6.2 Patterns of Inheritance: Exam-Focused Study Guide | 6.2 遗传模式考点突破

📚 6.2 Patterns of Inheritance: Exam-Focused Study Guide | 6.2 遗传模式考点突破

Understanding the patterns of inheritance is fundamental to genetics. From Mendel’s classic monohybrid crosses to complex epistatic interactions, these concepts form the core of many A-Level Biology exam questions. This guide will walk you through the most commonly tested inheritance patterns, equipping you with the knowledge and exam techniques needed to secure full marks.

理解遗传模式是遗传学的基础。从孟德尔经典的单基因杂交到复杂的上位效应，这些概念构成了许多A-Level生物考试题目的核心。本指南将带你梳理最常见的遗传模式考点，帮助你掌握必要的知识和考试技巧，从而稳拿满分。

1. Monohybrid Crosses and Dominance | 单基因杂交与显性

A monohybrid cross investigates the inheritance of a single gene with two alleles. Complete dominance occurs when the dominant allele fully masks the recessive allele in heterozygous individuals. For example, in pea plants, tall (T) is dominant over dwarf (t). When homozygous tall (TT) is crossed with homozygous dwarf (tt), all F₁ offspring are heterozygous tall (Tt). A subsequent self-cross of Tt produces a genotypic ratio of 1 TT : 2 Tt : 1 tt and a classic phenotypic ratio of 3 tall : 1 dwarf. A test cross with a homozygous recessive (tt) reveals the genotype of an unknown dominant individual: 100% tall if homozygous, or 1:1 if heterozygous.

单基因杂交研究一对等位基因的遗传。当显性等位基因在杂合子中完全遮盖隐性等位基因时，即为完全显性。例如，豌豆的高茎(T)对矮茎(t)为显性。纯合高茎(TT)与纯合矮茎(tt)杂交，F₁代全为杂合高茎(Tt)。F₁自交产生的F₂代基因型比例为1 TT : 2 Tt : 1 tt，表型比例为经典的3高 : 1矮。通过隐性纯合(tt)测交可鉴定未知显性个体的基因型：若全为高茎则为纯合，若比例为1:1则为杂合。

2. Codominance and Multiple Alleles | 共显性与复等位基因

Codominance is a pattern where both alleles are fully expressed in the heterozygote, producing a phenotype that shows both traits simultaneously. The ABO blood group system in humans is a prime example, which also illustrates multiple alleles (I^A, I^B, i). Alleles I^A and I^B are codominant, while both are dominant over i. The table below summarises the possible genotypes and phenotypes.

共显性是指杂合子中两个等位基因均充分表达，表型同时显现两种性状。人类的ABO血型系统是典型的共显性与复等位基因例子（等位基因 I^A、I^B、i）。I^A 和 I^B 为共显性，两者均对 i 为显性。下表总结了可能的基因型与表现型。

Genotype (基因型)	Phenotype (血型)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

When solving codominance problems, always write the alleles as superscripts on a base letter (e.g., C^R for red, C^W for white in snapdragons), and remember that the heterozygous phenotype is distinct from either homozygote.

在解决共显性问题时，务必将等位基因书写为基字母的上标（例如用 C^R 表示红花，C^W 表示白花），并记住杂合表型不同于任一纯合表型。

3. Sex Linkage | 性连锁遗传

Sex-linked genes are located on the sex chromosomes, most commonly on the X chromosome. Because males have only one X chromosome (XY), they are hemizygous for X-linked alleles and express the trait even if the allele is recessive. Red-green colour blindness is a classic X-linked recessive disorder. If X^B represents normal vision and X^b represents colour blindness, a carrier female (X^BX^b) and a normal male (X^BY) produce offspring with a 25% chance of an affected son (X^bY) and 0% affected daughters, though daughters can be carriers.

性连锁基因位于性染色体上，最常见于X染色体。由于雄性只有一条X染色体(XY)，他们是X连锁等位基因的半合子，即使该等位基因为隐性也会表现性状。红绿色盲是典型的X连锁隐性遗传病。若以 X^B 代表正常视觉，X^b 代表色盲，则携带者女性(X^BX^b)与正常男性(X^BY)婚配，子代中儿子有25%概率患病(X^bY)，女儿则不会患病，但可能成为携带者。

In a pedigree, X-linked recessive conditions appear more frequently in males, and affected fathers cannot pass the trait to their sons. For X-linked dominant disorders, affected fathers pass the condition to all daughters but no sons.

在系谱中，X连锁隐性遗传病在男性中的发病率更高，且患病父亲不会将性状传给儿子。对于X连锁显性遗传病，患病父亲将病症传给所有女儿而不传给儿子。

4. Dihybrid Crosses and Independent Assortment | 双因子杂交与自由组合

A dihybrid cross involves two genes located on different chromosomes. According to Mendel’s law of independent assortment, the alleles of these two genes segregate independently during gamete formation. The classic F₂ phenotypic ratio from a heterozygous dihybrid cross (e.g., RrYy × RrYy) is 9 : 3 : 3 : 1. This ratio represents 9/16 showing both dominant traits, 3/16 showing one dominant and one recessive, another 3/16 showing the other combination, and 1/16 showing both recessive traits. Always confirm that the genes are on separate chromosomes before applying the 9:3:3:1 ratio; otherwise, linkage must be considered.

双因子杂交涉及位于不同染色体上的两个基因。根据孟德尔自由组合定律，这两个基因的等位基因在配子形成过程中独立分离。杂合双因子杂交（如 RrYy × RrYy）的经典F₂表型比例为 9 : 3 : 3 : 1。该比例代表了 9/16 表现为两个显性性状，3/16 表现一个显性一个隐性，另外 3/16 为另一组合，1/16 表现为两个隐性性状。在套用9:3:3:1比例前，务必确认基因位于不同染色体上；否则需考虑连锁。

Quick check: gamete types from RrYy are RY, Ry, rY, ry in equal proportions. The use of Punnett square or probability multiplication helps deduce the ratios accurately. Ensure you can calculate the probability of a specific genotype, e.g., RRYY = ¼ × ¼ = 1/16.

快速检验：RrYy 产生的配子类型为 RY、Ry、rY、ry，比例相等。使用旁纳特方格或概率乘法有助于精确推导比例。务必能够计算特定基因型的概率，例如 RRYY = ¼ × ¼ = 1/16。

5. Autosomal Linkage and Crossing Over | 常染色体连锁与交叉互换

When two genes are located on the same autosome, they are linked and do not assort independently. Instead, the parental combinations of alleles tend to be inherited together. A cross between an organism heterozygous for two linked genes and a homozygous recessive individual will produce mostly parental phenotypes, with a small proportion of recombinant offspring due to crossing over during prophase I of meiosis. The recombination frequency is calculated as (number of recombinant offspring / total offspring) × 100%, which roughly corresponds to the distance between the loci in centimorgans.

当两个基因位于同一条常染色体上时，它们表现为连锁，不遵循自由组合定律。相反，亲本等位基因组合倾向于共同遗传。一个双基因杂合连锁个体与隐性纯合个体测交，子代主要为亲本表型，仅有少量重组子代出现，这是由于减数分裂前期I发生了交叉互换。重组频率 = (重组子代数 / 总子代数) × 100%，该数值大致对应基因座之间以厘摩为单位的距离。

For example, without crossing over, a heterozygote AB/ab produces only AB and ab gametes; with crossing over, small amounts of Ab and aB gametes appear. Exam questions often provide offspring numbers and ask you to identify linkage and calculate map distance. Remember: a recombination frequency > 50% indicates independent assortment.

例如，不发生交换时，杂合子 AB/ab 只产生 AB 和 ab 配子；发生交叉互换后，会出现少量 Ab 和 aB 配子。考试常给出子代数目，要求识别连锁并计算图距。记住：重组频率大于50%即表明基因自由组合。

6. Epistasis | 上位效应

Epistasis occurs when the expression of one gene is influenced by another gene. The gene that masks the effect is epistatic, while the masked gene is hypostatic. Several ratios deviate from the expected 9:3:3:1, and recognising them is crucial. Common forms include recessive epistasis (9:3:4), as seen in coat colour of Labrador retrievers, where the homozygous recessive state of one gene (ee) masks the expression of another (B_); dominant epistasis (12:3:1), as in summer squash fruit colour; and complementary gene interaction (9:7), where both dominant alleles are needed to produce a particular phenotype, as in sweet pea flower colour.

上位效应指一个基因的表达受另一个基因影响。起遮盖作用的基因称上位基因，被遮盖的基因称下位基因。多种比例偏离预期的9:3:3:1，识别这些比例至关重要。常见类型包括：隐性上位（9:3:4），如拉布拉多犬的毛色，一个基因的隐性纯合态(ee)遮盖了另一基因(B_)的表达；显性上位（12:3:1），如南瓜果实颜色；以及互补基因作用（9:7），需要两个显性等位基因共同作用才表现特定性状，如香豌豆花色。

The table below summarises the classic epistatic ratios seen in A-Level exams:

下表总结了A-Level考试中出现的经典上位比例：

Type of Epistasis	F₂ Phenotypic Ratio	Example
Recessive epistasis (隐性上位)	9 : 3 : 4	Labrador coat colour
Dominant epistasis (显性上位)	12 : 3 : 1	Summer squash colour
Complementary genes (互补基因)	9 : 7	Sweet pea flower colour
Duplicate genes (重复基因)	15 : 1	Shepherd’s purse seed capsule shape

7. Chi-Squared Test in Genetics | 遗传学中的卡方检验

The chi-squared (χ²) test is a statistical tool used to determine whether observed ratios differ significantly from expected Mendelian ratios. It is frequently assessed in practical-based questions. The formula is:

卡方检验(χ²)是一种统计工具，用于判断观测比例是否与预期孟德尔比例存在显著差异。它是实验类题目的常考点。公式如下：

χ² = Σ (O – E)² / E

Where O = observed frequency, E = expected frequency. After calculating χ², you compare it against a critical value from a table at a given probability level (usually p = 0.05) and degrees of freedom (n – 1). If χ² < critical value, you accept the null hypothesis (no significant difference). If χ² ≥ critical value, you reject the null hypothesis, suggesting that some factor other than chance is affecting the results, such as linkage or epistasis.

其中 O = 观测值，E = 期望值。计算出 χ² 后，需要将其与给定概率水平（通常为 p = 0.05）及自由度（n – 1）下的临界值进行比较。若 χ² < 临界值，则接受零假设（无显著差异）。若 χ² ≥ 临界值，则拒绝零假设，表明除随机因素外存在其他影响因素，如连锁或上位效应。

Important: always state your conclusion in terms of the biological context, not just “reject/accept null hypothesis”. Calculate expected numbers by multiplying the total offspring by the expected ratio fraction. Round to whole numbers for meaningful comparison.

重要提示：书写结论时务必结合生物学情境，切勿只写“拒绝/接受零假设”。计算期望值时，用总子代数乘以预期比例分数，并四舍五入为整数以便比较。

8. Pedigree Analysis | 系谱分析

Pedigree diagrams are used to trace the inheritance of traits through generations. You may be asked to determine whether a trait is dominant or recessive, and autosomal or sex-linked. Key clues: if the trait appears in every generation, it is likely dominant; if it skips generations, it is likely recessive. For autosomal recessive, unaffected parents can have affected children. For autosomal dominant, affected children must have at least one affected parent. For X-linked recessive, more males are affected, and affected fathers pass the allele to all daughters (carriers) but no sons.

系谱图用于追踪性状在世代间的遗传。考试可能要求判断性状是显性还是隐性，以及位于常染色体还是性染色体上。关键线索：若性状逐代出现，很可能为显性；若隔代出现，则很可能为隐性。常染色体隐性遗传中，表型正常的父母可生出患病子女。常染色体显性遗传中，患病子女必有一方亲本患病。X连锁隐性遗传中，患病男性较多，患病父亲将等位基因传给所有女儿（成为携带者）而不传给儿子。

To deduce genotypes, assign symbols and work backwards from affected individuals. Always indicate the reasoning clearly. Draw a small Punnett square as a check.

推导基因型时，先指定符号，并从患病个体逆向推导。务必清晰说明推理过程，并可借助旁纳特方格验证。

9. Genetic Diagrams and Ratios | 遗传图解与比例速查

Constructing clear, labelled genetic diagrams is essential for scoring full marks. Always state the phenotype and genotype of the parents, the gametes they produce, and the offspring genotypes and phenotypes with ratios. Use a Punnett square where appropriate. In codominance or blood group questions, use appropriate superscripts. The summary table below provides a quick reference for classic ratios and their implications.

绘制清晰、带标注的遗传图解是获取满分的关键。务必注明亲本表型和基因型、产生的配子类型，以及子代基因型和表型及比例。必要时使用旁纳特方格。在共显性或血型题目中，使用合适的上标。下表为经典比例及其含义提供速查参考。

Cross (杂交类型)	Expected Ratio (预期比例)	Indication (指示含义)
Monohybrid F₂ (单基因F₂)	3 : 1	Complete dominance
Test cross heterozygous (杂合测交)	1 : 1	Single gene heterozygous
Dihybrid F₂ (双因子F₂)	9 : 3 : 3 : 1	Independent assortment
Linked dihybrid test cross (连锁双因子测交)	Mostly parental, few recombinant	Autosomal linkage
Codominance (共显性)	1 : 2 : 1 (genotype = phenotype)	Both alleles expressed

10. Common Exam Pitfalls | 常见考试陷阱

Many students lose marks by forgetting to link genotype to phenotype, or by failing to express probabilities correctly. Always specify whether a ratio is phenotypic or genotypic. When calculating the chance of a specific offspring, consider the independence of events and use the ‘and/or’ rule correctly. Avoid vague language like ‘the gene is dominant’; instead, say ‘the dominant allele’.

许多学生因未能将基因型与表型对应或错误表达概率而失分。务必注明比例是表型比还是基因型比。在计算特定子代概率时，需考虑事件的独立性并正确运用“和/或”法则。避免“基因是显性的”这类模糊表述，应明确说“显性等位基因”。

Another common error is misinterpreting test cross results: if offspring show a 1:1 ratio, the parent is heterozygous, not ‘50% dominant’. Also, when performing chi-squared tests, do not forget to deduct one degree of freedom for the expected categories. Finally, in linkage problems, not all non-parental phenotypes necessarily arise from crossing over; double crossovers can occur, but at A-Level, single crossovers are the focus.

另一个常见错误是误解测交结果：若子代呈现1:1比例，说明亲本是杂合子，而非‘50%显性’。此外，进行卡方检验时，不要忘记自由度需根据期望类别数减一。最后，在连锁问题中，并非所有非亲本表型都由交叉互换引起；可能存在双交换，但在A-Level阶段，重点为单交换。

Practice drawing diagrams quickly and neatly. Underline or box your final answer. Use the correct symbols consistently, and always show your working for calculations—this can earn

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