📚 Mendelian Genetics & Laws of Inheritance | 遗传规律与孟德尔定律
Mendel’s experiments with garden peas laid the foundation of classical genetics. Understanding the laws of segregation and independent assortment is essential for solving inheritance problems in A Level biology.
孟德尔以豌豆为材料进行的实验奠定了经典遗传学的基础。掌握分离定律与自由组合定律，是解决 A Level 生物考试中遗传题目的关键。
1. The Historical Context and Mendel’s Experimental Design | 历史背景与孟德尔的实验设计
Gregor Mendel (1822–1884) worked with Pisum sativum, the garden pea, selecting seven contrasting traits that exhibited clear alternative forms such as tall versus dwarf stems and round versus wrinkled seeds.
孟德尔（1822–1884）以豌豆（Pisum sativum）为材料，选取了七对具有鲜明相对性状的特征，例如高茎对矮茎、圆粒对皱粒等。
The pea plant was ideal because it was self-fertilising, cheap to cultivate, and had a short generation time, and Mendel could strictly control crosses by emasculation and artificial pollination.
豌豆是理想的实验材料，因为它自花传粉、便于种植、世代周期短，孟德尔可以通过去雄和人工授粉严格控制杂交组合。
By counting large numbers of offspring and applying mathematics, Mendel was able to detect discrete ratios rather than blending inheritance.
通过对大量后代进行计数并运用数学分析，孟德尔观察到的不是融合遗传，而是彼此离散的比例。
2. Key Terminology: Genes, Alleles and Phenotype | 关键术语：基因、等位基因与表型
Alleles are alternative versions of the same gene that occupy the same locus on homologous chromosomes.
等位基因是位于同源染色体相同位点上的同一基因的不同版本。
An organism’s genotype describes its allelic constitution, while phenotype is the observable expression of the genotype interacting with the environment.
基因型描述生物的等位基因组成，而表型是基因型与环境共同作用后表现出的可观察特征。
| English Term 英文术语 | 中文翻译 | Definition 定义 |
|---|---|---|
| Gene | 基因 | A DNA segment coding for a specific polypeptide or functional RNA. |
| Allele | 等位基因 | An alternative version of a gene at the same locus. |
| Dominant | 显性 | An allele that is expressed in the heterozygote; shown by a capital letter. |
| Recessive | 隐性 | An allele whose effect is masked in the heterozygote; shown by a lower-case letter. |
| Homozygous | 纯合 | Carrying two identical alleles of a gene. |
| Heterozygous | 杂合 | Carrying two different alleles of a gene. |
| Phenotype | 表型 | The observable physical or biochemical expression of a trait. |
| Genotype | 基因型 | The allelic constitution of an organism at one or more loci. |
3. The Law of Segregation | 分离定律
The law of segregation states that for any given characteristic, the two alleles carried by a diploid parent separate equally into gametes, so each gamete receives only one allele.
分离定律指出：对于任何给定性状，二倍体亲本携带的两个等位基因在形成配子时彼此分离、均等分配，因此每个配子只携带一个等位基因。
That separation is based on the behaviour of homologous chromosomes during meiosis I, when homologous pairs segregate to opposite poles.
这种分离基于减数第一次分裂过程中同源染色体分别移向细胞两极的行为。
In a monohybrid cross between two heterozygous plants (Rr × Rr), Mendel predicted a 3:1 phenotypic ratio in the F₂ generation.
在杂合植株（Rr × Rr）之间的单因子杂交中，孟德尔预测 F₂ 代会呈现 3:1 的表型分离比。
Rr × Rr → 配子各为 ½ R 与 ½ r → F₂ 基因型比 1 : 2 : 1，表型比 3 : 1
4. Monohybrid Cross Diagrams | 单因子杂交遗传图解
The Punnett square shows all possible gamete fusions for a single gene. For a cross of Rr × Rr, the square has four cells.
庞纳特方格展示单个基因所有可能的配子结合方式。对于 Rr × Rr 杂交，方格共有四格。
| 配子 | R | r |
|---|---|---|
| R | RR 圆粒 | Rr 圆粒 |
| r | Rr 圆粒 | rr 皱粒 |
The genotypic ratio among F₂ offspring is 1 RR : 2 Rr : 1 rr, whereas the phenotypic ratio is 3 round : 1 wrinkled.
F₂ 群体的基因型比例为 1 RR : 2 Rr : 1 rr，而表型比例为 3 圆粒 : 1 皱粒。
Importantly, the 3:1 ratio only appears when large numbers of offspring are counted and when both sexes contribute equally.
值得注意的是，3:1 的表型比只有在后代数量足够大、且双亲贡献均等时才能稳定显现。
5. Test Crosses | 测交
Because dominant phenotypes may be homozygous (RR) or heterozygous (Rr), a test cross with a homozygous recessive parent distinguishes the two genotypes.
由于显性表型可能来自纯合体（RR）或杂合体（Rr），因此需要与隐性纯合体测交来区分这两种基因型。
If the unknown plant is RR, all offspring show the dominant phenotype. If it is Rr, the offspring will show a 1:1 ratio of dominant to recessive phenotypes.
若未知植株为 RR，则全部后代均为显性表型；若为 Rr，则后代表现为显性:隐性接近 1:1。
Rr × rr → ½ Rr（显性） : ½ rr（隐性），即 1 : 1
6. The Law of Independent Assortment | 自由组合定律
The law of independent assortment holds that alleles of genes on different chromosomes segregate independently during gamete formation.
自由组合定律指出，位于不同染色体上的等位基因在配子形成过程中彼此独立分离。
This applies only to unlinked genes; genes on the same chromosome tend to be inherited together and may require linkage analysis.
该定律仅适用于不连锁的基因；同一染色体上的基因往往倾向于一同遗传，需要通过连锁分析来研究。
The gametes produced by a double heterozygote AaBb are therefore AB, Ab, aB and ab in equal frequencies, provided the two loci are on different chromosomes.
因此，双杂合体 AaBb 产生的配子为 AB、Ab、aB、ab，频率相等——前提是这两个基因座位于不同染色体上。
7. Dihybrid Crosses and the 9:3:3:1 Ratio | 双因子杂交与 9:3:3:1 比例
In a dihybrid cross such as RrYy × RrYy, where R = round, r = wrinkled, Y = yellow and y = green, the two traits assort independently.
在双因子杂交 RrYy × RrYy 中，R 代表圆粒、r 代表皱粒，Y 代表黄色、y 代表绿色，两对性状独立分配。
Each parent produces four gamete types in equal proportions: RY, Ry, rY, ry.
每个亲本产生四种数量相等的配子类型：RY、Ry、rY、ry。
| 配子 | RY | Ry | rY | ry |
|---|---|---|---|---|
| RY | RRYY 圆黄 | RRYy 圆黄 | RrYY 圆黄 | RrYy 圆黄 |
| Ry | RRYy 圆黄 | RRyy 圆绿 | RrYy 圆黄 | Rryy 圆绿 |
| rY | RrYY 圆黄 | RrYy 圆黄 | rrYY 皱黄 | rrYy 皱黄 |
| ry | RrYy 圆黄 | Rryy 圆绿 | rrYy 皱黄 | rryy 皱绿 |
The F₂ phenotype ratio is 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.
F₂ 的表型比为 9 圆黄 : 3 圆绿 : 3 皱黄 : 1 皱绿。
8. Modifications to Mendelian Ratios: Incomplete Dominance and Codominance | 孟德尔比例的扩展：不完全显性与共显性
Not all genes display complete dominance. In incomplete dominance, the heterozygote has an intermediate phenotype, giving a 1:2:1 F₂ ratio.
并非所有基因都表现完全显性。在不完全显性中，杂合子呈现中间表型，F₂ 表现为 1:2:1。
In codominance, both alleles are fully expressed in the heterozygote, as in the ABO blood group system where A and B alleles both appear.
在共显性中，两个等位基因在杂合子中都会完全表达，例如 ABO 血型系统中 A 和 B 等位基因同时表现。
Codominance does not change the genotype ratio, but it changes the phenotype count because the heterozygote is a distinct class.
共显性并不改变基因型比例，但因杂合子成为独立可区分的类别，因此表型数目相应改变。
例：红色花 RR × 白色花 rr → F₁ 全部为粉色花 Rr（不完全显性）
9. Gene Interaction and Epistasis | 基因互作与上位效应
Epistasis occurs when one gene masks the effect of another non-allelic gene. Such interactions modify the classic 9:3:3:1 ratio.
上位效应指一个基因掩盖另一个非等位基因的表型效应。这类
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