GCSE OCR Science: Genetics | 遗传 考点精讲

📚 GCSE OCR Science: Genetics | 遗传 考点精讲

Genetics is the branch of biology that explains how traits are passed from parents to offspring. In the OCR GCSE Science specification, you need to understand the structure of DNA, how genes control characteristics, and how to predict inheritance using Punnett squares and family trees. This revision guide covers all the key concepts, including alleles, monohybrid crosses, sex determination, and inherited disorders, with clear explanations and exam tips.

遗传学是生物学的一个分支，解释性状如何从亲代传递给子代。在 OCR GCSE 科学大纲中，你需要理解 DNA 的结构、基因如何控制特征，以及如何使用旁纳特方格和家族系谱图预测遗传。本复习指南涵盖所有关键概念，包括等位基因、单基因杂交、性别决定和遗传病，并配有清晰的解释和考试技巧。

1. DNA and the Genome | DNA 与基因组

DNA (deoxyribonucleic acid) is a long, double-stranded molecule that carries the genetic instructions for all living organisms. It has a double helix structure, with each strand made of a sugar-phosphate backbone and nitrogenous bases (A, T, C, G). The bases pair specifically: A with T, and C with G, held together by hydrogen bonds.

DNA（脱氧核糖核酸）是一种长长的双链分子，携带着所有生物体的遗传指令。它具有双螺旋结构，每条链由糖-磷酸骨架和含氮碱基（A、T、C、G）组成。碱基特异性配对：A 与 T，C 与 G，通过氢键连接。

A gene is a short section of DNA that codes for a particular protein. The sequence of bases determines the order of amino acids in a protein, which then folds into a specific shape to carry out its function. This link between genes and proteins is how your genotype influences your phenotype.

基因是 DNA 的一个小片段，编码特定的蛋白质。碱基序列决定了蛋白质中氨基酸的排列顺序，然后折叠成特定形状以执行其功能。基因与蛋白质之间的这种联系，正是基因型影响表型的方式。

The entire set of genetic material in an organism is called the genome. Studying the human genome has helped scientists identify genes linked to inherited diseases and develop new medical treatments and genetic screening methods.

一个生物体中全部遗传物质的集合称为基因组。研究人类基因组帮助科学家鉴定出与遗传病相关的基因，并开发新的医学治疗和基因筛查方法。

2. Chromosomes and Genes | 染色体与基因

Inside the nucleus, DNA is tightly coiled and packaged into structures called chromosomes. Human body cells contain 46 chromosomes arranged in 23 pairs. One chromosome in each pair comes from the mother and the other from the father. These are homologous chromosomes – they carry the same genes in the same positions (loci), but may have different versions (alleles).

在细胞核内，DNA 紧密盘旋并被包装成称为染色体的结构。人体细胞含有 46 条染色体，排列成 23 对。每对染色体中一条来自母亲，另一条来自父亲。这些是同源染色体——它们在相同位置（基因座）上携带相同的基因，但可能具有不同的版本（等位基因）。

Gametes (sperm and egg cells) are produced by meiosis and are haploid: they contain only 23 unpaired chromosomes. At fertilisation, the nuclei of the sperm and egg fuse to restore the diploid number (46). This is why offspring inherit exactly half of their genetic information from each parent, leading to variation.

配子（精子和卵细胞）通过减数分裂产生，是单倍体：它们只含有 23 条不成对的染色体。受精时，精子和卵子的细胞核融合，恢复二倍体数目（46）。这就是为什么后代恰好从每个亲代继承一半的遗传信息，从而产生变异。

3. Alleles and Inheritance | 等位基因与遗传

Different forms of the same gene are called alleles. For example, the gene for eye colour has alleles for brown eyes or blue eyes. An individual inherits two alleles for each gene – one from each parent. If both alleles are the same, the individual is homozygous for that trait; if different, they are heterozygous.

同一基因的不同形式称为等位基因。例如，控制眼睛颜色的基因有棕色眼睛和蓝色眼睛的等位基因。个体每个基因遗传两个等位基因——一个来自父亲，一个来自母亲。如果两个等位基因相同，该个体在该性状上是纯合的；如果不同，则是杂合的。

Alleles can be dominant or recessive. A dominant allele is always expressed in the phenotype if present. A recessive allele is only expressed if an individual has two copies (homozygous recessive). This principle explains why some traits can skip generations.

等位基因可以是显性或隐性。显性等位基因只要存在就会在表型中表达。隐性等位基因仅在个体有两个拷贝（纯合隐性）时才会表达。这一原理解释了为什么某些性状会隔代出现。

When writing alleles, we use the same letter in upper and lower case, for instance ‘R’ and ‘r’. The dominant allele is written with a capital letter, and the recessive with a lower-case letter. The choice of letter is often based on the dominant trait (e.g., ‘T’ for tall).

书写等位基因时，我们使用同一个字母的大写和小写形式，例如 ‘R’ 和 ‘r’。显性等位基因用大写字母表示，隐性等位基因用小写字母表示。字母的选择通常基于显性性状（例如 ‘T’ 代表 tall，即高的）。

4. Dominant and Recessive Alleles | 显性与隐性等位基因

In genetic diagrams, the combination of alleles determines the appearance of the organism. A dominant allele masks the effect of a recessive allele in a heterozygous individual. Therefore, a heterozygous organism will show the dominant characteristic even though it carries one copy of the recessive allele.

在遗传图解中，等位基因的组合决定了生物体的外观。显性等位基因在杂合个体中会掩盖隐性等位基因的作用。因此，杂合生物体即使携带一个隐性等位基因的拷贝，仍然会表现出显性特征。

Recessive alleles are not ‘weaker’ versions; they simply code for a non-functional protein or no protein at all, while the dominant allele produces a working protein. The phenotype depends on whether a functional protein is produced.

隐性等位基因并非“较弱”的版本；它们只是编码了一种无功能的蛋白质或根本不产生蛋白质，而显性等位基因产生有功能的蛋白质。表型取决于是否产生功能性蛋白质。

It is important to remember that being dominant does not mean the allele is more common in the population. For example, the allele for polydactyly (extra digits

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