number can cause genetic disorders. Aneuploidy is the loss or gain of individual chromosomes, often due to nondisjunction during meiosis (e.g., Down syndrome is trisomy 21). Polyploidy involves entire extra sets of chromosomes, common in plants.
染色体结构或数目的改变可导致遗传病。非整倍体是单条染色体的丢失或获得，常因减数分裂不分离引起（例如唐氏综合征为 21 三体）。多倍体涉及整套额外的染色体，常见于植物。
Structural mutations include deletions, duplications, inversions, and translocations, each potentially altering gene expression and leading to phenotypic abnormalities.
结构突变包括缺失、重复、倒位和易位，每一种都可能改变基因表达并导致表型异常。
10. Population Genetics and Hardy-Weinberg Principle | 群体遗传学与哈代-温伯格原理
The Hardy-Weinberg principle provides a null model for allele and genotype frequencies in a non-evolving population. For a gene with two alleles A (frequency p) and a (frequency q), where p + q = 1, the expected genotype frequencies are:
哈代-温伯格原理为无进化群体中的等位基因和基因型频率提供了一个零假设模型。对于一个具有两个等位基因 A（频率 p）和 a（频率 q）的基因，p + q = 1，预期基因型频率为：
p² + 2pq + q² = 1
The conditions required: large population size, random mating, no mutation, no natural selection, and no gene flow. Exam questions often ask you to calculate carrier frequencies or test if a population is in equilibrium.
所需条件为：大群体、随机交配、无突变、无自然选择、无基因流动。考试题常要求计算携带者频率或检验群体是否处于平衡状态。
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