Mendelian Inheritance | 孟德尔遗传

📚 Mendelian Inheritance | 孟德尔遗传

Mendelian inheritance forms the foundation of classical genetics, describing how traits are passed from parents to offspring through discrete units called genes. In the CCEA A-Level Biology specification, you need to master terminology, monohybrid and dihybrid crosses, test crosses, codominance, sex linkage, autosomal linkage, epistasis and the chi‑squared test. This article provides a bilingual review of every key concept, with clear explanations and exam‑style examples.

孟德尔遗传是经典遗传学的基石，描述性状如何通过被称为基因的离散单位亲代传递给子代。在CCEA A-Level生物考纲中，你需要掌握术语、单因子杂交与双因子杂交、测交、共显性、性连锁、常染色体连锁、上位效应以及卡方检验。本文以中英双语精讲每一个核心概念，配有清晰的解释和贴近考题的例子。

1. Key Terminology | 关键术语

A gene is a segment of DNA coding for a functional product; an allele is a variant form of a gene; the genotype is the genetic makeup of an organism; the phenotype is the physical expression of the genotype; homozygous describes having two identical alleles; heterozygous describes having two different alleles; a dominant allele masks the effect of a recessive allele in a heterozygote; a recessive allele is expressed only in the homozygous state. The terms F1 (first filial) and F2 (second filial) describe successive generations of a cross.

基因是编码功能性产物的DNA片段；等位基因是基因的变体形式；基因型是生物体的遗传组成；表现型是基因型的物理表达；纯合子指拥有两个相同等位基因；杂合子指拥有两个不同等位基因；显性等位基因在杂合状态下掩盖隐性等位基因的效应；隐性等位基因仅在纯合状态下表达。F1（第一子代）和F2（第二子代）术语描述杂交的连续世代。

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2. Mendel’s Laws of Inheritance | 孟德尔遗传定律

Mendel’s first law, the Law of Segregation, states that each individual has two alleles for each gene, which segregate during gamete formation so each gamete carries only one allele. When gametes fuse at fertilisation, the diploid number is restored and the offspring’s genotype is determined by the combination of alleles received.

孟德尔第一定律——分离定律，指出每个个体每个基因都有两个等位基因，它们在配子形成时分离，使每个配子只携带一个等位基因。当配子在受精时融合，二倍体数目恢复，后代的基因型便由所获等位基因的组合决定。

Mendel’s second law, the Law of Independent Assortment, states that alleles of different genes assort independently of one another during gamete formation, provided the genes are located on different chromosomes. This generates new combinations of alleles in the offspring.

孟德尔第二定律——自由组合定律，指出不同基因的等位基因在配子形成过程中独立分配，前提是这些基因位于不同的染色体上。这使后代产生新的等位基因组合。

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3. Monohybrid Crosses | 单因子杂交

A monohybrid cross follows the inheritance of a single gene. For example, crossing a homozygous dominant (AA) with a homozygous recessive (aa) produces an F1 generation that is entirely heterozygous (Aa) and shows the dominant phenotype. Self‑pollinating the F1 gives an F2 generation with a phenotypic ratio of 3 dominant : 1 recessive and a genotypic ratio of 1 AA : 2 Aa : 1 aa.

单因子杂交追踪单个基因的遗传。例如，纯合显性（AA）与纯合隐性（aa）杂交，产生的F1代全部为杂合（Aa）并表现显性性状。将F1自交，F2代表现型比例为3显性 : 1隐性，基因型比例为1 AA : 2 Aa : 1 aa。

A Punnett square helps visualise the combinations:

旁氏表有助于可视化组合：

Gametes	A	a
A	AA	Aa
a	Aa	aa

Phenotypic ratio: 3 : 1

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4. Dihybrid Crosses | 双因子杂交

A dihybrid cross follows two genes on different chromosomes. Crossing two individuals heterozygous for both genes (e.g. RrYy × RrYy) yields a characteristic 9 : 3 : 3 : 1 phenotypic ratio in the F2 generation, provided the genes assort independently. The ratio arises because the probability of inheriting each trait combination is the product of the individual probabilities.

双因子杂交追踪位于不同染色体上的两个基因。两个双杂合个体杂交（如RrYy × RrYy），如果基因自由组合，F2代的经典表现型比例为9 : 3 : 3 : 1。该比例之所以产生，是因为每一性状组合的遗传概率是各自概率的乘积。

Gametes	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

Expected phenotypic ratio: 9 : 3 : 3 : 1

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5. Test Crosses | 测交

A test cross determines the genotype of an individual showing the dominant phenotype. The individual is crossed with a homozygous recessive. If all offspring show the dominant trait, the unknown parent is homozygous dominant. If half the offspring show the recessive trait, the unknown parent is heterozygous. This is a powerful tool for revealing hidden alleles.

测交用于确定表现显性性状个体的基因型。将该个体与隐性纯合子杂交。如果所有后代都表现显性性状，则未知亲本是显性纯合子；如果一半后代表现隐性性状，则未知亲本是杂合子。这是揭示隐蔽等位基因的有力工具。

For example, crossing a tall plant (T_) with a dwarf (tt): if all progeny are tall, the parent was TT; if a 1:1 ratio of tall to dwarf appears, the parent was Tt.

例如，高茎植物（T_）与矮茎（tt）杂交：如果所有后代都是高茎，则亲本为TT；如果出现1:1的高矮比，则亲本为Tt。

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

In codominance, both alleles in a heterozygote are fully expressed. The classic example is the human ABO blood group system, controlled by a single gene with three alleles: Iᴬ, Iᴮ and i. Alleles Iᴬ and Iᴮ are codominant; both produce functional antigens and are expressed together in type AB blood. Allele i is recessive and produces no antigen, giving type O.

在共显性中，杂合子的两个等位基因都充分表达。经典例子是人类ABO血型系统，由单个基因的三个等位基因控制：Iᴬ、Iᴮ和i。等位基因Iᴬ与Iᴮ为共显性；两者都产生功能性抗原，在AB型血中同时表达。等位基因i为隐性，不产生抗原，形成O型血。

Possible genotypes and phenotypes for the ABO system:

ABO系统的可能基因型与表现型：

Genotype	Phenotype
IᴬIᴬ or Iᴬi	Type A
IᴮIᴮ or Iᴮi	Type B
IᴬIᴮ	Type AB
ii	Type O

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7. Sex Linkage | 性连锁

Sex‑linked genes are carried on the X chromosome (or rarely the Y chromosome). Because males have only one X chromosome (XY), a recessive allele on their single X is always expressed, making males more likely to show sex‑linked recessive disorders such as haemophilia and red‑green colour blindness. Females (XX) can be carriers if they are heterozygous.

性连锁基因位于X染色体（极少在Y染色体）。由于男性只有一个X染色体（XY），他们唯一的X上若携带隐性等位基因便会表达，因此男性更易患血友病、红绿色盲等X连锁隐性遗传病。女性（XX）如果杂合，则可成为携带者。

Using the haemophilia allele (h) and normal allele (H): a carrier female is XᴴXʰ, a normal male is XᴴY, and an affected male is XʰY. A cross between a carrier female and a normal male can produce an affected son with a 25% probability.

以血友病等位基因h和正常等位基因H为例：女性携带者为XᴴXʰ，正常男性为XᴴY，患病男性为XʰY。女性携带者与正常男性婚配，有25%的概率生下患病的儿子。

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8. Autosomal Linkage | 常染色体连锁

When two genes are located on the same autosome and are close together, they do not assort independently; they are linked. Linked genes tend to be inherited together, and dihybrid crosses produce offspring ratios that deviate from the 9:3:3:1 expectation. The closer the genes, the lower the chance of crossing over separating them. Offspring phenotypes will show a higher proportion of parental combinations than recombinant ones.

当两个基因位于同一条常染色体上且相距很近时，它们不遵循自由组合，而是连锁的。连锁的基因倾向于一同遗传，双因子杂交产生的后代比例会偏离9:3:3:1。基因间距离越近，交叉互换将它们分开的概率越低。后代表现型中亲本组合的比例将高于重组型。

For example, if genes A and B are linked, a heterozygote (AB/ab) test‑crossed with a homozygous recessive (ab/ab) will yield mostly AB/ab and ab/ab progeny, with a small number of recombinant genotypes Ab/ab and aB/ab, reflecting crossing over during meiosis.

例如，若基因A与B连锁，杂合子（AB/ab）与隐性纯合子（ab/ab）测交，后代将主要为AB/ab和ab/ab，只有少量重组基因型Ab/ab和aB/ab，反映出减数分裂中发生了交叉互换。

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9. Epistasis | 上位效应

Epistasis occurs when the expression of one gene masks or modifies the expression of another gene at a different locus. In recessive epistasis, a homozygous recessive genotype at one locus masks the effect of alleles at a second locus. The coat colour of Labrador retrievers is a well‑known example. The gene E controls pigment deposition: the recessive e allele (ee) prevents any pigment deposition, resulting in a yellow coat regardless of the B gene (which determines black vs. chocolate). The B_E_ genotype gives black, bbE_ gives chocolate, and any ee combination gives yellow, producing a modified 9:3:4 ratio in a dihybrid cross.

当一个基因的表达掩盖或改变另一位点的另一基因的表达时，发生上位效应。隐性上位中，一个位点的隐性纯合基因型掩盖另一位点等位基因的效应。拉布拉多犬的毛色是典型例子。基因E控制色素沉积：隐性e等位基因（ee）阻止所有色素沉积，导致黄色毛色，而无论B基因（决定黑与巧克力色）如何。B_E_基因型为黑色，bbE_为巧克力色，任何ee组合均为黄色，双因子杂交产生修饰后的9:3:4比例。

Dominant epistasis, where a dominant allele at one locus masks the effect of another locus, gives ratios such as 12:3:1 (e.g. squash colour). These patterns must be recognised when analysing genetic crosses.

显性上位中，一个位点的显性等位基因掩盖另一位点的效应，产生如12:3:1的比例（如南瓜颜色）。分析遗传杂交时必须识别这些模式。

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10. Chi‑squared (χ²) Test in Genetics | 遗传学中的卡方检验

The chi‑squared test determines whether the observed phenotypic ratios fit the expected Mendelian ratios. A null hypothesis (H₀) is stated: there is no significant difference between observed and expected frequencies; any difference is due to chance. The test statistic is calculated as:

卡方检验用于判断观察到的表现型比例是否符合预期的孟德尔比例。提出零假设（H₀）：观察值与预期值之间无显著差异，任何差异由偶然造成。检验统计量计算如下：

χ² = Σ (O − E)² / E

where O is the observed number and E is the expected number for each class. The calculated χ² is compared with a critical value from a χ² table at the appropriate degrees of freedom

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