IGCSE AQA Biology: Genetics Key Points | IGCSE AQA 生物：遗传学考点精讲

📚 IGCSE AQA Biology: Genetics Key Points | IGCSE AQA 生物：遗传学考点精讲

Genetics is the study of how characteristics are passed from parents to offspring. In IGCSE AQA Biology, this topic covers DNA structure, genes, alleles, patterns of inheritance, sex determination, genetic disorders, variation, and evolution. Mastering these concepts is essential for understanding how life continues and diversifies.

遗传学研究特征如何从亲代传递给后代。在 IGCSE AQA 生物中，这一主题涵盖 DNA 结构、基因、等位基因、遗传模式、性别决定、遗传病、变异和进化。掌握这些概念对于理解生命的延续和多样性至关重要。

1. Introduction to Genetics | 遗传学简介

Genetics explains why offspring resemble their parents but are not identical to them. It involves the study of heredity and the variation of inherited characteristics.

遗传学解释了为什么后代与亲代相似却不完全相同，它涉及对遗传和可遗传特征变异的研究。

The genetic information that determines your features is carried in molecules called DNA, organised into structures called chromosomes within the nucleus of every cell.

决定你特征的遗传信息存在于称为 DNA 的分子中，这些分子在细胞核内组织成称为染色体的结构。

2. DNA and Chromosomes | DNA 与染色体

DNA (deoxyribonucleic acid) is a double‑stranded molecule twisted into a double helix. Each strand is made up of repeating units called nucleotides, which consist of a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C) or guanine (G).

DNA（脱氧核糖核酸）是一种双链分子，缠绕成双螺旋结构。每条链由称为核苷酸的重复单位组成，核苷酸包含一个脱氧核糖、一个磷酸基团以及含氮碱基之一：腺嘌呤(A)、胸腺嘧啶(T)、胞嘧啶(C)或鸟嘌呤(G)。

The bases pair complementarily: A always pairs with T, and C always pairs with G, held together by hydrogen bonds. This pairing ensures accurate replication.

碱基互补配对：A 总是与 T 配对，C 总是与 G 配对，由氢键连接。这种配对确保了准确复制。

In human body cells, there are 46 chromosomes arranged in 23 pairs. One chromosome of each pair comes from the mother and the other from the father. The 23rd pair determines sex (XX for female, XY for male).

人体细胞中有 46 条染色体，成对排列为 23 对。每对染色体中的一条来自母亲，另一条来自父亲。第 23 对决定性别（XX 为女性，XY 为男性）。

A gene is a small section of DNA on a chromosome that codes for a particular protein, which then contributes to a characteristic. For example, there is a gene for eye colour, another for the ability to roll your tongue.

基因是染色体上 DNA 的一小段，编码特定的蛋白质，从而决定某种特征。例如，有一种控制眼睛颜色的基因，另一种控制能否卷舌的基因。

3. Genes and Alleles | 基因与等位基因

Different forms of the same gene are called alleles. For a given gene, you inherit one allele from your mother and one from your father. The combination of alleles you have is your genotype, while the observable characteristic is your phenotype.

同一基因的不同形式称为等位基因。对于某个基因，你从母亲继承一个等位基因，从父亲继承另一个等位基因。你所拥有的等位基因组合称为基因型，而可观察到的特征称为表型。

If the two alleles for a gene are identical (e.g. BB or bb), the organism is homozygous for that trait. If the two alleles are different (e.g. Bb), the organism is heterozygous. Homozygous individuals are ‘pure‑breeding’ for that trait, while heterozygous individuals are ‘carriers’ of the recessive allele.

若某个基因的两个等位基因相同（如 BB 或 bb），该生物在该性状上是纯合的。若两个等位基因不同（如 Bb），则为杂合的。纯合个体在该性状上“纯种繁殖”，而杂合个体是隐性等位基因的“携带者”。

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

Alleles can be dominant or recessive. A dominant allele is always expressed in the phenotype even if only one copy is present. A recessive allele is only expressed if two copies are present (i.e. the organism is homozygous recessive). Dominant alleles are represented by a capital letter (e.g. A), recessive alleles by a lower‑case letter (e.g. a).

等位基因可以是显性的或隐性的。显性等位基因即使只有一个拷贝也会在表型中表达。隐性等位基因只有存在两个拷贝（即纯合隐性）时才表达。显性等位基因用大写字母表示（如 A），隐性等位基因用小写字母表示（如 a）。

For example, in pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). A plant with the genotype Pp will have purple flowers because the dominant allele masks the recessive one.

例如，在豌豆中，紫色花的等位基因 (P) 对白色花的等位基因 (p) 为显性。基因型为 Pp 的植株会开紫色花，因为显性等位基因掩盖了隐性等位基因。

5. Monohybrid Inheritance | 单基因遗传

Monohybrid inheritance involves the study of one gene at a time. By tracking a single characteristic, scientists can predict the probability of offspring showing certain traits.

单基因遗传涉及一次研究一个基因。通过追踪单一特征，科学家可以预测后代表现出某种性状的概率。

A genetic cross starts with the parental generation (P). Their offspring are the first filial generation (F₁). Interbreeding the F₁ generation produces the second filial generation (F₂). The ratios of phenotypes in the F₂ generation reveal the dominance relationships.

遗传杂交从亲代 (P) 开始。它们的后代是第一子代 (F₁)。让 F₁ 代杂交产生第二子代 (F₂)。F₂ 代的表型比例揭示了显隐性关系。

Consider a cross between a homozygous dominant tall pea plant (TT) and a homozygous recessive dwarf plant (tt). All F₁ offspring are heterozygous (Tt) and tall. When two Tt plants are crossed, the expected F₂ phenotype ratio is 3 tall : 1 dwarf.

考虑纯合显性高茎豌豆 (TT) 与纯合隐性矮茎豌豆 (tt) 的杂交。所有 F₁ 后代都是杂合子 (Tt)，表现为高茎。当两株 Tt 植株杂交时，预期的 F₂ 表型比例为 3 高：1 矮。

6. Punnett Squares | 庞纳特方格

A Punnett square is a grid used to determine the possible genotypes of offspring from a genetic cross. It shows the alleles each parent can pass on and how they combine.

庞纳特方格是一种用于确定遗传杂交中后代可能基因型的表格。它显示了每个亲本可以传递的等位基因及其组合方式。

Example: crossing two heterozygous (Tt) plants.

示例：两株杂合子 (Tt) 植株杂交。

	T	t
T	TT	Tt
t	Tt	tt

Genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 dwarf.

基因型：1 TT : 2 Tt : 1 tt。表型：3 高茎 : 1 矮茎。

Punnett squares only predict probabilities; actual offspring ratios may vary due to chance, especially with small sample sizes.

庞纳特方格仅预测概率；实际后代比例可能因随机性而有所不同，尤其是样本量较小时。

7. Sex Determination | 性别决定

In humans, sex is determined by the 23rd pair of chromosomes, called sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

在人类中，性别由第 23 对染色体（即性染色体）决定。女性有两条 X 染色体 (XX)，而男性有一条 X 和一条 Y 染色体 (XY)。

During gamete formation, a female can only produce eggs containing an X chromosome. A male produces sperm containing either an X or a Y chromosome. Therefore, the sperm determines the sex of the child.

在配子形成过程中，女性只能产生含有 X 染色体的卵细胞。男性产生含有 X 或 Y 染色体的精子。因此，精子决定了孩子的性别。

	X (sperm)	Y (sperm)
X (egg)	XX (female)	XY (male)

Thus, there is a 50% probability of having a male child and a 50% probability of having a female child at each pregnancy.

因此，每次怀孕生男孩的概率为 50%，生女孩的概率也为 50%。

8. Family Pedigrees | 家族系谱分析

A family pedigree is a chart that shows the inheritance of a trait over several generations. Males are represented by squares, females by circles. Shaded symbols indicate individuals who express the trait; unshaded symbols indicate those who do not.

家族系谱图是显示某一性状在几代人中遗传情况的图表。男性用方块表示，女性用圆圈表示。填充的符号表示表现该性状的个体，未填充的符号表示未表现该性状的个体。

By analysing a pedigree, you can often determine whether the trait is dominant or recessive and whether it is autosomal (on a non‑sex chromosome) or sex‑linked (usually X‑linked).

通过分析系谱图，通常可以确定该性状是显性还是隐性，以及是常染色体（非性染色体）还是性连锁（通常是 X 连锁）。

For example, if a trait appears in every generation and affected individuals have an affected parent, it is likely dominant. If it skips generations and affected children can have unaffected parents, it is likely recessive.

例如，若性状在每一代都出现且患病个体有患病的亲代，则很可能是显性的。若性状隔代出现且患病孩子的父母未患病，则很可能是隐性的。

9. Genetic Disorders: Cystic Fibrosis & Polydactyly | 遗传病：囊性纤维化与多指症

Cystic fibrosis (CF) is a recessive genetic disorder caused by a faulty allele on chromosome 7. It affects the production of a protein that regulates salt and water movement across cell membranes, leading to the buildup of thick, sticky mucus in the lungs and digestive system.

囊性纤维化 (CF) 是一种由第 7 号染色体上的缺陷等位基因引起的隐性遗传病。它影响一种调节盐和水跨细胞膜运输的蛋白质的生成，导致肺部和消化系统积聚粘稠的黏液。

A person must inherit two faulty alleles to have CF (genotype ff). Carriers (Ff) do not show symptoms but can pass the allele to their children. If both parents are carriers, there is a 25% chance their child will have CF, a 50% chance the child will be a carrier, and a 25% chance the child will be homozygous normal.

一个人必须继承两个缺陷等位基因才会患 CF（基因型 ff）。携带者 (Ff) 不表现症状，但可以将该等位基因传给子女。如果父母双方都是携带者，孩子有 25% 的概率患 CF，50% 的概率是携带者，25% 的概率为纯合正常。

Polydactyly is a dominant genetic disorder where a person has extra fingers or toes. It is caused by a dominant allele (P). Even one copy of the allele results in the condition. A heterozygous affected individual (Pp) has a 50% chance of passing the trait to each child if their partner is homozygous recessive (pp).

多指症是一种显性遗传病，患者有多余的手指或脚趾。它由显性等位基因 (P) 引起，即使只有一个拷贝也会发病。如果杂合患病个体 (Pp) 的伴侣是纯合隐性 (pp)，每个孩子有 50% 的概率遗传该性状。

10. Variation: Continuous and Discontinuous | 变异：连续与不连续

Variation describes the differences between individuals of the same species. It can be caused by genetic factors (inherited alleles), environmental factors (e.g. diet, climate), or a combination of both.

变异描述同一物种个体之间的差异。它可以由遗传因素（遗传的等位基因）、环境因素（如饮食、气候）或两者共同引起。

Continuous variation shows a range of phenotypes with no distinct categories. Examples include height, weight, and skin colour. These traits are usually controlled by many genes (polygenic) and are strongly influenced by the environment. When plotted on a graph, they form a bell‑shaped curve.

连续变异表现出一系列没有明显类别的表型。例如身高、体重和肤色。这些性状通常由多个基因控制（多基因），并且受环境强烈影响。在图表上绘制时，它们形成钟形曲线。

Discontinuous variation shows distinct, separate categories. Examples are blood groups (A, B, AB, O), eye colour (basic types), and the ability to roll your tongue. These traits are usually controlled by a single gene with no environmental influence. Data fall into clearly defined groups.

不连续变异表现出明显、独立的类别。例如血型（A、B、AB、O）、眼睛颜色（基本类型）和卷舌能力。这些性状通常由单个基因控制，不受环境影响。数据落入明确定义的组别中。

11. Mutation | 突变

A mutation is a change in the DNA base sequence. Mutations can occur spontaneously during DNA replication or be caused by mutagens such as ionising radiation (UV light, X‑rays) and certain chemicals (e.g. those in tobacco smoke).

突变是 DNA 碱基序列的变化。突变可以在 DNA 复制过程中自发发生，也可以由诱变剂引起，如电离辐射（紫外线、X 射线）和某些化学物质（如烟草烟雾中的化学物质）。

Most mutations have no effect on the phenotype because the genetic code is degenerate (different codons can code for the same amino acid) or because they occur in non‑coding DNA. However, some mutations can alter the amino acid sequence of a protein, potentially changing its shape and function.

大多数突变对表型没有影响，因为遗传密码具有简并性（不同的密码子可以编码相同的氨基酸），或者发生在非编码 DNA 中。然而，一些突变可以改变蛋白质的氨基酸序列，可能改变其形状和功能。

Occasionally, a mutation can produce a new allele that gives an organism a survival advantage. For example, a mutation in bacteria may confer resistance to an antibiotic. Such beneficial mutations are the raw material for evolution by natural selection.

偶尔，突变可以产生新的等位基因，赋予生物生存优势。例如，细菌中的突变可能使其对抗生素产生耐药性。这种有利突变是自然选择进化的原始材料。

12. Natural Selection and Evolution | 自然选择与进化

Natural selection is the process by which organisms with traits better suited to their environment are more likely to survive, reproduce, and pass on their advantageous alleles to the next generation. Over many generations, this can lead to evolution — a change in the inherited characteristics of a population over time.

自然选择是指具有更适合其环境的性状的生物更有可能生存、繁殖并将有利等位基因传给下一代的过程。经过许多代，这可能导致进化——即种群可遗传特征随时间的变化。

The development of antibiotic resistance in bacteria is a clear example. When a bacterial population is exposed to an antibiotic, most bacteria die, but a few that happen to have a resistance allele survive. These survivors reproduce, passing on the resistance allele. Over time, the population becomes dominated by resistant bacteria, making the antibiotic ineffective.

细菌产生抗生素耐药性就是一个明显的例子。当细菌种群暴露于抗生素时，大多数细菌死亡，但少数碰巧拥有耐药等位基因的细菌存活下来。这些幸存者繁殖，传递耐药等位基因。随着时间的推移，种群被耐药菌主导，使抗生素失效。

Note that mutations for resistance occur randomly and are not caused by the antibiotic. The antibiotic acts as a selection pressure, killing non‑resistant individuals.

请注意，耐药性突变是随机发生的，并非由抗生素引起。抗生素充当选择压力，杀死非耐药个体。

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IGCSE AQA Biology: Genetics Key Points | IGCSE AQA 生物：遗传学 考点精讲