Genetic Inheritance for OCR Science | OCR 科学：遗传考点精讲

📚 Genetic Inheritance for OCR Science | OCR 科学：遗传考点精讲

Genetic inheritance is the process by which traits are passed from parents to offspring through genes. Understanding how characteristics are inherited is a fundamental part of biology, and it forms a key topic in OCR GCSE Science. The following guide covers essential concepts, from DNA and alleles to monohybrid crosses and inherited disorders, helping you master the topic step by step.

遗传是亲本通过基因将性状传递给后代的过程。理解性状如何遗传是生物学的基础，也是 OCR GCSE 科学中的关键考点。以下指南涵盖从 DNA 与等位基因到单基因杂交和遗传病的基本概念，帮助你逐步掌握该主题。

1. DNA, Genes and Chromosomes | DNA、基因与染色体

Deoxyribonucleic acid (DNA) is a long molecule that carries the genetic instructions used in the growth, development and reproduction of all living organisms. A gene is a small section of DNA that codes for a particular protein, which in turn determines a characteristic. Genes are arranged along thread-like structures called chromosomes, which are found in the nucleus of most cells.

脱氧核糖核酸（DNA）是一种携带所有生物生长、发育和繁殖所需遗传指令的长分子。基因是 DNA 上的一个小片段，编码特定的蛋白质，进而决定性状。基因沿被称为染色体的线状结构排列，染色体存在于大多数细胞的细胞核中。

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 the sex of the individual.

人体细胞中有 46 条染色体，排列成 23 对。每对染色体中的一条来自母亲，另一条来自父亲。第 23 对染色体决定个体的性别。

2. Alleles and Genotype | 等位基因与基因型

An allele is a different version of the same gene. For example, the gene that controls eye colour may have an allele for brown eyes and an allele for blue eyes. Each individual inherits two alleles for each gene – one from each parent.

等位基因是同一基因的不同版本。例如，控制眼睛颜色的基因可能有一个产生棕色眼睛的等位基因和一个产生蓝色眼睛的等位基因。每个个体从每个亲本各继承一个等位基因，因此每个基因有两个等位基因。

The combination of alleles an organism possesses is called its genotype. The observable characteristics that result from the genotype are known as the phenotype.

生物体所拥有的等位基因组合称为其基因型。由基因型导致的 observable 特征称为表型。

3. Dominant and Recessive Alleles | 显性等位基因与隐性等位基因

Some alleles are dominant, meaning that they will express their characteristic even if only one copy is present. Recessive alleles only show their effect if two copies are present (i.e. when no dominant allele is present). In genetic diagrams, dominant alleles are represented by uppercase letters (e.g. A), while recessive alleles are written in lowercase (e.g. a).

有些等位基因是显性的，意味着即使只存在一个拷贝，它们也会表现其性状。隐性等位基因只有在存在两个拷贝（即没有显性等位基因时）才表现效应。在遗传图解中，显性等位基因用大写字母（如 A）表示，隐性等位基因用小写字母（如 a）表示。

A person who inherits at least one dominant allele for a trait will show the dominant phenotype. The recessive phenotype only appears in individuals with two recessive alleles.

对于一个性状，只要遗传到至少一个显性等位基因，个体就会显示显性表型。隐性表型仅出现在拥有两个隐性等位基因的个体中。

4. Homozygous and Heterozygous | 纯合与杂合

When an organism has two identical alleles for a gene – both dominant or both recessive – it is said to be homozygous for that trait (e.g. AA or aa). If the two alleles are different (e.g. Aa), the organism is heterozygous.

当生物体的某个基因拥有两个相同的等位基因——同为显性或同为隐性——则称该性状为纯合的（如 AA 或 aa）。如果两个等位基因不同（如 Aa），则该生物体为杂合的。

Heterozygous individuals carry a recessive allele but do not express it; they are sometimes called carriers, especially in the context of genetic disorders.

杂合个体携带着一个隐性等位基因但不表现它；他们有时被称为携带者，尤其在遗传病的语境中。

5. Monohybrid Inheritance | 单基因遗传

Monohybrid inheritance is the study of how a single characteristic is passed from one generation to the next. It involves one gene with two alleles, one dominant and one recessive. Gregor Mendel’s experiments with pea plants laid the foundation for our understanding of monohybrid crosses.

单基因遗传研究的是单一性状如何从一代传到下一代。它涉及一个具有两个等位基因的基因，一个显性一个隐性。格雷戈尔·孟德尔的豌豆实验为我们理解单基因杂交奠定了基础。

In a typical monohybrid cross between two heterozygous parents (Aa × Aa), the expected offspring genotypes follow a 1:2:1 ratio (AA : Aa : aa), giving a 3:1 phenotypic ratio of dominant to recessive traits.

在典型的两个杂合亲本（Aa × Aa）的单基因杂交中，预期的子代基因型比例为 1:2:1（AA : Aa : aa），显性与隐性性状的表型比例为 3:1。

6. Punnett Squares | 旁氏方格

A Punnett square is a grid used to predict the possible genotypes of offspring from a genetic cross. The alleles from one parent are written along the top, and the alleles from the other parent are written down the side. The combination inside each box shows a possible offspring genotype.

旁氏方格是一种用于预测杂交后代可能基因型的表格。一个亲本的等位基因写在顶部，另一个亲本的等位基因写在侧边。方框内的组合表示一种可能的子代基因型。

For example, crossing a homozygous dominant parent (AA) with a homozygous recessive parent (aa) produces offspring that are all heterozygous (Aa) and show the dominant phenotype.

例如，将一个纯合显性的亲本（AA）与一个纯合隐性的亲本（aa）杂交，产生的后代全部为杂合子（Aa），并表现显性表型。

7. Predicting Genotypic and Phenotypic Ratios | 预测基因型与表型比例

The ratio of different genotypes among offspring can be calculated using a Punnett square. In a cross between two heterozygotes (Aa × Aa), the genotypic ratio is 1 AA : 2 Aa : 1 aa. Because AA and Aa both show the dominant trait, the phenotypic ratio becomes 3 dominant : 1 recessive.

子代中不同基因型的比例可通过旁氏方格计算。在两个杂合子（Aa × Aa）之间的杂交中，基因型比例为 1 AA : 2 Aa : 1 aa。由于 AA 和 Aa 均表现显性性状，表型比例变为 3 显性 : 1 隐性。

These ratios are probability predictions based on large numbers of offspring; in small families, actual numbers may differ significantly from these expectations.

这些比例是基于大量后代数量的概率预测；在小家庭中，实际数量可能与这些预期有显著差异。

8. Sex Determination | 性别决定

In humans, sex is determined by the 23rd pair of chromosomes: females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The sex of a baby is determined by the sperm: an egg always carries an X chromosome, but a sperm can carry either an X or a Y.

人类性别由第 23 对染色体决定：女性有两条 X 染色体（XX），男性有一条 X 和一条 Y 染色体（XY）。婴儿的性别由精子决定：卵子总是携带一条 X 染色体，而精子可以携带 X 或 Y 染色体。

A Punnett square for sex determination shows that there is a 50% chance of having a male (XY) and a 50% chance of having a female (XX) in each pregnancy.

性别决定的旁氏方格显示，每次怀孕有 50% 的概率为男性（XY），50% 的概率为女性（XX）。

9. Inherited Disorders – Cystic Fibrosis | 遗传病——囊性纤维化

Cystic fibrosis (CF) is a recessive genetic disorder caused by a faulty allele of the CFTR gene. It leads to the production of thick, sticky mucus in the lungs and digestive system, causing breathing difficulties and problems with digestion.

囊性纤维化（CF）是一种隐性遗传病，由 CFTR 基因的一个缺陷等位基因引起。它导致肺部和消化系统中产生浓稠黏液，造成呼吸困难和消化问题。

For a child to have CF, both parents must be carriers (heterozygous Ff) or affected. If both parents are carriers, there is a 25% chance the child will have the disorder (ff), a 50% chance of being a carrier (Ff), and a 25% chance of being unaffected and not a carrier (FF).

孩子若要患上囊性纤维化，父母双方都必须是携带者（杂合子 Ff）或者患者。如果父母双方都是携带者，孩子有 25% 的概率患病（ff），50% 的概率成为携带者（Ff），以及 25% 的几率既未患病也不是携带者（FF）。

10. Inherited Disorders – Polydactyly | 遗传病——多指（趾）症

Polydactyly is a dominant genetic disorder that causes a person to be born with extra fingers or toes. It is caused by a dominant allele (P), so only one copy of the allele is needed for the condition to appear.

多指（趾）症是一种显性遗传病，患者出生时即有多余的手指或脚趾。该病由一个显性等位基因（P）引起，因此只需一个等位基因即可表现症状。

A person with polydactyly will be either heterozygous (Pp) or homozygous dominant (PP). If a heterozygous affected person (Pp) has children with a non-affected person (pp), there is a 50% chance that each child will inherit the disorder.

多指症患者可能为杂合子（Pp）或纯合显性（PP）。如果一个杂合患者（Pp）与非患者（pp）生育子女，每个孩子有 50% 的几率遗传该病。

11. Family Pedigrees | 家族系谱图

A family pedigree chart is a diagram that shows how a trait is inherited across multiple generations. Males are represented by squares and females by circles. Shaded symbols usually indicate individuals who express the trait, while unshaded symbols represent those who do not.

家族系谱图是一种显示性状如何跨多代遗传的图表。男性用正方形表示，女性用圆形表示。阴影符号通常表示表现性状的个体，无阴影符号表示不表现的个体。

Pedigrees can help determine whether a trait is dominant or recessive. If two unaffected parents have an affected child, the trait must be recessive, and both parents are carriers.

系谱图有助于判断性状是显性还是隐性。如果两个未患病的父母生出一个患病的孩子，则该性状必定是隐性的，而且父母双方都是携带者。

12. Summary and Exam Tips | 总结与应试技巧

Master genetic vocabulary: be able to define allele, genotype, phenotype, homozygous, heterozygous, dominant and recessive accurately. In exams, always use the correct letters when drawing Punnett squares and show all possible gametes from each parent.

掌握遗传学词汇：能准确定义等位基因、基因型、表型、纯合、杂合、显性和隐性。在考试中，绘制旁氏方格时要使用正确的字母，并标明每个亲本所有可能的配子。

When explaining monohybrid crosses, state the phenotypic ratio clearly and relate it to the probability for each child, not just the number among siblings. Practice interpreting pedigree charts and use genetic diagrams to explain how recessive and dominant disorders are inherited.

在解释单基因杂交时，要清晰地陈述表型比例，并将其与每个孩子的概率联系起来，而不是仅仅用兄弟姐妹中的数量。练习解读系谱图，并利用遗传图解解释隐性和显性遗传病是如何遗传的。

For cystic fibrosis and polydactyly, be ready to construct genetic crosses, identify carrier status, and calculate the chances of a child inheriting the condition. Always check whether you are being asked for genotype or phenotype in an exam question.

对于囊性纤维化和多指（趾）症，要准备好构建遗传杂交图、识别携带者状态，并计算孩子患病的概率。在答题时，请始终检查你是否被问到的是基因型还是表型。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Genetic Inheritance for OCR Science | OCR 科学：遗传 考点精讲