📚 Inheritance | 遗传
Inheritance is the process by which genetic information is passed from parents to their offspring. It explains why offspring resemble their parents but also show variation. Understanding inheritance is fundamental in biology, as it underpins topics such as evolution, selective breeding and genetic disorders.
遗传是指遗传信息从亲代传递给子代的过程。它解释了为什么子代既与亲代相似又存在差异。理解遗传是生物学的基础，为进化、选择性育种和遗传病等主题提供了支撑。
1. Introduction to Inheritance | 遗传概述
Every living organism inherits a set of instructions encoded in DNA. These instructions determine its characteristics, from eye colour to metabolic pathways. The study of inheritance, or genetics, started with the work of Gregor Mendel in the 19th century, who used pea plants to uncover the basic principles of heredity.
每种生物都继承了一套编码在 DNA 中的指令。这些指令决定了从眼色到代谢途径等特征。对遗传（遗传学）的研究始于 19 世纪孟德尔的工作，他利用豌豆植物揭示了遗传的基本原理。
A gene is a section of DNA that codes for a particular protein. Genes exist in different forms called alleles, and the combination of alleles an organism carries gives rise to its traits. The transmission of alleles follows predictable patterns, which we can model using genetic diagrams.
基因是 DNA 上编码特定蛋白质的一个片段。基因以不同的形式存在，称为等位基因。生物体携带的等位基因组合决定了它的性状。等位基因的传递遵循可预测的模式，我们可以用遗传图解来模拟。
2. Chromosomes, Genes and Alleles | 染色体、基因与等位基因
In the nucleus of eukaryotic cells, DNA is organised into structures called chromosomes. In humans, each body cell contains 46 chromosomes arranged in 23 pairs. One chromosome of each pair comes from the mother and the other from the father. These pairs are called homologous chromosomes.
在真核生物的细胞核中，DNA 被组装成称为染色体的结构。人类每个体细胞含有 46 条染色体，排成 23 对。每对染色体中的一条来自母亲，另一条来自父亲，它们被称为同源染色体。
A gene occupies a specific position, or locus, on a chromosome. Since homologous chromosomes carry the same genes in the same order, an individual has two copies of each gene – one on each homologous chromosome. An allele is an alternative version of a gene. For example, the gene for plant height in peas has a tall allele and a dwarf allele.
基因占据染色体上特定的位置，即基因座。由于同源染色体以相同的顺序携带相同的基因，因此每个个体对每个基因都有两个拷贝——分别位于两条同源染色体上。等位基因是基因的替代版本。例如，豌豆株高基因就有一个高茎等位基因和一个矮茎等位基因。
3. Genotype and Phenotype | 基因型与表现型
The genotype is the combination of alleles an organism possesses for a particular gene. It is the genetic make‑up, often represented by letters – dominant alleles are shown with capital letters and recessive alleles with lower‑case letters. The phenotype is the observable characteristic that results from the interaction of the genotype with the environment.
基因型是生物体就某一特定基因所拥有的等位基因组合，是其遗传构成，通常用字母表示——显性等位基因用大写字母，隐性等位基因用小写字母。表现型是通过基因型与环境相互作用而产生的可观察特征。
If the two alleles are identical, the organism is homozygous (e.g. TT or tt). If the two alleles are different, it is heterozygous (e.g. Tt). In a heterozygous individual, the dominant allele usually masks the effect of the recessive allele, but the recessive allele can still be passed on to the next generation.
如果两个等位基因相同，则该生物是纯合的（例如 TT 或 tt）。如果两个等位基因不同，则是杂合的（例如 Tt）。在杂合子中，显性等位基因通常会掩盖隐性等位基因的效应，但隐性等位基因仍能传递给下一代。
4. Monohybrid Inheritance and Punnett Squares | 单基因杂交与庞纳特方格
Monohybrid inheritance involves a single gene controlling one characteristic. Mendel's classic experiment crossed pure‑breeding tall (TT) and dwarf (tt) pea plants. All the first‑generation (F₁) offspring were tall (Tt). When these F₁ plants were self‑pollinated, the second‑generation (F₂) showed a 3:1 ratio of tall to dwarf plants.
单基因杂交涉及控制一个性状的单个基因。孟德尔的经典实验将纯种高茎（TT）和矮茎（tt）豌豆植株杂交。第一代（F₁）后代全部为高茎（Tt）。当这些 F₁ 植株自花传粉后，第二代（F₂）出现了高茎与矮茎 3:1 的比例。
ng using a Punnett square. For a cross between two heterozygous tall pea plants (Tt × Tt):
我们可以用庞纳特方格预测后代的基因型和表现型。以两株杂合高茎豌豆（Tt × Tt）的杂交为例：
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
The genotypic ratio is 1 TT : 2 Tt : 1 tt, and the phenotypic ratio is 3 tall : 1 dwarf. This 3:1 ratio is characteristic of a monohybrid cross involving a dominant and a recessive allele.
基因型比例为 1 TT : 2 Tt : 1 tt，表现型比例为 3 高 : 1 矮。这一 3:1 的比例是涉及显性和隐性等位基因的单基因杂交的典型特征。
Mendel's laws of segregation state that allele pairs separate during gamete formation, so each gamete carries only one allele for each gene. This principle, when combined with random fertilisation, produces the predictable ratios seen in offspring.
孟德尔分离定律指出，等位基因对在配子形成过程中分离，每个配子只携带每个基因的一个等位基因。该原理与随机受精结合后，即可在后代中产生可预测的比例。
5. Test Cross | 测交
A test cross is used to determine the genotype of an individual showing a dominant phenotype but whose genotype is unknown. The individual is crossed with a homozygous recessive individual. If any offspring show the recessive trait, the parent of unknown genotype must be heterozygous.
测交用于确定一个表现显性性状但基因型未知的个体的基因型。将该个体与纯合隐性个体杂交。如果后代中出现隐性性状，那么基因型未知的亲本一定是杂合的。
For example, a tall pea plant could be TT or Tt. Crossing it with a dwarf plant (tt) will give all tall offspring if the parent is TT, or a 1:1 ratio of tall to dwarf offspring if the parent is Tt. This simple cross provides conclusive evidence of the hidden allele.
例如，一株高茎豌豆可能是 TT 或 Tt。将其与矮茎植株（tt）杂交，如果亲本是 TT，则后代全为高茎；如果亲本是 Tt，后代高茎与矮茎的比例为 1:1。这一简单的杂交就为隐藏等位基因提供了确凿证据。
6. Co‑dominance | 共显性
Co‑dominance occurs when both alleles in a heterozygous individual are expressed equally in the phenotype. Neither allele is dominant or recessive; instead, both contribute to the trait. This pattern differs from complete dominance, where one allele masks the other.
共显性是指杂合子中两个等位基因均在表现型中同等表达。两个等位基因既非显性也非隐性，而是共同参与性状。这一模式不同于一个等位基因掩盖另一个的完全显性。
A common example is the coat colour in shorthorn cattle. The allele for red coat (CR) and the allele for white coat (CW) are co‑dominant. A heterozygous individual (CRCW) has a roan coat, where red and white hairs are intermixed.
常见的例子是短角牛的毛色。控制红色毛的等位基因（CR）和控制白色毛的等位基因（CW）是共显性的。杂合个体（CRCW）呈现红白毛相间的沙毛。
In co‑dominance, genetic diagrams still use capital letters with superscripts to distinguish alleles. The genotypic and phenotypic ratios are identical because each genotype produces a distinct phenotype.
在共显性中，遗传图解仍然使用带右上标的大写字母区分等位基因。基因型比例与表现型比例相同，因为每种基因型都产生独特的表型。
7. Multiple Alleles and Blood Groups | 复等位基因与血型
Although any individual carries only two alleles for a gene, a population may contain more than two alleles. This is called multiple alleles. The ABO blood group system in humans is a classic example, determined by three alleles: IA, IB and i.
虽然任何个体对某一基因只携带两个等位基因，但一个群体中可能存在两个以上的等位基因，这称为复等位基因。人类 ABO 血型系统就是典型例子，由三个等位基因决定：IA、IB 和 i。
The alleles IA and IB are co‑dominant to each other, while both are dominant over i. This produces four possible phenotypes:
等位基因 IA 和 IB 互为共显性，且两者对 i 均为显性。由此产生四种可能的表型：
| Genotype | Blood group (phenotype) |
| IAIA or IAi | A |
| IBIB or IBi | B |
| IAIB | AB |
| ii | O |
Understanding blood group inheritance is crucial for safe blood transfusions. A person can only receive blood that lacks antigens their own immune system would attack. Blood group O is the universal donor, and AB is the universal recipient.
理解血型遗传对安全输血至关重要。一个人只能接受不含会触发其自身免疫系统攻击的抗原的血液。O 型血是万能输血者，AB 型血是万能受血者。
8. Sex Determination | 性别决定
In humans and many other organisms, sex is determined by sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The remaining 22 pairs of chromosomes are called autosomes and are identical in both sexes.
在人类和许多其他生物中，性别由性染色体决定。女性有两条 X 染色体（XX），男性有一条 X 和一条 Y 染色体（XY）。其余的 22 对染色体称为常染色体，在男女中相同。
During gamete formation, females produce eggs that all contain a single X chromosome. Males produce sperm, half of which carry an X chromosome and half carry a Y chromosome. The sex of the offspring is therefore determined by the sperm that fertilises the egg.
在配子形成过程中，女性产生的卵子全部含有一条 X 染色体。男性产生的精子中，一半携带 X 染色体，一半携带 Y 染色体。因此，后代的性别由使卵子受精的精子决定。
A Punnett square for sex determination shows a 1:1 ratio of XX (female) to XY (male) offspring. Thus, there is always an equal chance of having a boy or a girl in each pregnancy.
庞纳特方格显示的性别决定比例是 XX（女性）与 XY（男性）1:1。因此，每次怀孕生男生女的概率总是相等的。
9. Variation and Mutation | 变异与突变
Variation refers to the differences between individuals of the same species. It can be continuous, such as height or skin colour, or discontinuous, such as blood group or eye colour. Genetic variation arises from the reshuffling of alleles during meiosis, random fertilisation and mutations.
变异是指同一物种个体之间的差异。它可以是连续的，如身高或肤色，也可以是不连续的，如血型或眼色。遗传变异源自减数分裂中等位基因的重组、随机受精和突变。
A mutation is a permanent change in the DNA sequence of a gene. Mutations can be neutral, harmful or occasionally beneficial. They are the original source of new alleles and drive evolution by natural selection. Environmental factors such as radiation and certain chemicals can increase the rate of mutation.
突变是基因 DNA 序列的永久性改变。突变可能是中性的、有害的，偶尔也可能是有利的。它们是新等位基因的原始来源，通过自然选择推动进化。辐射和某些化学物质等环境因素会提高突变率。
In the context of inheritance, mutations that occur in gametes can be passed to offspring, potentially causing genetic disorders. Examples include cystic fibrosis, caused by a recessive allele of the CFTR gene, and Huntington's disease, caused by a dominant allele. Understanding the inheritance patterns of such conditions allows genetic counselling.
在遗传背景下，发生在配子中的突变可传递给后代，从而可能导致遗传病。例如，由 CFTR 基因隐性等位基因引起的囊性纤维化，以及由显性等位基因引起的亨廷顿病。了解这些疾病的遗传模式使遗传咨询成为可能。
10. Pedigree Analysis and Genetic Disorders | 系谱分析与遗传病
A pedigree chart is a diagram that shows the inheritance of a particular trait over several generations. Symbols represent individuals, and shading indicates whether they express the trait. Pedigrees help geneticists determine whether an allele is dominant, recessive or sex‑linked.
系谱图是展示某一特定性状在数代内遗传情况的图表。符号代表个体，阴影表示是否表现出该性状。系谱图帮助遗传学家判断等位基因是显性、隐性还是性连锁遗传。
For example, cystic fibrosis shows a recessive pattern, where two unaffected parents can have an affected child. In contrast, Huntington's disease shows a dominant pattern, where every affected person has at least one affected parent. Pedigree analysis allows predictions of the risk of inheriting such conditions.
例如，囊性纤维化呈现隐性遗传模式，两个无病的父母可能生下患病的孩子。亨廷顿病则呈现显性模式，每个患者至少有一位患病的亲代。系谱分析可以预测遗传此类疾病的风险。
Modern genetic testing and prenatal diagnosis have enabled families to make informed reproductive decisions. However, ethical considerations are essential, particularly regarding confidentiality and the right not to know one's genetic status.
现代基因检测和产前诊断使家庭能够做出知情的生育决策。然而，伦理考量至关重要，特别是在保密性和“不知情权”方面。
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