📚 Mendelian Genetics – IGCSE OCR Biology | IGCSE OCR 生物：孟德尔遗传 考点精讲
Mendelian genetics forms the foundation of modern inheritance studies. For IGCSE OCR Biology, you must understand how traits are passed from parents to offspring, use correct genetic terminology, predict outcomes of monohybrid crosses with Punnett squares, and interpret genetic diagrams. This article breaks down every key concept into clear, bilingual explanations to help you excel in exams.
孟德尔遗传学构成了现代遗传学的基础。在 IGCSE OCR 生物中，你必须理解性状如何从亲代传递给后代，使用正确的遗传学术语，用庞纳特方格预测单因子杂交的结果，并解读遗传图解。本文将每个核心概念拆解为清晰的双语解释，助你在考试中脱颖而出。
1. Who Was Gregor Mendel? | 孟德尔是谁？
Gregor Mendel was an Austrian monk who performed ground-breaking experiments on pea plants (Pisum sativum) in the mid-19th century. Although his work was initially overlooked, he is now celebrated as the father of genetics because he discovered the fundamental principles of heredity.
格雷戈尔·孟德尔是 19 世纪中期的一位奥地利修道士，他在豌豆（Pisum sativum）上进行了开创性的实验。尽管他的研究最初被忽视，但现在他被誉为遗传学之父，因为他发现了遗传的基本原理。
He chose pea plants because they are easy to grow, have a short generation time, and display several clearly contrasting traits, such as tall vs short stems and purple vs white flowers. Crucially, pea plants can self-pollinate or be artificially cross-pollinated, allowing Mendel to control breeding.
他选择豌豆是因为它们易于种植、世代时间短，并且表现出多种鲜明对比的性状，例如高茎与矮茎、紫花与白花。至关重要的是，豌豆可以自花授粉也可以人工异花授粉，这使得孟德尔能够控制繁殖。
2. Key Terms in Genetics | 遗传学关键术语
Gene: A section of DNA that codes for a particular protein and thus determines a specific characteristic.
基因（Gene）： 一段编码特定蛋白质的 DNA，从而决定了一个特定的性状。
Allele: One of two or more alternative forms of a gene. For example, the gene for flower colour has a purple allele and a white allele.
等位基因（Allele）： 一个基因的两个或多个替代形式之一。例如，花色基因有紫色等位基因和白色等位基因。
Dominant allele: An allele whose characteristic is always expressed in the phenotype, even if only one copy is present. It is represented by an uppercase letter (e.g., P for purple).
显性等位基因： 即使只有一个拷贝存在，其性状也总会在表型中表达出来的等位基因。它用大写字母表示（例如，P 代表紫色）。
Recessive allele: An allele whose characteristic is only expressed when two copies are present (i.e., no dominant allele is present). It is represented by a lowercase letter (e.g., p for white).
隐性等位基因： 只有当两个拷贝都存在（即不存在显性等位基因）时，其性状才会表达出来的等位基因。它用小写字母表示（例如，p 代表白色）。
Genotype: The genetic makeup of an organism – the combination of alleles it possesses (e.g., PP, Pp, pp).
基因型（Genotype）： 一个生物体的基因构成——它所拥有的等位基因组合（例如，PP、Pp、pp）。
Phenotype: The observable physical or biochemical characteristics of an organism (e.g., purple flowers).
表型（Phenotype）： 生物体可观察到的物理或生化特征（例如，紫花）。
Homozygous: Having two identical alleles for a gene (e.g., PP or pp).
纯合子（Homozygous）： 一个基因拥有两个相同等位基因（例如，PP 或 pp）。
Heterozygous: Having two different alleles for a gene (e.g., Pp). The dominant allele determines the phenotype in a heterozygote.
杂合子（Heterozygous）： 一个基因拥有两个不同等位基因（例如，Pp）。在杂合子中，显性等位基因决定表型。
3. Mendel’s Monohybrid Experiment | 孟德尔的单因子实验
Mendel started with pure-breeding (homozygous) plants. He crossed a purple-flowered plant (PP) with a white-flowered plant (pp). This is the parental (P) generation.
孟德尔从纯种（纯合）植株开始。他将一棵紫花植株（PP）与一棵白花植株（pp）杂交。这是亲代（P）。
All the offspring in the first filial (F₁) generation were purple (Pp). The white trait seemed to disappear. The F₁ plants were all heterozygous.
第一子代（F₁）中的所有后代都是紫色的（Pp）。白色性状似乎消失了。F₁ 植株全部是杂合子。
Mendel then allowed the F₁ plants to self-pollinate. The second filial (F₂) generation showed both purple and white flowers in a ratio of approximately 3 purple : 1 white. The white trait reappeared, proving it was masked, not destroyed, in the F₁ generation.
然后孟德尔让 F₁ 植株自花授粉。第二子代（F₂）中出现了紫花和白花，比例大约为 3 紫 : 1 白。白色性状重新出现，证明了它在 F₁ 代中被掩盖而非被消灭。
4. The Law of Segregation | 分离定律
Mendel’s first law states that each organism carries two alleles for each characteristic, but these alleles separate (segregate) during the formation of gametes. As a result, each gamete carries only one allele for each gene.
孟德尔第一定律指出，每个生物体对每种性状都携带两个等位基因，但这些等位基因在配子形成时会彼此分离。因此，每个配子只携带每个基因的一个等位基因。
When gametes fuse at fertilisation, the offspring inherits one allele from each parent, restoring the pair. This explains how recessive traits can be masked and reappear in later generations.
当配子在受精时结合，后代从每个亲本各继承一个等位基因，恢复成对。这就解释了隐性性状如何被遮盖并在后代中重新出现。
5. How to Use a Punnett Square | 如何使用庞纳特方格
A Punnett square is a grid that helps predict the genotypes and phenotypes of offspring from a genetic cross. Follow these steps:
庞纳特方格是一种帮助预测杂交后代基因型和表型的网格。请遵循以下步骤：
1. Write the genotype of the parents (e.g., Pp x Pp).
1. 写下亲本的基因型（例如，Pp × Pp）。
2. Determine the possible gametes each parent can produce. A heterozygous parent (Pp) produces gametes P and p.
2. 确定每个亲本能产生的可能配子。杂合亲本（Pp）产生配子 P 和 p。
3. Draw a grid with one parent’s gametes along the top and the other parent’s gametes down the side.
3. 绘制一个网格，将一个亲本的配子写在顶部，另一个亲本的配子写在侧边。
4. Fill in the squares by combining the alleles from the top and side. This gives the potential genotypes of the offspring.
4. 将顶部和侧边的等位基因组合填入方格内，得到后代可能的基因型。
Here is a Punnett square for a heterozygous cross (Pp x Pp):
下面是杂合子杂交（Pp × Pp）的庞纳特方格：
| P | p | |
| P | PP | Pp |
| p | Pp | pp |
From this square, you can see the genotypic ratio: 1 PP : 2 Pp : 1 pp, and the phenotypic ratio: 3 purple : 1 white (if P is dominant for purple colour).
从这个方格中，你可以看到基因型比例：1 PP : 2 Pp : 1 pp，以及表型比例：3 紫 : 1 白（假设 P 对紫色为显性）。
6. Typical Monohybrid Cross Ratios | 典型的单因子杂交比例
In a cross between two heterozygous individuals (Pp x Pp), the expected phenotypic ratio is 3:1 (dominant : recessive). This is the classic Mendelian ratio for a monohybrid cross with complete dominance.
在两个杂合子个体的杂交（Pp × Pp）中，预期的表型比例为 3:1（显性 : 隐性）。这是具有完全显性的单因子杂交的经典孟德尔比例。
If a homozygous dominant (PP) is crossed with a homozygous recessive (pp), all F₁ offspring are heterozygous (Pp) and show the dominant phenotype – a 100% dominant ratio.
如果将纯合显性（PP）与纯合隐性（pp）杂交，所有 F₁ 后代都是杂合子（Pp）并显示显性表型——比例为 100% 显性。
If a heterozygous (Pp) is crossed with a homozygous recessive (pp), the offspring ratio is 1:1, producing half dominant and half recessive phenotypes. This is known as a test cross ratio.
如果将杂合子（Pp）与纯合隐性（pp）杂交，后代比例为 1:1，即一半显性表型一半隐性表型。这就是测交比例。
7. Determining an Unknown Genotype – The Test Cross | 确定未知基因型 – 测交
A test cross is used to find out whether an organism showing the dominant phenotype is homozygous (PP) or heterozygous (Pp). The individual is crossed with a homozygous recessive (pp) individual.
测交用于确定一个显示显性表型的生物体是纯合子（PP）还是杂合子（Pp）。将该个体与纯合隐性（pp）个体杂交。
If any offspring show the recessive phenotype, the unknown parent must be heterozygous (Pp), because it has contributed a recessive allele. If all offspring are dominant, the unknown parent is likely homozygous dominant (PP).
如果任何后代表现出隐性表型，那么未知亲本一定是杂合子（Pp），因为它贡献了一个隐性等位基因。如果所有后代都是显性，则未知亲本很可能是纯合显性（PP）。
For example, crossing a purple-flowered plant of unknown genotype with a white (pp) plant: if the result is 100% purple, the parent is PP; if 50% purple and 50% white, the parent is Pp.
例如，将一株未知基因型的紫花植株与白花（pp）植株杂交：如果结果是 100% 紫花，则亲本为 PP；如果是 50% 紫花和 50% 白花，则亲本为 Pp。
8. Extensions: Codominance and Incomplete Dominance | 拓展：共显性与不完全显性
Mendel studied complete dominance, but some alleles interact differently. In co-dominance, both alleles are expressed equally in the phenotype of a heterozygote. The classic example is the ABO blood group system in humans, where alleles Iᴬ and Iᴮ are co-dominant over i. A genotype IᴬIᴮ results in the AB blood type, expressing both A and B antigens.
孟德尔研究的是完全显性，但有些等位基因以不同方式作用。在共显性中，两个等位基因在杂合子的表型中同等表达。经典例子是人类的 ABO 血型系统，等位基因 Iᴬ 和 Iᴮ 对 i 呈共显性。基因型 IᴬIᴮ 产生 AB 血型，同时表达 A 和 B 抗原。
In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes. For instance, crossing a red snapdragon (RR) with a white snapdragon (WW) yields all pink offspring (RW). The phenotype is intermediate.
在不完全显性中，杂合子的表型是两个纯合子表型的混合。例如，将红色金鱼草（RR）与白色金鱼草（WW）杂交，后代全部为粉色（RW）。表型是中间型。
9. Genetic Diagrams for Exams | 考试中的遗传图解
In IGCSE OCR exams, you are expected to draw clear genetic diagrams. Always include: the parental phenotypes and genotypes, the gametes (with circles around each allele if specified by your teacher), a Punnett square or line diagram, the offspring genotypes and phenotypes, and the final phenotypic ratio.
在 IGCSE OCR 考试中，你需要绘制清晰的遗传图解。务必包括：亲代表型和基因型、配子（如有要求，将每个等位基因圈出）、庞纳特方格或线条图、后代的基因型和表型，以及最终的表型比例。
Use standard formatting: mark the generations P, F₁, F₂ where relevant. Write the cross clearly, e.g., ‘Purple (PP) x White (pp)’. Show all steps logically. Avoid mixing arrows and randomly placed text.
使用标准格式：在相关位置标注世代 P、F₁、F₂。清楚地写出杂交，例如“紫花（PP）× 白花（pp）”。逻辑清晰地展示所有步骤。避免箭头和随意放置文本的混杂。
10. Common Pitfalls and Exam Tips | 常见陷阱与考试技巧
Don’t confuse genotype with phenotype. Genotype is the allele combination (PP, Pp, pp); phenotype is the physical trait (purple, white). Many marks are lost by using uppercase/recessive incorrectly – always define your letters.
不要混淆基因型与表型。基因型是等位基因组合（PP、Pp、pp）；表型是物理性状（紫色、白色）。很多分数因错误使用大/小写而丢失——务必定义你的字母。
When predicting ratios, large offspring numbers give ratios closer to expected Mendelian ratios. If a question gives a 3:1 ratio in actual data, it strongly suggests both parents were heterozygous.
在预测比例时，后代数量较大时比例会接近预期的孟德尔比例。如果题目给出实际数据为 3:1 比例，强烈暗示双亲均为杂合子。
For pedigree charts, work backwards from affected individuals who show the recessive trait. They must be homozygous recessive. Use this to deduce possible genotypes of parents and carriers. Always check if the trait is dominant or recessive before solving.
对于家系图，从表现出隐性性状的患病个体倒推。他们一定是隐性纯合子。利用这一点推断亲本和携带者的可能基因型。在解题前务必确认该性状是显性还是隐性。
Finally, always read the question carefully – some questions specify using certain letter codes, and you must follow them exactly.
最后，务必仔细读题——有些题目指定了特定的字母代码，你必须严格遵循。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导