📚 Topic 3.9.2: Monohybrid Inheritance | 主题3.9.2：单基因遗传
Monohybrid inheritance is the study of how a single characteristic is passed from parents to offspring. This topic is a cornerstone of genetics in the Edexcel IGCSE Science syllabus, helping you understand how traits like eye colour, seed shape, or certain genetic diseases are inherited. By focusing on one gene at a time, we can predict the probability of offspring showing specific traits using simple models.
单基因遗传研究的是单一性状如何从亲代传递给子代。这是Edexcel IGCSE科学课程中遗传学的基础内容，能帮助你理解眼色、种子形状或某些遗传病等性状是如何遗传的。每次只关注一个基因，我们可以用简单的模型预测子代表现出特定性状的概率。
1. Genes and Chromosomes | 基因与染色体
Genes are sections of DNA found on chromosomes inside the nucleus of a cell. Each gene carries the code for a particular protein, which in turn determines a characteristic. In humans, we have 46 chromosomes arranged in 23 pairs; one chromosome of each pair comes from the mother, the other from the father. The position of a gene on a chromosome is called its locus.
基因是位于细胞核内染色体上的DNA片段。每个基因都带有合成特定蛋白质的密码，从而决定一种性状。人类有46条染色体，组成23对；每对染色体中一条来自母亲，另一条来自父亲。基因在染色体上的位置称为基因座。
2. Alleles and Variation | 等位基因与变异
Different forms of the same gene are called alleles. For example, the gene for eye colour has an allele for brown eyes and an allele for blue eyes. Since we have two copies of each chromosome, we have two alleles for each gene – one inherited from each parent. The combination of alleles an individual possesses is their genotype, while the physical expression is the phenotype.
同一基因的不同形式称为等位基因。例如，控制眼色的基因有褐色等位基因和蓝色等位基因。由于每种染色体都有两个拷贝，每个基因我们也就拥有两个等位基因——各来自父母一方。个体拥有的等位基因组合是基因型，而外在表现则是表现型。
3. 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 if present, even if only one copy is inherited. A recessive allele is only expressed when two copies are present – that is, when the dominant allele is absent. We use capital letters for dominant alleles (e.g., B for brown eyes) and lowercase letters for recessive alleles (e.g., b for blue eyes).
等位基因可以是显性或隐性的。显性等位基因只要存在就会在表现型中显现，即使只有一个拷贝。隐性等位基因只有在拥有两个拷贝（即没有显性等位基因）时才会表达。我们用大写字母表示显性等位基因（如B代表褐色眼），用小写字母表示隐性等位基因（如b代表蓝色眼）。
4. Homozygous and Heterozygous | 纯合与杂合
An individual with two identical alleles for a gene is homozygous (e.g., BB or bb). An individual with two different alleles is heterozygous (e.g., Bb). Homozygous dominant and heterozygous individuals show the dominant trait, while only homozygous recessive individuals show the recessive trait. Understanding these terms is essential for predicting outcomes of genetic crosses.
拥有两个相同等位基因的个体是纯合的（如BB或bb）。拥有两个不同等位基因的个体是杂合的（如Bb）。纯合显性和杂合个体都表现显性性状，只有纯合隐性个体才表现隐性性状。理解这些术语是预测遗传杂交结果的基础。
5. The Punnett Square Method | 庞纳特方格法
A Punnett square is a simple diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. The alleles from one parent are written along the top, and those from the other parent along the left side. The grid squares are then filled in by combining the alleles, showing all possible combinations and their expected ratios.
庞纳特方格是一种用来预测遗传杂交中子代基因型和表现型的简单图表。把一方亲本的等位基因写在顶部，另一方亲本的等位基因写在左侧。方格内通过组合等位基因填写，展示所有可能的组合及其预期比例。
6. Monohybrid Cross Example: Pea Plant Height | 单基因杂交示例：豌豆株高
Mendel's classic experiment involved crossing a pure-breeding tall pea plant (TT) with a pure-breeding short pea plant (tt). All the first-generation (F₁) offspring were tall and heterozygous (Tt). When two F₁ plants were crossed, the F₂ generation showed a 3:1 ratio of tall to short plants. The genotypes were 1 TT : 2 Tt : 1 tt.
孟德尔的经典实验是将纯种高茎豌豆（TT）与纯种矮茎豌豆（tt）杂交。第一代（F₁）子代全部是高茎且杂合（Tt）。当两株F₁植物杂交，F₂代表现出高茎与矮茎3:1的比例。基因型比例为1 TT : 2 Tt : 1 tt。
7. Predicting Genotypic and Phenotypic Ratios | 预测基因型比与表现型比
gous individual is crossed with a homozygous recessive (Bb × bb), the offspring will be 50% heterozygous dominant and 50% homozygous recessive, giving a 1:1 phenotypic ratio.
在杂合亲本间的单基因杂交（Bb × Bb）中，预期基因型为BB、Bb、Bb和bb，基因型比例为1:2:1。因为BB和Bb都表现显性性状，表现型比例为3:1。若杂合个体与纯合隐性杂交（Bb × bb），子代50%为杂合显性，50%为纯合隐性，表现型比例为1:1。
8. Test Crosses | 测交
A test cross is used to determine the genotype of an individual showing the dominant trait. The unknown individual is crossed with a homozygous recessive individual. If any offspring show the recessive trait, the parent must be heterozygous. If all offspring show the dominant trait, the parent is likely homozygous dominant.
测交用于确定表现显性性状个体的基因型。将未知个体与一个纯合隐性个体杂交。如果任何子代表现隐性性状，则亲本必为杂合。如果所有子代均表现显性性状，则亲本很可能为纯合显性。
9. Inherited Diseases and Monohybrid Inheritance | 遗传病与单基因遗传
Some human diseases are caused by a single recessive allele, such as cystic fibrosis and sickle cell anaemia. A person must inherit two copies of the faulty allele to have the disease. Carriers – heterozygous individuals – do not show symptoms but can pass the allele to their children. Punnett squares can help couples assess the probability of their child inheriting the condition.
一些人类疾病由单个隐性等位基因引起，如囊性纤维化和镰状细胞贫血。一个人必须遗传两个致病等位基因才会患病。携带者——即杂合个体——没有症状，但能将等位基因传给孩子。庞纳特方格可以帮助夫妻评估孩子遗传该病的概率。
10. Codominance | 共显性
Sometimes, neither allele is completely dominant over the other; this is called codominance. Both alleles are expressed equally in the phenotype. A common example is the ABO blood group system: alleles IA and IB are codominant, while allele i is recessive. A person with genotype IAIB has blood type AB, expressing both antigens.
有时，没有哪个等位基因完全显性于另一个，这就是共显性。两个等位基因在表现型中同等表达。常见例子是ABO血型系统：等位基因 IA 和 IB 是共显性的，而等位基因 i 是隐性的。基因型为 IAIB 的人血型为AB型，同时表达两种抗原。
11. Family Pedigrees | 家族系谱
A pedigree chart is a graphical way of tracing the inheritance of a trait through several generations. Squares represent males, circles represent females, and shaded symbols indicate individuals showing the trait. By analysing pedigrees, we can determine whether a trait is dominant or recessive, and predict the genotypes of family members.
系谱图是通过几代人追踪性状遗传的图示方法。方框代表男性，圆圈代表女性，阴影符号表示表现出该性状的个体。通过分析系谱，我们可以判断性状是显性还是隐性，并预测家族成员的基因型。
12. Summary of Key Terms | 关键术语总结
Mastering monohybrid inheritance requires a clear understanding of the following: gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, Punnett square, and pedigree. These terms form the foundation for more advanced topics such as dihybrid crosses and sex-linked inheritance, which you may encounter in further study.
掌握单基因遗传需要清晰理解以下概念：基因、等位基因、显性、隐性、纯合、杂合、基因型、表现型、庞纳特方格和系谱。这些术语为更深入的主题（如双基因杂交和伴性遗传）打下基础，你可能会在进一步学习中遇到这些内容。
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