A-Level Biology: Genetics Key Points | A-Level 生物：遗传学 考点精讲

📚 A-Level Biology: Genetics Key Points | A-Level 生物：遗传学 考点精讲

Genetics is a central topic in A-Level Biology, exploring how traits are inherited from one generation to the next. This guide covers fundamental concepts such as Mendelian inheritance, gene interactions, sex linkage, pedigree analysis, and population genetics, providing clear explanations and key exam tips.

遗传学是 A-Level 生物的核心主题，研究性状如何从一代传递到下一代。本指南涵盖孟德尔遗传、基因互作、伴性遗传、系谱分析和群体遗传学等基本概念，提供清晰的解释和关键考试技巧。

1. Mendelian Inheritance | 孟德尔遗传

Gregor Mendel’s experiments with pea plants established the basic laws of inheritance. He discovered that traits are determined by discrete units, now called genes, which exist in different forms called alleles.

格雷戈尔·孟德尔的豌豆实验奠定了遗传的基本规律。他发现性状由离散的单位（现在称为基因）决定，基因的存在形式称为等位基因。

Mendel’s Law of Segregation states that during gamete formation, the two alleles for a gene separate so that each gamete carries only one allele.

孟德尔的分离定律指出，在配子形成过程中，一个基因的两个等位基因彼此分离，每个配子只携带一个等位基因。

Mendel’s Law of Independent Assortment states that alleles of different genes are distributed to gametes independently of one another, provided the genes are on different chromosomes.

孟德尔的自由组合定律指出，不同基因的等位基因在配子中独立分配，前提是这些基因位于不同的染色体上。

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2. Monohybrid Crosses and Segregation | 单基因杂交与分离

A monohybrid cross involves one gene with contrasting alleles. For example, crossing a homozygous dominant tall pea plant (TT) with a homozygous recessive short plant (tt) produces heterozygous (Tt) offspring, all tall. The F2 generation shows a 3:1 phenotypic ratio.

单基因杂交涉及具有相对性状的一个基因。例如，纯合显性高茎豌豆（TT）与纯合隐性矮茎豌豆（tt）杂交，产生杂合子（Tt）后代，全部为高茎。F2 代表型比例为 3:1。

To perform a monohybrid cross, use a Punnett square to combine gametes. This predicts the genotypic ratio 1:2:1 (TT:Tt:tt) and the expected phenotypic ratio.

进行单基因杂交时，使用庞纳特方格组合配子。可预测基因型比例 1:2:1（TT:Tt:tt）和预期的表型比例。

The test cross (crossing an unknown dominant phenotype with a homozygous recessive) reveals whether the unknown is homozygous or heterozygous.

测交（将未知显性表型个体与纯合隐性个体杂交）可揭示该未知个体是纯合子还是杂合子。

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3. Dihybrid Crosses and Independent Assortment | 双基因杂交与自由组合

A dihybrid cross examines two genes simultaneously. When crossing two heterozygous individuals (e.g., TtYy × TtYy), and if genes are unlinked, the offspring phenotypic ratio is 9:3:3:1 (9 both dominant traits, 3 first dominant second recessive, 3 first recessive second dominant, 1 both recessive).

双基因杂交同时研究两个基因。当两个杂合子（如 TtYy × TtYy）杂交，如果基因不连锁，后代表型比例为 9:3:3:1（9 个双显性，3 个显隐，3 个隐显，1 个双隐性）。

The 9:3:3:1 ratio is typical for unlinked genes showing complete dominance. Deviations from this ratio suggest linkage, epistasis, or other interactions.

9:3:3:1 比例是典型的不连锁且完全显性的情况。偏离这一比例可能暗示连锁、上位效应或其他相互作用。

Using a Punnett square for dihybrid crosses helps visualise how independent assortment leads to four types of gametes in equal frequency (TY, Ty, tY, ty).

使用庞纳特方格进行双基因杂交有助于可视化自由组合如何导致四种类型配子以相等频率产生（TY、Ty、tY、ty）。

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4. Codominance and Multiple Alleles | 共显性与多等位基因

In codominance, both alleles are fully expressed in the heterozygote. The ABO blood group system is a classic example, with alleles Iᴬ, Iᴮ, and i. Iᴬ and Iᴮ are codominant (both antigens expressed), and both are dominant over i.

在共显性中，杂合子中两个等位基因均完全表达。ABO 血型系统是经典例子，等位基因 Iᴬ、Iᴮ 和 i。Iᴬ 与 Iᴮ 共显性（两种抗原均表达），且两者对 i 为显性。

The genotypes and phenotypes are:

基因型与表型如下：

Genotype 基因型	Phenotype (Blood Group) 表型（血型）
Iᴬ Iᴬ or Iᴬ i	A
Iᴮ Iᴮ or Iᴮ i	B
Iᴬ Iᴮ	AB
ii	O

Other examples include sickle-cell anaemia, where heterozygotes (HbA HbS) produce both normal and sickle-shaped red blood cells, showing codominance at the molecular level, but the overall health effect is more complex (incomplete dominance for disease).

其他例子包括镰状细胞贫血，杂合子（HbA HbS）同时产生正常和镰状红细胞，在分子水平上表现出共显性，但对疾病的影响更复杂（不完全显性）。

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5. Sex-linked Inheritance | 伴性遗传

Genes located on sex chromosomes (X or Y) show distinct inheritance patterns. Most sex-linked traits are X-linked because the X chromosome carries many genes, while the Y chromosome is small with few genes.

位于性染色体（X 或 Y）上的基因表现出独特的遗传模式。大多数伴性性状是 X 连锁的，因为 X 染色体携带许多基因，而 Y 染色体较小且基因少。

In X-linked recessive disorders (e.g., haemophilia, red-green colour blindness), males are more frequently affected because they have only one X chromosome. A single recessive allele on the X causes the trait.

在 X 连锁隐性遗传病中（如血友病、红绿色盲），男性更常受影响，因为他们只有一条 X 染色体。X 染色体上的单个隐性等位基因就会导致性状出现。

Females can be carriers (heterozygous) and pass the allele to offspring without being affected. A carrier female has a 50% chance of passing the recessive allele to a son (who will be affected) or to a daughter (who becomes a carrier if she inherits the dominant allele from father).

女性可以是携带者（杂合子），并将等位基因传给后代而不发病。女性携带者有 50% 的概率将隐性等位基因传给儿子（儿子将患病）或传给女儿（如果女儿从父亲处获得显性等位基因，她将成为携带者）。

X-linked dominant traits are rarer; affected males pass the trait to all daughters but no sons.

X 连锁显性性状较罕见；患病男性会将性状传给所有女儿，但不传给儿子。

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