📚 IB Science: Genetics Key Points | IB 科学：遗传考点精讲
Genetics is a cornerstone of IB Science, encompassing the molecular mechanisms of heredity, patterns of inheritance, and modern biotechnologies. This guide systematically covers the essential concepts and examinable skills required for high achievement.
遗传学是 IB 科学的核心内容，涵盖遗传的分子机制、遗传模式以及现代生物技术。本指南系统梳理了必须掌握的核心概念和考试技巧，助你取得高分。
1. DNA Structure and Replication | DNA 结构与复制
DNA is a double-stranded helix formed by two antiparallel strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G).
DNA 是由两条反向平行的核苷酸链组成的双螺旋结构。每个核苷酸包含一个脱氧核糖、一个磷酸基团以及四种含氮碱基之一：腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 或鸟嘌呤 (G)。
Hydrogen bonds between complementary bases – A with T (two bonds) and C with G (three bonds) – hold the two strands together. The sugar-phosphate backbones provide structural stability.
互补碱基之间的氢键将两条链连接在一起：A 与 T 配对（两个氢键），C 与 G 配对（三个氢键）。糖-磷酸主链提供了结构稳定性。
During semi-conservative replication, helicase unwinds the double helix, and DNA polymerase synthesises new complementary strands using each original strand as a template. Replication occurs in the 5′ to 3′ direction on the leading strand continuously and on the lagging strand discontinuously as Okazaki fragments.
在半保留复制过程中，解旋酶解开双螺旋，DNA 聚合酶以每条原始链为模板合成新的互补链。复制沿 5′ 到 3′ 方向进行，前导链连续合成，后随链不连续地形成冈崎片段。
Meselson and Stahl’s experiment using heavy nitrogen (¹⁵N) confirmed the semi-conservative model by showing that replicated DNA contains one parental and one newly synthesised strand after one generation in ¹⁴N medium.
梅塞尔森和斯塔尔利用重氮 (¹⁵N) 的实验证明了半保留复制模型，在 ¹⁴N 培养基中培养一代后，复制的 DNA 含有一条母链和一条新合成链。
2. Transcription and Translation | 转录与翻译
Transcription is the synthesis of mRNA from a DNA template. RNA polymerase binds to the promoter region, unwinds the DNA, and assembles an RNA strand complementary to the template strand, substituting uracil (U) for thymine.
转录是以 DNA 为模板合成 mRNA 的过程。RNA 聚合酶与启动子区域结合，解开 DNA，并组装一条与模板链互补的 RNA 链，其中尿嘧啶 (U) 替代胸腺嘧啶。
In eukaryotes, pre-mRNA undergoes splicing: introns are removed and exons are joined to form mature mRNA before translation. Prokaryotes lack introns.
在真核生物中，前体 mRNA 经过剪接：内含子被切除，外显子连接形成成熟 mRNA 再进行翻译。原核生物没有内含子。
Translation occurs at ribosomes. mRNA codons (three-base sequences) are read, and tRNA molecules with complementary anticodons deliver specific amino acids. The ribosome catalyses peptide bond formation, creating a polypeptide chain until a stop codon is reached.
翻译在核糖体上进行。mRNA 密码子（三个碱基的序列）被读取，具有互补反密码子的 tRNA 分子运送特定的氨基酸。核糖体催化肽键形成，生成多肽链，直至遇到终止密码子。
The genetic code is universal among nearly all organisms and is degenerate, meaning most amino acids are encoded by more than one codon. The start codon AUG codes for methionine and initiates translation.
遗传密码几乎是所有生物通用的，且具有简并性，即大多数氨基酸由多个密码子编码。起始密码子 AUG 编码甲硫氨酸并启动翻译。
3. Mendelian Genetics | 孟德尔遗传学
Gametes carry one allele per gene due to meiosis; offspring inherit one allele from each parent. Dominant alleles mask recessive ones in heterozygotes. True-breeding plants were used by Mendel to establish the laws of inheritance.
由于减数分裂，配子每个基因只携带一个等位基因；子代从每个亲本各继承一个等位基因。显性等位基因在杂合子中遮盖隐性等位基因。孟德尔用纯合植株建立了遗传定律。
Mendel’s Law of Segregation states that allele pairs separate during gamete formation, and each gamete receives one allele. The Law of Independent Assortment states that alleles of different genes segregate independently when genes are on different chromosomes.
孟德尔的分离定律指出，等位基因对在配子形成时分离，每个配子得到一个等位基因。自由组合定律指出，位于不同染色体上的基因的等位基因独立分配。
Monohybrid crosses produce a 3:1 phenotypic ratio in the F₂ generation for a dominant-recessive trait. A Punnett square can predict expected genotypic and phenotypic ratios. The chi-squared test assesses whether observed frequencies deviate significantly from expected Mendelian ratios.
单因子杂交在 F₂ 代产生 3:1 的表型比（显隐性性状）。旁氏表可预测预期的基因型和表型比。卡方检验用于评估观测频率是否与预期孟德尔比率存在显著差异。
4. Dihybrid Crosses and Linked Genes | 双因子杂交与连锁基因
For unlinked genes, a dihybrid cross between two heterozygous parents (AaBb x AaBb) produces a 9:3:3:1 phenotypic ratio. This reflects independent assortment of the two gene pairs.
对于不连锁的基因，两个杂合亲本 (AaBb x AaBb) 的双因子杂交产生 9:3:3:1 的表型比，体现了两个基因对的自由组合。
Linked genes are located close together on the same chromosome and tend to be inherited together, violating independent assortment. Recombination can still occur through crossing over during prophase I of meiosis, generating new combinations of alleles.
连锁基因位于同一染色体上且距离较近，倾向一同遗传，不遵循自由组合。然而，在减数分裂前期 I 通过交叉互换仍可发生重组，产生新的等位基因组合。
Recombination frequency can be used to map the relative positions of genes on a chromosome; 1% recombination equals 1 map unit (centimorgan). Double crossovers can complicate mapping but are resolved by comparing recombination data from three-point crosses.
重组频率可用于绘制基因在染色体上的相对位置；1% 的重组率等于 1 个图距单位（厘摩）。双交换会复杂化图谱绘制，但通过三点测交比较数据可以解决。
5. Sex Determination and Sex-Linked Inheritance | 性别决定与伴性遗传
In humans and many organisms, sex is determined by the X and Y chromosomes: females are XX, males are XY. The SRY gene on the Y chromosome triggers male development.
在人类和许多生物中，性别由 X 和 Y 染色体决定：女性为 XX，男性为 XY。Y 染色体上的 SRY 基因触发男性发育。
X-linked recessive disorders, such as haemophilia and red-green colour blindness, occur more frequently in males because they have only one X chromosome. A carrier female (heterozygous) has a 50% chance of passing the recessive allele to a son, who will be affected, and a 50% chance to a daughter, who becomes a carrier.
X 连锁隐性遗传病，如血友病和红绿色盲，在男性中更常见，因为他们只有一条 X 染色体。携带者女性（杂合）有 50% 的概率将隐性等位基因传给儿子（儿子患病），传给女儿则女儿成为携带者。
X-linked dominant inheritance is rare; affected fathers pass the trait to all daughters but no sons. Pedigree analysis helps deduce mode of inheritance: absence of father-to-son transmission suggests X-linked recessive.
X 连锁显性遗传罕见；患病父亲将性状传给所有女儿，但不传给儿子。系谱分析有助于推断遗传方式：无父传子现象提示 X 连锁隐性遗传。
6. Mutations and Genetic Variation | 突变与遗传变异
A gene mutation is a permanent change in the DNA sequence. Base substitution can lead to silent, missense, or nonsense mutations. Frameshift mutations (insertions or deletions) alter the reading frame, often resulting in nonfunctional proteins.
基因突变是 DNA 序列的永久性改变。碱基替换可导致沉默突变、错义突变或无义突变。移码突变（插入或缺失）改变阅读框，常产生无功能蛋白质。
Sickle cell anaemia is caused by a single base substitution (GAG → GTG) in the beta-globin gene, changing glutamic acid to valine, which alters haemoglobin structure. This mutation offers heterozygote advantage against malaria in certain regions.
镰刀型细胞贫血症由 beta-珠蛋白基因的单碱基替换 (GAG → GTG) 引起，谷氨酸变为缬氨酸，改变了血红蛋白结构。该突变在特定地区为杂合子提供了抗疟疾的杂合优势。
Mutagens such as UV radiation, ionising radiation, and certain chemicals increase mutation rates. Mutations provide the raw material for natural selection and can be beneficial, neutral, or harmful depending on the environment.
诱变剂如紫外线、电离辐射和某些化学物质会增加突变率。突变为自然选择提供了原始材料，根据环境不同，可能是有益的、中性的或有害的。
7. Gene Pools and Evolution | 基因库与进化
A gene pool is the total collection of alleles in a population. The Hardy-Weinberg principle states that allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary influences:
基因库是指种群中所有等位基因的总和。哈代-温伯格平衡原理指出，在没有进化影响的条件下，等位基因和基因型频率世代保持恒定：
p + q = 1 and p² + 2pq + q² = 1
Evolution occurs when the Hardy-Weinberg conditions are not met. Factors that change allele frequencies include natural selection, genetic drift (particularly bottleneck and founder effects), gene flow, and non-random mating.
当不满足哈代-温伯格条件时，进化就会发生。改变等位基因频率的因素包括自然选择、遗传漂变（特别是瓶颈效应和奠基者效应）、基因流和非随机交配。
Genetic variation within a species arises from mutation, sexual reproduction (meiosis and fertilisation), and crossing over. It is essential for a population’s ability to adapt to changing environments.
物种内的遗传变异源于突变、有性生殖（减数分裂和受精）以及交叉互换。这是种群适应变化环境能力的基础。
8. Genetic Engineering and Biotechnology | 基因工程与生物技术Published by TutorHao | IB Science Revision Series | aleveler.com
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