📚 Monohybrid Inheritance | 单基因遗传
Welcome to this comprehensive revision guide on monohybrid inheritance, designed for Edexcel IGCSE Biology. Monohybrid inheritance involves the study of how a single characteristic is transmitted from parents to offspring through alleles. By mastering the key concepts – dominant and recessive alleles, homozygous and heterozygous genotypes, and the use of Punnett squares – you can predict the probability of offspring displaying certain traits. This guide will take you step by step through the essential theory, worked examples, and common human genetic disorders, helping you build confidence for your exams.
欢迎阅读这篇为Edexcel IGCSE生物学设计的单基因遗传全面复习指南。单基因遗传研究的是单一性状如何通过等位基因从亲代传给子代。掌握显性与隐性等位基因、纯合与杂合基因型以及庞纳特方格等核心概念，你就能预测后代表现特定性状的概率。本指南将逐步带你学习基本理论、实例分析以及常见人类遗传病，助你在考试中树立信心。
1. What is Monohybrid Inheritance? | 什么是单基因遗传？
Monohybrid inheritance refers to the inheritance pattern of a single characteristic determined by one gene. The term 'mono' means one, and 'hybrid' means a cross between two individuals with different alleles for that gene. A classic example is the height of pea plants studied by Gregor Mendel, where tall and dwarf varieties were crossed to observe patterns in the offspring.
单基因遗传指的是由一对等位基因决定的单一性状的遗传模式。'Mono'意为单一，'hybrid'指两个具有不同等位基因的个体之间的杂交。经典例子是格里哥·孟德尔研究的豌豆植株高度，他将高茎和矮茎品种进行杂交，观察后代中的遗传模式。
2. Key Genetic Terms | 关键遗传术语
To understand monohybrid inheritance, you must become comfortable with a set of fundamental terms. A gene is a section of DNA that codes for a specific protein and determines a characteristic. An allele is an alternative form of a gene. For example, the gene for plant height has a tall allele and a dwarf allele. The genome refers to the entire DNA of an organism. Chromosomes, located in the nucleus, carry many genes lined up along their length.
要理解单基因遗传，你必须熟悉一系列基本术语。基因是编码特定蛋白质并决定性状的一段DNA。等位基因是基因的不同形式。例如控制植株高度的基因就有高茎等位基因和矮茎等位基因。基因组是指一个生物体的全部DNA。位于细胞核中的染色体携带着许多沿其长轴排列的基因。
3. Alleles: Dominant and Recessive | 等位基因：显性与隐性
Alleles can be dominant or recessive. A dominant allele is always expressed in the phenotype even if only one copy is present. We represent it with an uppercase letter, such as T for tall. A recessive allele is only expressed if two copies are present (homozygous recessive), and is represented by a lowercase letter, e.g. t for dwarf. In a heterozygous individual (Tt), the dominant tall allele masks the effect of the recessive allele, so the plant grows tall.
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现型中表达。我们用大写字母表示，如T代表高茎。隐性等位基因只有当两个拷贝同时存在（隐性纯合）时才表达，用小写字母表示，例如t代表矮茎。在杂合个体（Tt）中，显性高茎等位基因掩盖了隐性等位基因的作用，因此植株表现为高茎。
4. Homozygous and Heterozygous Genotypes | 纯合与杂合基因型
The combination of alleles an organism possesses is called its genotype. A homozygous genotype has two identical alleles for a gene – either both dominant (TT) or both recessive (tt). A heterozygous genotype has two different alleles (Tt). In monohybrid crosses, the genotypes of parents and offspring determine how traits are passed on.
一个生物体拥有的等位基因组合称为基因型。纯合基因型拥有两个相同的等位基因——要么都是显性（TT），要么都是隐性（tt）。杂合基因型则拥有两个不同的等位基因（Tt）。在单基因杂交中，亲代和子代的基因型决定了性状的传递方式。
5. Phenotype vs Genotype | 表现型与基因型
The phenotype is the observable characteristic resulting from the genotype and its interaction with the environment. For instance, plants with genotypes TT and Tt both show a tall phenotype, while only tt shows dwarf. Understanding the distinction between genotype and phenotype is crucial when analysing inheritance patterns and predicting offspring ratios.
表现型是由基因型及其与环境相互作用产生的可观察性状。例如，基因型为TT和Tt的植株都表现出高茎表现型，而只有tt表现为矮茎。理解基因型与表现型之间的区别对于分析遗传模式和预测后代比例至关重要。
6. Mendel's Pea Plant Experiments | 孟德尔的豌豆实验
Gregor Mendel's work on pea plants laid the foundation of genetics. He crossed pure-breeding tall plants (TT) with pure-breeding dwarf plants (tt). The first filial (F1) generation were all tall (Tt). When he self-pollinated the F1 plants, the second filial (F2) generation showed a phenotypic ratio of approximately 3 tall : 1 dwarf. This demonstrated that traits are inherited as discrete units (genes) and that recessive alleles can be carried without being expressed.
格里哥·孟德尔的豌豆实验奠定了遗传学的基础。他将纯种高茎植株（TT）与纯种矮茎植株（tt）杂交。子一代（F1）全部为高茎（Tt）。当他让F1植株自花授粉后，子二代（F2）出现了约3高茎 : 1矮茎的表现型比例。这证明性状是作为离散单位（基因）遗传的，并且隐性等位基因可以在不表达的情况下被携带。
7. Using Punnett Squares | 使用庞纳特方格
A Punnett square is a grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross. You place the gametes of one parent along the top and the gametes of the other parent along the side. Filling in the squares shows all the possible combinations when fertilisation occurs at random.
庞纳特方格是一种用来预测遗传杂交后代可能基因型和表现型的网格工具。你将一个亲本的配子放在顶部，另一个亲本的配子放在侧边。填充方格即可展示随机受精时所有可能的组合。
Example cross between two heterozygous tall plants (Tt × Tt):
两个杂合高茎植株（Tt × Tt）之间的杂交示例：
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
The Punnett square above shows a genotypic ratio of 1 TT : 2 Tt : 1 tt.
上方的庞纳特方格展示的基因型比例为1 TT : 2 Tt : 1 tt。
8. Predicting Offspring Ratios | 预测后代比例
ross, the expected phenotypic ratio is typically 3 : 1 (dominant phenotype : recessive phenotype), provided that dominance is complete. The genotypic ratio is 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. In a test cross, where an organism showing the dominant trait is crossed with a homozygous recessive individual, the offspring ratio reveals the unknown genotype: if any offspring show the recessive trait, the tested parent must be heterozygous.
在杂合子与杂合子的杂交中，只要存在完全显性，预期的表现型比例通常为3 : 1（显性表现型 : 隐性表现型）。基因型比例为1纯合显性 : 2杂合 : 1纯合隐性。在测交中，将表现显性性状的个体与隐性纯合个体杂交，后代表现型比例能揭示未知的基因型：如果任何后代出现隐性性状，则被测试的亲本必定是杂合子。
Phenotypic ratio: 3 dominant : 1 recessive
表现型比例：3 显性 : 1 隐性
9. Understanding Pedigree Charts | 理解家族谱系图
Pedigree charts are diagrams that show the occurrence of a trait through several generations of a family. Squares represent males, circles represent females. Shaded symbols indicate individuals who express the trait, and unshaded symbols indicate those who do not. By tracing a condition through a pedigree, you can often determine whether it is caused by a dominant or recessive allele. If two unaffected parents produce an affected child, the trait is recessive, and both parents are heterozygous carriers.
家族谱系图是显示某个性状在一个家族几代人中发生情况的图表。方块代表男性，圆圈代表女性。实心符号表示表现出该性状的个体，空心符号表示未表现该性状的个体。通过追踪某种疾病在谱系中的传递，通常可以判断其是显性还是隐性等位基因所致。如果两个未患病的父母生出一个患病的孩子，则该性状是隐性的，并且父母双方都是杂合携带者。
10. Monohybrid Crosses in Humans: Cystic Fibrosis | 人类单基因杂交：囊性纤维化
Cystic fibrosis (CF) is an inherited disorder caused by a recessive allele of the CFTR gene. We represent the normal allele as F and the faulty allele as f. Individuals with the genotype ff develop CF, producing thick, sticky mucus that affects the lungs and digestive system. Heterozygous individuals (Ff) are carriers and do not show symptoms. A cross between two carriers has a 25% chance of producing a child with CF.
囊性纤维化是由CFTR基因的一个隐性等位基因引起的遗传病。我们用F表示正常等位基因，f表示缺陷等位基因。基因型为ff的个体患囊性纤维化，会产生影响肺部和消化系统的黏稠粘液。杂合子个体（Ff）是携带者，不表现症状。两个携带者之间的杂交有25%的概率生下患病孩子。
| F | f | |
| F | FF | Ff |
| f | Ff | ff |
11. Monohybrid Crosses: Sickle Cell Disease | 单基因杂交：镰状细胞病
Sickle cell disease is another example of monohybrid inheritance, caused by a recessive allele of the HBB gene. The normal allele is often written as HbA and the sickle allele as HbS. People with two sickle alleles (HbS HbS) suffer from sickle cell anaemia, where red blood cells become sickle-shaped. Heterozygotes (HbA HbS) have sickle cell trait and are resistant to malaria, demonstrating the concept of heterozygote advantage in certain regions. A cross between two heterozygotes gives a 1 in 4 chance of having a child with sickle cell disease.
镰状细胞病是另一个单基因遗传实例，由HBB基因的隐性等位基因引起。正常等位基因常写作HbA，镰状等位基因写作HbS。拥有两个镰状等位基因（HbS HbS）的人患镰状细胞贫血，红细胞呈镰刀形。杂合子（HbA HbS）具有镰状细胞特征，且对疟疾有抵抗力，这体现了某些地区杂合子优势的概念。两个杂合子之间的杂交有1/4的概率生下镰状细胞病患儿。
12. Limitations and Importance | 局限性与重要性
While monohybrid crosses illustrate basic inheritance patterns perfectly, many real-world traits are influenced by multiple genes (polygenic inheritance) or display incomplete dominance and codominance. However, understanding monohybrid inheritance remains essential. It allows genetic counsellors to predict the risk of inherited disorders, helps farmers and breeders select desirable characteristics, and forms the backbone of modern genetic analysis. For your Edexcel IGCSE, practising Punnett squares and interpreting pedigree charts will be invaluable.
虽然单基因杂交完美地阐明了基本的遗传模式，但现实世界中许多性状受多基因影响（多基因遗传），或表现出不完全显性和共显性。然而，理解单基因遗传依然至关重要。它使遗传咨询师能够预测遗传病的风险，帮助农民和育种者筛选优良性状，并构成现代遗传分析的基石。对于你的Edexcel IGCSE考试，练习庞纳特方格和解读家族谱系图将极具价值。
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