📚 IB & WJEC Biology: Genetics Key Points | IB WJEC 生物：遗传学 考点精讲
Genetics forms the backbone of modern biology, explaining how traits are passed from one generation to the next and how variation arises. This article distils the essential concepts required for both IB and WJEC biology specifications, providing clear explanations and exam-focused insights.
遗传学是现代生物学的支柱，它解释了性状如何从一代传到下一代以及变异如何产生。本文浓缩了IB和WJEC生物课程中必备的核心概念，提供清晰的解释和面向考点的精讲。
1. Mendelian Inheritance | 孟德尔遗传定律
Gregor Mendel's experiments with pea plants established the fundamental laws of inheritance. The Law of Segregation states that each organism possesses two alleles for a trait, which separate during gamete formation so that each gamete carries only one allele.
格雷戈尔·孟德尔通过豌豆实验确立了遗传的基本定律。分离定律指出，每个生物体的每个性状都有两个等位基因，在配子形成时它们分离，因此每个配子只携带一个等位基因。
The Law of Independent Assortment states that alleles for different traits are distributed to gametes independently of one another, provided the genes are on different chromosomes or far apart on the same chromosome.
自由组合定律表明，不同性状的等位基因在配子中随机组合，只要这些基因位于不同染色体上或在同一染色体上相距很远。
ybrid cross (AaBb × AaBb) gives the classic 9:3:3:1 ratio when genes assort independently.
孟德尔还观察到一些等位基因对另一些呈显性，在杂合子中掩盖隐性等位基因的效应。两个杂合子的单因子杂交(Aa × Aa)产生3:1的表型比，而双因子杂交(AaBb × AaBb)在自由组合时产生典型的9:3:3:1比率。
2. Alleles, Genotype and Phenotype | 等位基因、基因型与表型
An allele is a variant form of a gene. Diploid organisms have two alleles for each gene, one inherited from each parent. The specific combination of alleles is the genotype, while the observable characteristic is the phenotype.
等位基因是基因的一种变异形式。二倍体生物的每个基因有两个等位基因，分别来自父母。等位基因的特定组合为基因型，而可观察到的特征为表型。
Homozygous individuals have two identical alleles (e.g., AA or aa), whereas heterozygous individuals possess two different alleles (e.g., Aa). The phenotype results from the interaction of genotype with the environment; many traits, such as height or skin colour, are polygenic and show continuous variation.
纯合子有两个相同的等位基因(如AA或aa)，杂合子有两个不同的等位基因(如Aa)。表型是基因型与环境相互作用的结果；许多性状如身高或肤色是多基因遗传，呈现连续变异。
Test crosses are used to determine an unknown genotype. A dominant phenotype individual is crossed with a homozygous recessive; if any offspring show the recessive trait, the unknown parent must be heterozygous.
测交用于确定未知基因型。将显性表型个体与隐性纯合子杂交；若后代中出现隐性性状，则未知亲本必为杂合子。
3. Co-dominance and Incomplete Dominance | 共显性与不完全显性
Not all alleles follow the simple dominant-recessive pattern. In co-dominance, both alleles are fully expressed in the heterozygote. A classic example is the AB blood group in humans, where both A and B alleles produce their respective antigens on red blood cells.
并非所有等位基因都遵循简单的显隐性模式。在共显性中，杂合子中两个等位基因都完全表达。典型例子是人类AB血型，其中A和B等位基因在红细胞上产生各自的抗原。
In incomplete dominance, the heterozygote shows an intermediate phenotype. For instance, crossing red-flowered snapdragons (CR CR) with white-flowered ones (CW CW) produces pink flowers (CR CW), where neither allele is completely dominant.
在不完全显性中，杂合子表现出中间表型。例如，将红花金鱼草(CR CR)与白花金鱼草(CW CW)杂交，产生粉红色花(CR CW)，此时没有一个等位基因完全显性。
These patterns can affect genotypic and phenotypic ratios in a cross. For co-dominance and incomplete dominance, the 1:2:1 genotypic ratio in a monohybrid cross is directly reflected in the phenotypic ratio, as each genotype has a distinct phenotype.
这些模式会影响杂交中的基因型比和表型比。对于共显性和不完全显性，单因子杂交中的1:2:1基因型比直接反映在表型比上，因为每个基因型都有独特的表型。
4. Sex-linked Inheritance | 性连锁遗传
Sex-linked genes are located on the sex chromosomes, typically the X chromosome in mammals. Because males have only one X chromosome (XY), they are hemizygous for X-linked genes, meaning a single recessive allele on the X chromosome will be expressed. This explains why conditions like red-green colour blindness and haemophilia are more common in males.
性连锁基因位于性染色体上，在哺乳动物中通常位于X染色体上。由于男性只有一条X染色体(XY)，他们对X连锁基因是半合子，这意味着X染色体上的单个隐性等位基因就会表达。这解释了为什么红绿色盲和血友病等病症在男性中更常见。
In a cross involving an X-linked recessive trait, a carrier female (XB Xb) and a normal male (XB Y) produce offspring where 50% of sons inherit the Xb allele and express the trait, while 50% of daughters are carriers but not affected.
在涉及X连锁隐性性状的杂交中，携带者女性(XB Xb)与正常男性(XB Y)产生的后代中，50%的儿子继承了Xb等位基因并表达该性状，而50%的女儿是携带者但不受影响。
Pedigree charts for sex-linked recessive traits show a distinct pattern: affected males cannot pass the trait to their sons (since sons inherit the Y chromosome from the father), but all daughters of an affected male will be carriers if the mother is normal.
X连锁隐性性状的系谱图显示出独特模式：患病男性不能将性状传给儿子（因为儿子从父亲那里继承Y染色体），但如果母亲正常，患病男性的所有女儿都将是携带者。
5. Pedigree Analysis | 系谱分析
A pedigree is a diagram showing the inheritance of a trait through several generations. Circles represent females, squares represent males, shaded symbols indicate individuals expressing the trait. Examining a pedigree allows geneticists to deduce whether a trait is dominant, recessive, autosomal, or sex-linked.
系谱是显示一个性状在几代中遗传情况的图解。圆圈代表女性，方块代表男性，涂黑的符号表示表现出该性状的个体。通过检查系谱，遗传学家可以推断性状是显性、隐性、常染色体还是性连锁。
Key clues for autosomal recessive traits: affected individuals can appear in offspring of unaffected parents; males and females are equally likely to be affected. For autosomal dominant traits: every affected individual usually has at least one affected parent; the trait does not skip generations.
常染色体隐性性状的关键线索：患病个体可出现在未患病父母的后代中；男性和女性患病概率相同。对于常染色体显性性状：每个患病个体通常至少有一个患病亲本；性状不会隔代出现。
For X-linked recessive patterns, more males than females are affected; an affected male passing the trait to a grandson through a carrier daughter is a strong indicator. Y-linked traits appear only in males and are passed from father to all sons.
对于X连锁隐性模式，男性患者多于女性；患病男性通过携带者女儿将性状传给外孙是一个强有力的指标。Y连锁性状仅出现在男性中，由父亲传给所有儿子。
6. Gene Linkage and Recombination | 基因连锁与重组
Genes located close together on the same chromosome tend to be inherited together, a phenomenon called linkage. During prophase I of meiosis, crossing over between homologous chromosomes can exchange segments, producing recombinant gametes. The frequency of recombination between two genes is proportional to the distance between them, allowing construction of chromosome maps.
位于同一染色体上相距较近的基因倾向于一起遗传，这一现象称为连锁。在减数第一次分裂前期，同源染色体之间发生交叉，可以交换片段，产生重组配子。两个基因之间的重组频率与它们之间的距离成正比，从而可以绘制染色体图谱。
frequency (RF) is calculated as (number of recombinant offspring / total offspring) × 100%. A RF of 1% corresponds to a map distance of one centimorgan (cM).
如果连锁基因的双因子杂交产生的后代比率显著偏离自由组合预期的9:3:3:1，则可能存在连锁。重组频率(RF)计算为 (重组后代数 / 总后代数) × 100%。1%的RF对应一个厘摩(cM)的图距。
In Drosophila, linked genes for body colour and wing size do not assort independently; test crosses reveal a higher proportion of parental types. Understanding linkage is essential for mapping disease genes and in plant and animal breeding programmes.
在果蝇中，体色和翅膀大小连锁基因并不自由组合；测交揭示亲本类型比例更高。理解连锁对于定位疾病基因以及动植物育种计划至关重要。
7. Chromosomal Mutations | 染色体突变
Changes in chromosome structure or number can cause significant genetic disorders. Structural changes include deletions (loss of a segment), duplications (repeat of a segment), inversions (reversal of a segment), and translocations (transfer of a segment to a non-homologous chromosome).
染色体结构或数目的改变可导致严重的遗传病。结构改变包括缺失(片段丢失)、重复(片段重复)、倒位(片段翻转)和易位(片段转移到非同源染色体上)。
Aneuploidy results from nondisjunction during meiosis, leading to an abnormal number of chromosomes. Down syndrome (trisomy 21) is a well-known example where an individual has three copies of chromosome 21. Turner syndrome (45, X) and Klinefelter syndrome (47, XXY) are sex chromosome aneuploidies.
非整倍体来源于减数分裂中的不分离，导致染色体数目异常。唐氏综合征(21三体)是一个众所周知的例子，个体拥有三条21号染色体。特纳综合征(45, X)和克氏综合征(47, XXY)是性染色体非整倍体。
Polyploidy, having more than two complete sets of chromosomes, is common in plants and can be induced in agriculture to produce larger fruits or seedless varieties. However, in animals, polyploidy is usually lethal.
多倍体，即拥有超过两套完整的染色体组，在植物中常见，并可在农业中诱导产生更大的果实或无籽品种。但在动物中，多倍体通常是致死的。
8. Gene Expression and Regulation | 基因表达与调控
The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into messenger RNA (mRNA), which is then translated into a polypeptide chain. Transcription involves RNA polymerase binding to a promoter region and synthesizing a complementary RNA strand.
分子生物学的中心法则描述了遗传信息的流向：DNA转录为信使RNA(mRNA)，然后翻译为多肽链。转录涉及RNA聚合酶与启动子区域结合并合成互补的RNA链。
In eukaryotes, mRNA undergoes post-transcriptional modification: addition of a 5' cap and poly-A tail, and splicing to remove introns. The mature mRNA then exits the nucleus. Translation occurs on ribosomes, where transfer RNA (tRNA) molecules bring specific amino acids matching the mRNA codons.
在真核生物中，mRNA经历转录后修饰：添加5'帽和多聚A尾，以及剪接切除内含子。成熟的mRNA随后离开细胞核。翻译在核糖体上进行，转运RNA(tRNA)分子根据mRNA密码子带来特定的氨基酸。
Gene expression is tightly regulated. In prokaryotes, operons such as the lac operon allow coordinated control of genes involved in lactose metabolism. In eukaryotes, transcription factors, enhancers, and epigenetic modifications (e.g., DNA methylation, histone acetylation) control which genes are expressed in different cell types.
基因表达受到严格调控。在原核生物中，像乳糖操纵子这样的操作子允许对参与乳糖代谢的基因进行协调控制。在真核生物中，转录因子、增强子以及表观遗传修饰(如DNA甲基化、组蛋白乙酰化)控制不同细胞类型中哪些基因表达。
9. Molecular Genetics Techniques | 分子遗传学技术
Polymerase Chain Reaction (PCR) is used to amplify specific DNA sequences. It involves repeated cycles of denaturation (separating DNA strands by heat), annealing (short primers bind to complementary sequences), and extension (Taq polymerase synthesizes new strands). PCR is crucial for forensic analysis, genetic testing, and research.
聚合酶链式反应(PCR)用于扩增特定的DNA序列。它涉及重复的循环：变性(通过加热使DNA链分离)、退火(短引物与互补序列结合)和延伸(Taq聚合酶合成新链)。PCR对于法医分析、遗传检测和研究至关重要。
Gel electrophoresis separates DNA fragments by size. An electric current is applied across a gel matrix; negatively charged DNA migrates towards the positive electrode. Smaller fragments travel faster and further. The resulting bands can be compared to a DNA ladder to determine fragment sizes.
凝胶电泳按大小分离DNA片段。在凝胶基质上施加电流；带负电的DNA向正极移动。较小的片段迁移得更快更远。得到的条带可与DNA ladder比较以确定片段大小。
Restriction enzymes cut DNA at specific recognition sequences, generating fragments with 'sticky ends' or 'blunt ends'. Together with ligase enzymes, these tools enable the creation of recombinant DNA molecules for genetic engineering.
限制酶在特定的识别序列处切割DNA，产生具有“粘性末端”或“平末端”的片段。与连接酶一起，这些工具能够创建用于基因工程的重组DNA分子。
10. Genetic Engineering and Biotechnology | 遗传工程与生物技术
Genetic modification involves altering the genetic material of an organism. A gene of interest can be inserted into a plasmid vector, which is then introduced into bacterial cells. The bacteria multiply and produce the desired protein, such as human insulin for diabetes treatment.
遗传修饰涉及改变生物体的遗传物质。目标基因可插入质粒载体，然后将其导入细菌细胞。细菌繁殖并产生所需的蛋白质，例如用于治疗糖尿病的人类胰岛素。
CRISPR-Cas9 is a revolutionary genome-editing tool that allows precise modifications to DNA. The Cas9 enzyme is guided by a single guide RNA to a specific sequence, where it creates a double-strand break. The cell's repair machinery can then introduce insertions, deletions, or replacements, offering powerful applications in research and medicine.
CRISPR-Cas9是一种革命性的基因组编辑工具，可对DNA进行精确修饰。Cas9酶由单个引导RNA引导到特定序列，在那里制造双链断裂。细胞的修复机制随后可引入插入、缺失或替换，为研究和医学提供了强大的应用前景。
Ethical and safety issues surround genetic technologies. Concerns include potential unintended effects on non-target organisms, the ethics of germline editing in humans, and access to genetic information. IB and WJEC syllabi require students to evaluate the benefits and risks of genetic manipulation.
基因技术涉及伦理和安全问题。担忧包括对非目标生物的潜在意外影响、人类生殖系编辑的伦理以及遗传信息的获取。IB和WJEC大纲要求学生评估遗传操作的益处和风险。
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