📚 Genetics | 遗传学
Genetics is the branch of biology that explains how characteristics are passed from parents to offspring and how variation arises within populations. Mastering inheritance patterns, gene interactions and statistical testing is essential for A-Level Biology.
遗传学是生物学的一个分支，解释性状如何从亲代传递给子代，以及种群内变异如何产生。掌握遗传模式、基因互作和统计检验对于 A-Level 生物至关重要。
1. Key Terms in Genetics | 遗传学关键术语
Genetics uses precise vocabulary that every A-Level candidate must master. A gene is a length of DNA that codes for a polypeptide or a functional RNA molecule. An allele is one of two or more alternative forms of a gene found at the same locus on homologous chromosomes.
遗传学使用每个 A-Level 考生都必须掌握的精确术语。基因是一段编码多肽或功能性 RNA 分子的 DNA。等位基因是位于同源染色体相同基因座上的基因的两种或多种替代形式之一。
The genotype describes the alleles an organism possesses, while the phenotype is the observable characteristic produced by the genotype and the environment. Homozygous means having two identical alleles for a trait; heterozygous means having two different alleles. A dominant allele is expressed when only one copy is present, whereas a recessive allele is expressed only when both copies are present.
基因型描述生物体拥有的等位基因，而表型是由基因型和环境共同产生的可观察特征。纯合子指某一性状有两个相同的等位基因；杂合子指有两个不同的等位基因。显性等位基因只需一个拷贝即可表达，而隐性等位基因只有在两个拷贝都存在时才表达。
2. Monohybrid Inheritance | 单基因杂交遗传
A monohybrid cross tracks one gene with two alleles. When a pure-breeding dominant individual (TT) is crossed with a pure-breeding recessive individual (tt), all F1 offspring are heterozygous (Tt) and show the dominant phenotype.
单基因杂交追踪一个带有两个等位基因的基因。当纯合显性个体（TT）与纯合隐性个体（tt）杂交时，所有 F1 后代都是杂合子（Tt），并表现出显性表型。
Intercrossing the F1 heterozygotes produces an F2 genotypic ratio of 1 TT : 2 Tt : 1 tt. If dominance is complete, this gives a phenotypic ratio of 3 dominant : 1 recessive. Large sample sizes are essential for observed ratios to match expected ratios closely.
将 F1 杂合子相互杂交，F2 基因型比例为 1 TT : 2 Tt : 1 tt。如果为完全显性，则表型比例为 3 显性 : 1 隐性。足够大的样本量对于观察比例与预期比例高度吻合至关重要。
3. Dihybrid Inheritance and Independent Assortment | 双基因杂交与自由组合
A dihybrid cross examines two genes located on different autosomes. Each F1 double heterozygote (AaBb) produces four gamete types in equal numbers: AB, Ab, aB and ab. The random fusion of these gametes generates 16 equally likely combinations.
双基因杂交研究位于不同常染色体上的两个基因。每个 F1 双杂合子（AaBb）产生四种数量相等的配子类型：AB、Ab、aB 和 ab。这些配子的随机融合产生 16 种等可能的组合。
For two completely dominant genes, the F2 phenotypic ratio is 9 : 3 : 3 : 1. This ratio is diagnostic of independent assortment, meaning the alleles of one gene segregate without influencing the segregation of the other gene.
对于两个完全显性的基因，F2 表型比例为 9 : 3 : 3 : 1。这一比例是自由组合的标志，意味着一个基因的等位基因分离不会影响另一个基因的分离。
4. Codominance and Multiple Alleles | 共显性与复等位基因
Codominance means both alleles in a heterozygote are fully expressed, so neither allele masks the other. For example, in human ABO blood groups, the alleles IA and IB are codominant; a heterozygote has blood type AB because both antigens are present on red blood cells.
共显性指杂合子中两个等位基因都完全表达，因此任何一个都不会掩盖另一个。例如，在人类 ABO 血型中，等位基因 IA 和 IB 是共显性的；杂合子的血型为 AB，因为红细胞上同时存在两种抗原。
Incomplete dominance is different: the heterozygote shows an intermediate phenotype. Multiple alleles occur when more than two alleles exist in a population, although an individual still carries only two. The ABO system has three alleles: IA, IB and IO.
不完全显性则不同：杂合子表现出中间表型。复等位基因指种群中存在两个以上等位基因，但一个个体仍然只携带两个。ABO 系统有三个等位基因：IA、IB 和 IO。
5. Sex Linkage | 性连锁遗传
In humans, the X chromosome carries many genes that are absent from the smaller Y chromosome. A male has only one X chromosome, so he has only one allele for each X-linked gene. A recessive X-linked allele will therefore be expressed in males, because there is no corresponding allele on the Y chromosome to mask it.
在人类中，X 染色体携带许多在较小的 Y 染色体上不存在的基因。男性只有一条 X 染色体，因此每个 X 连锁基因只有一个等位基因。因此，隐性 X 连锁等位基因会在男性中表达，因为 Y 染色体上没有相应的等位基因可以掩盖它。
A female has two X chromosomes, so she must inherit two recessive alleles to show the trait; with one recessive allele she is a carrier. Red-green colour blindness and haemophilia are classic examples. A carrier female crossed with an unaffected male produces sons with a 50% chance of being affected.
女性有两条 X 染色体，因此必须遗传两个隐性等位基因才会表现该性状；只有一个隐性等位基因则是携带者。红绿色盲和血友病是经典例子。携带者女性与正常男性杂交，儿子有 50% 的概率患病。
6. Epistasis and Gene Interaction | 上位效应与基因互作
Epistasis is a form of gene interaction in which one gene masks or modifies the expression of another gene. In recessive epistasis, the presence of two recessive alleles at one locus prevents the expression of a second locus. A dihybrid cross can therefore produce a 9 : 3 : 4 phenotypic ratio instead of 9 : 3 : 3 : 1.
上位效应是一种基因互作形式，其中一个基因掩盖或改变另一个基因的表达。在隐性上位中，一个基因座上的两个隐性等位基因会阻止第二个基因座的表达。因此，双基因杂交可产生 9 : 3 : 4 的表型比例，而不是 9 : 3 : 3 : 1。
In dominant epistasis, a dominant allele at one locus masks the second locus, producing a 12 : 3 : 1 ratio. In complementary gene action, both dominant alleles are required for the phenotype, giving a 9 : 7 ratio. Recognising these modified ratios helps explain many biochemical pathways.
在显性上位中，一个基因座的显性等位基因掩盖第二个基因座，产生 12 : 3 : 1 的比例。在互补基因作用中，两个显性等位基因都是产生表型所必需的，产生 9 : 7 的比例。识别这些改变后的比例有助于解释许多生化途径。
7. Autosomal Linkage and Recombination | 常染色体连锁与重组
Genes on the same autosome are physically linked and tend to be
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