📚 Inherited Change | 遗传的变异
Inherited change refers to the passing of genetic information from one generation to the next, along with the variation that arises due to genetic recombination, mutation and sexual reproduction. In A-Level Biology, this topic explores how genes, alleles and chromosomes behave during meiosis and fertilisation, and how patterns of inheritance can be predicted, analysed and tested.
遗传的变异是指遗传信息从亲代传递到子代，同时伴随基因重组、突变和有性生殖产生的变异。在A-Level生物中，这一主题涵盖了基因、等位基因和染色体在减数分裂与受精过程中的行为，以及如何预测、分析和检验遗传模式。
1. Introduction to Inherited Change | 遗传变异简介
A gene is a length of DNA that codes for a specific polypeptide. Genes occupy fixed positions, called loci, on chromosomes. Different forms of a gene are known as alleles. The combination of alleles an organism possesses is its genotype, while the observable characteristics form the phenotype. An individual with two identical alleles at a locus is homozygous; one with two different alleles is heterozygous.
基因是编码特定多肽的一段DNA。基因在染色体上占据固定的位置，称为基因座。同一基因的不同形式叫做等位基因。生物体所拥有的等位基因组合构成基因型，而可观察的特征则为表现型。某基因座上拥有两个相同等位基因的个体是纯合子；拥有两个不同等位基因的个体是杂合子。
Dominant alleles are expressed in the phenotype even when only one copy is present, whereas recessive alleles require two copies to be expressed. The terms ‘dominant’ and ‘recessive’ describe the relationship between alleles, not their frequency in a population.
显性等位基因在仅存在一个拷贝时就能在表现型中表达，而隐性等位基因需要两个拷贝才能表达。‘显性’和‘隐性’描述的是等位基因之间的关系，而非它们在群体中的频率。
2. Monohybrid Inheritance | 单基因遗传
Monohybrid inheritance involves a single gene with two alleles. Gregor Mendel’s experiments on pea plants demonstrated that when pure-breeding (homozygous) parents with contrasting traits were crossed, the F1 generation was uniformly heterozygous and showed the dominant phenotype. Self-pollination of the F1 generation produced an F2 generation with a phenotypic ratio of 3 dominant : 1 recessive.
单基因遗传涉及一个有两个等位基因的基因。孟德尔在豌豆实验中发现，当具有相对性状的纯种（纯合）亲本杂交时，F1代全部为杂合且表现显性性状。F1代自花授粉产生的F2代表现型比例为3显性 : 1隐性。
The underlying genotypic ratio in F2 is 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. A test cross (crossing an individual of unknown genotype with a homozygous recessive individual) can reveal the unknown genotype by analysing the offspring phenotypes.
F2的基因型比例实际为1纯合显性 : 2杂合 : 1纯合隐性。测交（将未知基因型个体与纯合隐性个体杂交）可通过分析后代表现型来推断未知基因型。
3. Codominance and Multiple Alleles | 共显性与复等位基因
In codominance, both alleles in a heterozygote are fully expressed in the phenotype. The human ABO blood group system is a classic example. The gene has three common alleles: Iᴬ, Iᴮ and i. The alleles Iᴬ and Iᴮ are codominant to each other, and both are dominant over i.
在共显性中，杂合子中的两个等位基因都在表现型中充分表达。人类ABO血型系统是一个经典例子。该基因有三种常见等位基因：Iᴬ、Iᴮ和i。等位基因Iᴬ和Iᴮ互为共显性，且两者都对i为显性。
| Phenotype (Blood group) | Possible genotypes |
|---|---|
| A | IᴬIᴬ or Iᴬi |
| B | IᴮIᴮ or Iᴮi |
| AB | IᴬIᴮ |
| O | ii |
Because multiple alleles exist in the population, a greater number of phenotypic classes can be observed. Codominance demonstrates that not all dominant–recessive relationships are complete.
由于群体中存在复等位基因，能够观察到更多的表现型类别。共显性表明并非所有的显隐性关系都是完全的。
4. Dihybrid Inheritance | 双基因遗传
Dihybrid inheritance considers two genes located on different chromosomes. According to Mendel’s law of independent assortment, alleles of each gene separate independently during gamete formation. A cross between two F1 dihybrids (e.g., RrYy × RrYy, where R = round, r = wrinkled, Y = yellow, y = green in peas) yields an F2 phenotypic ratio of 9 : 3 : 3 : 1.
双基因遗传考虑位于不同染色体上的两个基因。根据孟德尔自由组合定律，形成配子时每个基因的等位基因独立分离。两个F1双杂合子杂交（例如RrYy × RrYy，R=圆粒、r=皱粒、Y=黄色、y=绿色），F2表现型比例为9 : 3 : 3 : 1。
This ratio arises because four types of gametes (RY, Ry, rY, ry) are produced in equal proportions, and fertilisation is random. The 9 : 3 : 3 : 1 ratio is a hallmark of unlinked genes. Deviations from this ratio can indicate gene interaction or linkage.
这一比例的产生是因为四种配子（RY、Ry、rY、ry）以相等比例生成，且受精是随机的。9 : 3 : 3 : 1比例是非连锁基因的特征。偏离这一比例可能说明存在基因互作或连锁。
5. Linkage and Crossing Over | 连锁与交换
Genes located on the same chromosome tend to be inherited together and are said to be linked. Linkage reduces the number of recombinant phenotypes in the offspring. For example, in a dihybrid test cross with linked genes, the parental phenotypes are far more frequent than the recombinant ones.
位于同一条染色体上的基因倾向于一起遗传，称为连锁。连锁会减少后代中重组表现型的数目。例如，在连锁基因的双杂合测交中，亲本表现型远比重组型常见。
During prophase I of meiosis, crossing over between homologous chromosomes can exchange alleles between linked genes, producing new combinations. The frequency of recombination between two linked genes can be used to map their relative positions: a 1% recombination frequency corresponds to one map unit (centimorgan).
在减数分裂前期I，同源染色体间的交叉互换可以在连锁基因之间交换等位基因，产生新的组合。两个连锁基因之间的重组频率可用于绘制它们的相对位置：1%的重组频率相当于一个图距单位（厘摩）。
6. Sex Linkage | 伴性遗传
Sex linkage refers to genes located on the sex chromosomes, most commonly the X chromosome. Because males (XY) have only one X chromosome, any recessive allele on the X is expressed in the phenotype, even if it is rare in females. Examples include red-green colour blindness and haemophilia A.
伴性遗传指位于性染色体（最常见为X染色体）上的基因。由于男性（XY）仅有一条X染色体，X染色体上的任何隐性等位基因都会在表现型中表达，即便在女性中少见。例子包括红绿色盲和血友病A。
We represent alleles using superscripts on the X chromosome, e.g., Xᴴ (normal clotting) and Xʰ (haemophilia). A carrier female (XᴴXʰ) has a 50% chance of passing the recessive allele to each son, who would then be affected. Affected fathers cannot pass the trait to their sons (since they give the Y chromosome), but all daughters will inherit the affected X and become carriers if the mother is normal.
我们用X染色体加上标表示等位基因，如Xᴴ（正常凝血）和Xʰ（血友病）。携带者女性（XᴴXʰ）有50%的概率将隐性等位基因传给每个儿子，儿子将患病。患病的父亲不能将此性状传给儿子（因为他提供的是Y染色体），但若母亲正常，所有女儿将遗传该患病X染色体并成为携带者。
7. Epistasis | 上位效应
Epistasis occurs when the allele of one gene masks or modifies the expression of another gene at a different locus. In recessive epistasis, the homozygous recessive state of one gene masks the expression of the other. For instance, in Labrador coat colour, the E gene controls pigment deposition: genotype ee produces a yellow coat regardless of the B gene alleles. The F2 ratio becomes 9 black : 3 chocolate : 4 yellow.
当某一基因的等位基因掩盖或修饰另一基因座的基因表达时，就发生上位效应。在隐性上位中，一个基因的纯合隐性状态会掩盖另一基因的表达。例如，拉布拉多犬的毛色中，E基因控制色素沉积：ee基因型产生黄色毛色，无论B基因的等位基因如何。F2比例变为9黑色 : 3巧克力色 : 4黄色。
In dominant epistasis, a single dominant allele of one gene masks the effect of another gene, leading to ratios such as 12 : 3 : 1 (as in squash fruit colour). Epistasis demonstrates that many phenotypes are the result of interactions between multiple genes.
在显性上位中，一个基因的单个显性等位基因掩盖另一基因的效应，产生如12 : 3 : 1的比例（如南瓜果色）。上位效应说明许多表现型是多基因互作的结果。
8. Chi-squared (χ²) Test | 卡方检验
The chi-squared test is a statistical tool used to determine whether the difference between observed and expected results is due to chance or is statistically significant. It is frequently applied in genetics to test Mendelian ratios. The formula is:
卡方检验是一种统计工具，用于判断观察值与期望值之间的差异是由偶然引起还是具有统计学显著性。在遗传学中常用于检验孟德尔比例。公式为：
χ² = Σ (O − E)² / E
where O = observed frequency, E = expected frequency. The null hypothesis assumes no significant difference. After calculating χ², the value is compared against a critical value at a chosen probability level (usually p = 0.05) and appropriate degrees of freedom (number of phenotypic classes − 1).
其中O = 观察频数，E = 期望频数。零假设假定无显著差异。计算χ²后，将所得值与选定概率水平（通常p = 0.05）和适当自由度（表型类别数−1）下的临界值进行比较。
If χ² < critical value, we accept the null hypothesis; if χ² > critical value, we reject it and conclude that the difference is significant, suggesting factors such as linkage, epistasis or sampling error may be at play.
若χ² < 临界值，接受零假设；若χ² > 临界值，拒绝零假设并认为差异显著，暗示可能存在连锁、上位效应或取样误差等因素。
9. Gene Mutations | 基因突变
Gene mutations are changes in the base sequence of DNA and are a primary source of new alleles. Substitution mutations replace one base with another; they may be silent (no change in amino acid), missense (change to a different amino acid) or nonsense (introduce a premature stop codon). Insertion and deletion mutations cause frameshifts, altering the entire downstream amino acid sequence.
基因突变是DNA碱基序列的改变，是新等位基因的主要来源。替换突变以另一个碱基替换一个碱基；可能是沉默（不改变氨基酸）、错义（变为不同氨基酸）或无义（引入提前终止密码子）突变。插入和缺失突变引起移码，改变整个下游氨基酸序列。
Sickle cell anaemia results from a single nucleotide substitution in the gene for haemoglobin, changing the amino acid glutamic acid to valine. This alters the shape of red blood cells under low oxygen conditions. Mutations can be neutral, harmful or beneficial, contributing to genetic variation acted upon by natural selection.
镰刀形细胞贫血症是由血红蛋白基因中的一个碱基替换引起的，将谷氨酸变为缬氨酸。这改变了低氧条件下红细胞的形状。突变可以是中性的、有害的或有利的，为自然选择提供遗传变异。
10. Chromosome Mutations | 染色体突变
Chromosome mutations involve changes in chromosome structure or number. Structural changes include deletion (loss of a segment), duplication (repetition of a segment), inversion (reversal of a segment) and translocation (transfer of a segment to a non-homologous chromosome). These can disrupt gene function and lead to phenotypic abnormalities.
染色体突变涉及染色体结构或数目的变化。结构变化包括缺失（丢失一段）、重复（增加一段）、倒位（一段颠倒）和易位（一段转移到另一非同源染色体）。这些突变会扰乱基因功能并导致表型异常。
Numerical mutations produce aneuploidy, where one or more chromosomes are missing or extra, often due to non-disjunction during meiosis. Down syndrome (trisomy 21) is an example of aneuploidy. Polyploidy, the possession of more than two complete sets of chromosomes, is common in plants and can be induced to produce seedless varieties.
数目突变产生非整倍体，即缺失或多出整条染色体，通常因减数分裂中的不分离引起。唐氏综合征（21三体）是非整倍体的一个例子。多倍体指拥有两套以上完整染色体组，在植物中常见并可被诱导产生无籽品种。
11. The Role of Meiosis in Generating Variation | 减数分裂在产生变异中的作用
Meiosis is essential for sexual reproduction and introduces genetic variation in two main ways: independent assortment and crossing over. During metaphase I, bivalents align randomly, so maternal and paternal chromosomes assort independently into gametes. For n chromosome pairs, this produces 2ⁿ possible combinations of chromosomes per gamete (excluding crossing over).
减数分裂对有性生殖至关重要，通过两种主要方式引入遗传变异：独立分配和交叉互换。中期I时，二价体随机排列，因此母源和父源染色体独立组合进入配子。对于n对染色体，这产生每个配子2ⁿ种可能的染色体组合（不包括交换）。
Crossing over during prophase I further reshuffles alleles between homologous chromosomes, creating new linkage groups. These processes ensure that each gamete is genetically unique. Random fertilisation then multiplies the variation, producing offspring that differ from both parents and siblings.
前期I的交叉互换进一步在同源染色体间重排等位基因，形成新的连锁群。这些过程确保每个配子在遗传上都是独特的。随机的受精则进一步放大变异，产生与双亲和兄弟姐妹都不相同的子代。
12. Sources of Genetic Variation | 遗传变异的来源
The ultimate sources of inherited variation are mutation, which creates new alleles, and recombination during meiosis, which shuffles existing alleles into new combinations. While the environment can influence the expression of genes (the phenotype), only changes in DNA sequences are heritable and contribute to inherited change.
遗传变异的根本来源是突变（产生新等位基因）和减数分裂中的重组（将现有等位基因重新洗牌形成新组合）。虽然环境能影响基因的表达（表现型），但只有DNA序列的改变才是可遗传的，才构成遗传的变异。
Other factors such as gene flow (migration) and genetic drift also alter allele frequencies in populations over generations, linking inherited change at the family level to evolution at the population level. In A-Level studies, the focus remains on the mechanisms that generate genetic diversity in offspring.
其他因素如基因流（迁移）和遗传漂变也会在世代间改变群体中的等位基因频率，将家族水平的遗传变异与群体水平的进化联系起来。在A-Level学习中，重点仍然是产生子代遗传多样性的机制。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导