📚 Genotype Affects Phenotype | 基因型影响表型
In A-level Biology, one of the central ideas is that the genetic makeup of an organism (genotype) determines much of its observable characteristics (phenotype). However, the relationship is not always one-to-one: dominance, gene interactions, sex linkage and the environment all shape how a genotype is expressed.
在A-level生物中，核心概念之一是生物体的遗传组成（基因型）在很大程度上决定了其可观察特征（表型）。但这种关系并不总是一一对应：显隐性、基因互作、伴性遗传和环境都会影响基因型的表达方式。
1. The Basic Genotype–Phenotype Distinction | 基因型与表型的基本区别
The genotype is the complete set of alleles carried by an organism, whereas the phenotype is the observable physical or biochemical expression of those alleles. The phenotype includes morphology, development, behaviour and physiological properties.
基因型是生物体携带的全部等位基因，而表型是这些等位基因的可观察物理或生化表现。表型包括形态、发育、行为和生理特性。
In a simple system, a particular allele can produce a particular protein, such as an enzyme or pigment, which then influences the phenotype. However, the pathway from DNA to phenotype is often indirect and can be modified at several steps.
在简单体系中，特定等位基因可以产生特定蛋白质，如酶或色素，进而影响表型。但从DNA到表型的途径往往是间接的，并且可以在多个步骤中被修饰。
2. Mendelian Dominance and Recessive Phenotypes | 孟德尔显性与隐性表型
In a diploid organism, two alleles at a locus may be dominant or recessive. A dominant allele is expressed in the phenotype even when only one copy is present, while a recessive allele is expressed only when two copies are present.
在二倍体生物中，同一基因座上的两个等位基因可能为显性或隐性。显性等位基因只要存在一个拷贝就能在表型中表现，而隐性等位基因只有存在两个拷贝时才会表现。
For example, if T represents the tall allele and t represents the dwarf allele in pea plants, the genotypes TT and Tt both produce tall plants, while tt produces dwarf plants. A monohybrid cross Tt × Tt gives a phenotypic ratio of 3 tall : 1 dwarf.
例如，若T代表豌豆高茎等位基因、t代表矮茎等位基因，则基因型TT和Tt都表现为高茎，而tt表现为矮茎。单基因杂合交配Tt × Tt产生3高茎 : 1矮茎的表型比例。
The hidden recessive allele in a heterozygote shows that phenotype is not simply a blend of the two alleles; one allele can be completely masked at the phenotypic level while still being passed to offspring.
杂合子中被隐藏的隐性等位基因表明，表型并不是两个等位基因的简单混合；一个等位基因可以在表型层面被完全掩盖，但仍能传递给后代。
3. Codominance and Multiple Alleles | 共显性与复等位基因
Some alleles show codominance, where both alleles are expressed equally in a heterozygote. This differs from incomplete dominance, where the phenotype is an intermediate blend of the two alleles.
一些等位基因表现为共显性，即杂合子中两个等位基因都同样表达。这与不完全显性不同，后者表型是两个等位基因的中间混合。
The human ABO blood group is controlled by three alleles: IA, IB and i. The alleles IA and IB are codominant, so the genotype IAIB produces the AB phenotype, while i is recessive to both.
人类ABO血型由三个等位基因控制：IA、IB和i。等位基因IA与IB为共显性，因此基因型IAIB产生AB型表型，而i对两者都是隐性。
Another example is roan coat colour in cattle, where a heterozygous animal has a mixture of red and white hairs, so both alleles are visible in the phenotype at the same time.
另一个例子是牛的沙毛色，杂合子个体同时具有红色和白色毛发，因此两个等位基因同时出现在表型中。
4. Sex-linked Inheritance | 伴性遗传
Sex-linked genes are located on the sex chromosomes, usually the X chromosome. Because males have only one X chromosome (XY), a recessive allele on the X chromosome is expressed in males even if it is rare in the population.
伴性基因位于性染色体上，通常是X染色体。由于男性只有一条X染色体（XY），X染色体上的隐性等位基因即使在群体中很稀有，也会在男性中表达。
For red-green colour blindness, let XR represent the normal allele and Xr the mutant allele. A carrier female has genotype XRXr and normal vision, while a male with genotype XrY is colour blind.
以红绿色盲为例，设XR代表正常等位基因，Xr代表突变等位基因。携带者女性的基因型为XRXr且色觉正常，而基因型为XrY的男性则患色盲。
In a cross between a carrier female and a normal male, half of the sons are expected to be colour blind. This shows that a genotype on the X chromosome directly alters the phenotype in a sex-specific pattern.
在携带者女性与正常男性的交配中，预计一半儿子会患色盲。这表明X染色体上的基因型以性别特异性方式直接改变表型。
5. Polygenic Inheritance and Continuous Variation | 多基因遗传与连续变异
Many phenotypic traits are polygenic, meaning they are controlled by several genes at different loci. Each gene adds a small effect, producing continuous variation rather than discrete categories.
许多表型性状是多基因控制的，即由不同基因座上的多个基因共同控制。每个基因产生微小效应，从而产生连续变异而非离散类别。
Human height, skin colour and seed mass in some plants are polygenic traits. Their distribution in a population often follows a bell-shaped curve because many combinations of alleles are possible.
人类身高、肤色以及某些植物的种子质量都是多基因性状。这些性状在群体中的分布常呈钟形曲线，因为存在许多等位基因组合。
Environmental factors also influence polygenic traits more visibly; therefore, the same polygenic genotype may produce a range of phenotypes under different conditions.
环境因素对多基因性状的影响也更明显；因此，相同的多基因基因型在不同条件下可能产生一系列表型。
6. Epistasis | 上位效应
Epistasis occurs when the allele at one gene locus masks or modifies the effect of an allele at a different locus. This changes the expected Mendelian phenotypic ratios.
上位效应发生在一个基因座上的等位基因掩盖或修饰另一个基因座上等位基因的效应。这会改变预期的孟德尔表型比率。
A classic example is coat colour in mice. Gene B controls whether the pigment is black (B) or brown (b), but gene C controls pigment deposition. The genotype cc masks the effect of B/b, producing albino mice.
经典例子是小鼠的毛色。B基因控制色素是黑色（B
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