Genetics Revision for AQA GCSE Science | 遗传考点精讲

📚 Genetics Revision for AQA GCSE Science | 遗传考点精讲

In AQA GCSE Science, understanding genetics is fundamental to explaining how characteristics are inherited. This revision guide covers key topics from DNA structure to inherited disorders, allele interactions, and sex-linked inheritance. Each concept is broken down with clear explanations and exam-focused insights.

在AQA GCSE科学中，理解遗传学是解释性状如何遗传的基础。本复习指南涵盖从DNA结构到遗传病、等位基因相互作用及伴性遗传等关键主题。每个概念均辅以清晰的解释和应试要点。

1. DNA, Genes and Chromosomes | DNA、基因和染色体

DNA (deoxyribonucleic acid) is a double-stranded helix that carries the genetic instructions for all living organisms. The monomers of DNA are nucleotides, each containing a sugar, a phosphate group, and a base (A, T, C, or G). Bases pair specifically: A with T, C with G.

DNA（脱氧核糖核酸）是一种双螺旋结构，携带着所有生物体的遗传指令。DNA的单体是核苷酸，每个核苷酸包含一个糖、一个磷酸基团和一个碱基（A、T、C或G）。碱基配对具有特异性：A与T配对，C与G配对。

A gene is a section of DNA that codes for a specific protein. Proteins determine most of our characteristics, from eye colour to enzyme function. Genes are found on chromosomes, which are long, coiled molecules of DNA located in the nucleus of cells. Humans have 23 pairs of chromosomes in most body cells.

基因是编码特定蛋白质的DNA片段。蛋白质决定了我们从眼睛颜色到酶功能的大多数特征。基因位于染色体上，染色体是位于细胞核中的长而卷曲的DNA分子。人类大多数体细胞中有23对染色体。

2. Alleles and Genetic Terms | 等位基因与遗传学术语

Different forms of the same gene are called alleles. For instance, the gene for eye colour has alleles for brown, blue, etc. Each individual inherits two alleles for each gene, one from each parent.

同一基因的不同形式称为等位基因。例如，眼睛颜色的基因有棕色、蓝色等位基因。每个个体从父母各继承一个等位基因，因此每个基因拥有两个等位基因。

If the two alleles are identical, the organism is homozygous for that trait; if they are different, it is heterozygous. The dominant allele is expressed even if only one copy is present (heterozygous), and is represented by a capital letter (e.g. B for brown eyes). The recessive allele is only expressed when two copies are present (homozygous), and is represented by a lower-case letter (e.g. b for blue eyes).

如果两个等位基因相同，则个体对该性状是纯合的；如果不同，则是杂合的。显性等位基因即使只有一个拷贝（杂合）也会表达，用大写字母表示（如B代表棕色眼）。隐性等位基因只有在两个拷贝都存在（纯合）时才会表达，用小写字母表示（如b代表蓝色眼）。

Genotype is the genetic makeup (e.g. BB, Bb, bb), while phenotype is the observable characteristic (e.g. brown eyes or blue eyes).

基因型是遗传组成（如BB、Bb、bb），而表型是可观察到的特征（如棕色眼睛或蓝色眼睛）。

3. Monohybrid Inheritance | 单基因遗传

Monohybrid inheritance involves the study of a single characteristic controlled by one gene with two alleles. An example is the height of pea plants, with tall (T) dominant over dwarf (t).

单基因遗传研究由一对等位基因控制的单一特征。例如豌豆植株高度，高茎（T）对矮茎（t）显性。

When a homozygous tall plant (TT) is crossed with a homozygous dwarf plant (tt), the first filial (F₁) generation are all heterozygous (Tt) and tall. Crossing two F₁ plants (Tt × Tt) produces an F₂ generation with a phenotypic ratio of 3 tall : 1 dwarf.

当纯合高茎植株（TT）与纯合矮茎植株（tt）杂交时，子一代（F₁）全部为杂合（Tt）且高茎。将两株F₁植株（Tt × Tt）杂交，产生子二代（F₂），其表型比例为3高茎:1矮茎。

TT × tt → F₁ all Tt (tall); Tt × Tt → F₂ 1 TT : 2 Tt : 1 tt (3 tall : 1 dwarf)

4. Punnett Squares and Predictions | 庞纳特方格与预测

A Punnett square is a grid used to predict the possible genotypes of offspring from a genetic cross. The alleles from one parent are placed along the top, and those from the other parent along the side. The combinations in the boxes show the possible offspring genotypes.

庞纳特方格是一种用于预测遗传杂交后代可能基因型的表格。将一个亲本的等位基因放在顶部，另一亲本的等位基因放在侧边。方格中的组合显示后代可能的基因型。

Example: crossing two heterozygous parents (Bb × Bb):

示例：两个杂合亲本杂交（Bb × Bb）：

The genotypic ratio is 1 BB : 2 Bb : 1 bb, and the phenotypic ratio is 3 dominant : 1 recessive if B is dominant. Punnett squares allow you to calculate the probability of inheriting a particular trait.

基因型比例为1 BB : 2 Bb : 1 bb，如果B为显性，表型比例为3显性:1隐性。庞纳特方格可计算遗传特定性状的概率。

5. Family Pedigrees | 家族遗传图谱

A family pedigree chart tracks the inheritance of a trait through several generations. Males are represented by squares, females by circles. Shaded symbols indicate individuals expressing the trait; unshaded symbols show those without it. A horizontal line between a male and female represents mating, and vertical lines connect parents to offspring.

家族遗传图谱追踪某一性状在几代人中的遗传方式。男性用方形表示，女性用圆形表示。涂色的符号表示表达该性状的个体，未涂色的表示不表达的个体。男女之间的横线表示婚配，垂直线连接亲代与子代。

By analysing a pedigree, you can deduce whether a trait is dominant or recessive, and whether it is autosomal or sex-linked. For example, if a trait appears in every generation, it is likely dominant; if it skips generations, it is likely recessive.

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