📚 IB Biology: Mendelian Genetics Key Concepts | IB 生物：孟德尔遗传 考点精讲
Mendelian genetics forms the foundation of modern inheritance studies, explaining how traits are passed from parents to offspring through discrete units called genes. In IB Biology, mastery of Mendel’s laws, genetic terminology, and predictive tools like Punnett squares is essential for tackling inheritance problems, interpreting pedigree charts, and understanding extensions such as co-dominance and multiple alleles. This article breaks down the key concepts you need to excel in your IB exams.
孟德尔遗传学是现代遗传学的基础，它解释了性状如何通过被称为基因的离散单位从亲代传递给子代。在 IB 生物考试中，掌握孟德尔定律、遗传学术语以及旁氏方格等预测工具，对于解决遗传问题、解读系谱图以及理解共显性和复等位基因等扩展内容至关重要。本文将详细拆解你需要掌握的核心考点。
1. Key Genetic Terminology | 关键遗传学术语
Before diving into Mendel’s experiments, it is crucial to understand the precise language of genetics. A gene is a heritable factor that controls a specific characteristic, located at a particular locus on a chromosome. Alleles are alternative forms of the same gene that occupy the same locus; they arise through mutation. An organism’s genotype is the combination of alleles it possesses (e.g., TT, Tt, tt), while its phenotype is the observable trait resulting from that genotype. A dominant allele masks the effect of a recessive allele in a heterozygote. When both alleles are identical, the organism is homozygous; when they are different, it is heterozygous.
在深入孟德尔实验之前，准确理解遗传学术语至关重要。基因是控制某一特定性状的可遗传因子，位于染色体上的特定基因座。等位基因是同一基因的不同形式，占据相同基因座，它们通过突变产生。生物的基因型是其拥有的等位基因组合（如 TT、Tt、tt），而表型是由该基因型产生的可观察性状。在杂合子中，显性等位基因掩盖隐性等位基因的效应。当两个等位基因相同时，该个体是纯合的；当它们不同时，则是杂合的。
2. Mendel’s Experiments and the Choice of Peas | 孟德尔的实验与豌豆的选择
Gregor Mendel conducted his pioneering work on the garden pea (Pisum sativum) because it exhibited several distinct, easily observable traits with two contrasting forms (e.g., tall vs. short, purple vs. white flowers). Peas can be self-fertilised or cross-pollinated manually, allowing Mendel to control matings precisely. He focused on seven characters, each determined by a single gene. By starting with true-breeding lines and tracking traits over generations, he deduced the particulate nature of inheritance, rejecting the blending hypothesis.
格雷戈尔·孟德尔选择豌豆（Pisum sativum）进行开创性研究，因为它具有多个明显且容易观察的性状，每个性状有两种对比鲜明的形态（如高茎与矮茎、紫花与白花）。豌豆可以自花授粉，也可进行人工异花授粉，使孟德尔能够精确控制交配。他研究了七个性状，每个性状由单一基因决定。通过使用纯品系并追踪性状在世代中的传递，他推断了遗传的颗粒性本质，否定了融合遗传假说。
3. Monohybrid Crosses and the Law of Segregation | 单因子杂交与分离定律
In a monohybrid cross, Mendel followed the inheritance of one trait. He crossed true-breeding tall plants (TT) with true-breeding short plants (tt). The F₁ generation were all tall (Tt). When F₁ plants were self-pollinated, the F₂ generation showed a phenotypic ratio of approximately 3 tall : 1 short. This led to the Law of Segregation: each organism carries two alleles for each trait, and these alleles separate (segregate) during gamete formation, so each gamete receives only one allele. The 3:1 ratio arises because the recessive trait reappears when two recessive alleles combine.
在单因子杂交中，孟德尔追踪一个性状的遗传。他将高茎纯合（TT）与矮茎纯合（tt）植株杂交，F₁ 代全部为高茎（Tt）。当 F₁ 植株自花授粉，F₂ 代表型比例约为 3 高茎 : 1 矮茎。由此他提出了分离定律：每个生物体的每个性状都带有两个等位基因，这些等位基因在配子形成时分离，使得每个配子只含有一个等位基因。3:1 的比例是因为隐性性状在两个隐性等位基因组合时重新出现。
4. Using Punnett Squares for Monohybrid Crosses | 运用旁氏方格预测单因子杂交
A Punnett square is a grid that helps predict the genotypic and phenotypic ratios of offspring. For a cross between two heterozygous (Tt) pea plants, the gametes are T and t from each parent. The resulting squares show genotypes: 1 TT, 2 Tt, 1 tt, giving a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1. Punnett squares are essential for solving IB exam questions; always label gametes, genotypes, and phenotypes clearly.
旁氏方格是一种帮助预测后代基因型和表型比例的网格工具。以两个杂合（Tt）豌豆植株的杂交为例，每个亲本产生的配子为 T 和 t。方格显示基因型为：1 TT、2 Tt、1 tt，基因型比例为 1:2:1，表型比例为 3:1。旁氏方格在 IB 考试解题中不可或缺；务必清晰标注配子、基因型和表型。
5. Dihybrid Crosses and the Law of Independent Assortment | 双因子杂交与自由组合定律
Mendel extended his work to crosses involving two traits, such as seed shape (round R vs. wrinkled r) and seed colour (yellow Y vs. green y). A cross between true-breeding round-yellow (RRYY) and wrinkled-green (rryy) plants gave an F₁ generation all round-yellow (RrYy). Selfing the F₁ yielded an F₂ phenotypic ratio of 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green (9:3:3:1). This ratio led to the Law of Independent Assortment: alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes. This principle applies only to unlinked genes.
孟德尔将研究扩展到涉及两个性状的杂交，例如种子形状（圆粒 R 对皱粒 r）和种子颜色（黄色 Y 对绿色 y）。用纯合圆粒黄色（RRYY）与皱粒绿色（rryy）杂交，F₁ 代全为圆粒黄色（RrYy）。将 F₁ 自交，F₂ 表型比例为 9 圆黄 : 3 圆绿 : 3 皱黄 : 1 皱绿（9:3:3:1）。这一比例引出了自由组合定律：在配子形成过程中，不同基因的等位基因彼此独立分配，前提是基因位于不同染色体上。该原理仅适用于非连锁基因。
6. The Test Cross | 测交
A test cross is used to determine the genotype of an individual showing a dominant phenotype but whose genotype is unknown (could be homozygous dominant or heterozygous). The individual is crossed with a homozygous recessive (e.g., tt or rryy). If any offspring show the recessive phenotype, the unknown parent must be heterozygous. If all offspring show the dominant phenotype, the parent is likely homozygous dominant, but statistical certainty increases with larger sample sizes. Test crosses are a classic IB exam application of Mendelian principles.
测交用于确定表现出显性表型但基因型未知的个体（可能是显性纯合或杂合）的基因型。将该个体与隐性纯合（如 tt 或 rryy）杂交。如果后代出现隐性表型，则未知亲本必为杂合子。如果所有后代都表现显性性状，则该亲本可能为显性纯合，但统计确定性需依赖较大的样本量。测交是 IB 考试中孟德尔原理的经典应用。
7. Incomplete Dominance and Co-dominance | 不完全显性与共显性
Not all traits follow a strict dominant-recessive pattern. In incomplete dominance, the heterozygote shows a blended phenotype. For example, crossing red-flowered (RR) and white-flowered (rr) snapdragons gives pink F₁ (Rr); the F₂ ratio becomes 1 red : 2 pink : 1 white, reflecting the genotypic ratio. In co-dominance, both alleles are fully expressed in the heterozygote with no blending. Human ABO blood groups are a prime example: alleles Iᴬ and Iᴮ are co-dominant, while i is recessive. A person with IᴬIᴮ has both A and B antigens on red blood cells (blood type AB).
并非所有性状都遵循严格的显隐性模式。在不完全显性中，杂合子表现出混合表型。例如，用红花（RR）与白花（rr）金鱼草杂交，F₁ 为粉色（Rr）；F₂ 比例为 1 红 : 2 粉 : 1 白，与基因型比例一致。在共显性中，杂合子的两个等位基因均完全表达，没有混合。人类 ABO 血型是典型例子：等位基因 Iᴬ 与 Iᴮ 为共显性，i 为隐性。IᴬIᴮ 基因型个体的红细胞上同时具有 A 和 B 抗原（AB 血型）。
8. Multiple Alleles and ABO Blood Groups | 复等位基因与 ABO 血型
While Mendel studied genes with only two alleles, many gene loci in populations have more than two possible alleles due to mutations. The ABO system involves a single gene with three alleles: Iᴬ, Iᴮ, and i. This creates six genotypes and four phenotypes. Understanding the immunological basis (antigens and antibodies) links genetics to human physiology. In exam problems, you must be able to predict possible blood types of offspring from given parental phenotypes and genotypes using Punnett squares.
尽管孟德尔只研究了具有两个等位基因的基因，但群体中许多基因座由于突变而存在两个以上的等位基因。ABO 系统由一个具有三个等位基因（Iᴬ、Iᴮ 和 i）的基因控制，产生六种基因型和四种表型。理解其免疫学基础（抗原和抗体）将遗传学与人体生理学联系起来。在考试解题中，你必须能够运用旁氏方格根据给定的亲代表型和基因型预测后代可能的血型。
9. Sex-linked Inheritance (Non-Mendelian Extension) | 性连锁遗传（孟德尔遗传的扩展）
Genes located on sex chromosomes (usually the X chromosome) show distinct inheritance patterns because males (XY) have only one allele for such loci. Classic examples include red-green colour blindness and haemophilia in humans. A recessive allele on the X chromosome is more likely to be expressed in males. A carrier female (XᴺXⁿ) crossed with a normal male (XᴺY) can produce an affected son (XⁿY) with a 50% chance. While not strictly Mendelian, sex-linked inheritance is tested alongside Mendelian concepts in IB. Use Punnett squares with X and Y chromosomes, and always indicate the allele as a superscript on the X.
位于性染色体（通常是 X 染色体）上的基因表现出独特的遗传模式，因为男性（XY）对于这些基因座只有一个等位基因。经典实例包括红绿色盲和血友病。X 染色体上的隐性等位基因在男性中更容易表达。女性携带者（XᴺXⁿ）与正常男性（XᴺY）杂交，生下患病儿子（XⁿY）的概率为 50%。虽然性连锁遗传并非严格的孟德尔遗传，但 IB 会将它与孟德尔概念一同考查。解题时需使用带有 X 和 Y 染色体的旁氏方格，并始终将等位基因标注为 X 的上标。
10. Pedigree Charts and Pattern Recognition | 系谱图与遗传模式识别
Pedigree charts are graphical representations of family inheritance over several generations. IB candidates must determine whether a trait is autosomal dominant, autosomal recessive, sex-linked recessive, or sex-linked dominant based on the pattern. Key clues: autosomal recessive traits often skip generations and appear in offspring of unaffected parents; autosomal dominant traits appear in every generation; sex-linked recessive traits affect more males and often transmitted through carrier females. Practice interpreting shaded symbols, identifying genotypes where possible, and calculating probabilities for specific individuals.
系谱图是家族数代遗传的图形表示。IB 考生必须根据模式判断某一性状是常染色体显性、常染色体隐性、X 连锁隐性还是 X 连锁显性。关键线索：常染色体隐性性状常隔代出现，并出现在未患病父母的后代中；常染色体显性性状每代均出现；X 连锁隐性性状影响更多男性，常通过女性携带者传递。要练习解读阴影符号、推断可能的基因型，并计算特定个体的患病概率。
11. Applying Mendelian Ratios to Real Data: The Chi-squared Test | 孟德尔比例与实际数据的拟合：卡方检验
In IB Biology, you may need to perform a chi-squared (χ²) test to determine whether observed phenotypic ratios match expected Mendelian ratios. The formula is χ² = Σ (observed − expected)² / expected. A small χ² value suggests that deviations are due to chance alone, and the null hypothesis (data fit the Mendelian ratio) is not rejected. Compare the calculated χ² to a critical value from a table at p = 0.05 with degrees of freedom (number of classes − 1). This statistical tool reinforces the understanding that Mendelian ratios emerge from large sample sizes.
在 IB 生物中，你可能需要进行卡方（χ²）检验，以判断观察到的表型比例是否与预期的孟德尔比例相符。公式为 χ² = Σ（观察值 − 期望值）² / 期望值。小的 χ² 值表明偏差可能仅由偶然因素引起，不拒绝零假设（数据符合孟德尔比例）。将计算所得的 χ² 值与 p = 0.05 且自由度为（类别数 − 1）的临界值表进行比较。这一统计工具强化了孟德尔比例在大样本量下才会呈现的认识。
12. Common Misconceptions and Exam Tips | 常见误区与考试技巧
Students often confuse genotype with phenotype or believe dominant allele is more common in a population (frequency depends on allele distribution, not dominance). Another pitfall is assuming that a 3:1 ratio guarantees a single-gene trait; environmental factors and gene interactions can modify ratios. Always use standard notation (upper case for dominant, lower case for recessive, same letter) unless instructed otherwise. In dihybrid crosses, remember that the 9:3:3:1 ratio only holds for unlinked genes with complete dominance. Draw Punnett squares systematically, and double-check gamete formation.
学生常混淆基因型与表型，或认为显性等位基因在群体中更常见（其实频率取决于等位基因分布，而非显性）。另一个误区是认为 3:1 比例一定表示单基因性状；环境因素和基因互作会改变比例。除非题目另有要求，始终使用标准符号（同一字母，大写表示显性，小写表示隐性）。在双因子杂交中，请记住 9:3:3:1 的比例仅适用于非连锁基因的完全显性。有条理地绘制旁氏方格，并仔细检查配子的形成。
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