📚 Inherited Change | 遗传变化
Inherited change refers to the transmission of genetic information from parents to offspring. At A Level, this topic links the behaviour of chromosomes in meiosis with the patterns of inheritance first described by Mendel. You need to understand how alleles interact, how sex-linked traits are inherited, and how statistical tests such as the chi-squared test are used to judge whether observed ratios fit expected ratios.
遗传变化指遗传信息从亲代传递给子代的过程。在A Level阶段，这一主题将减数分裂中染色体的行为与孟德尔最早描述的遗传模式联系起来。你需要理解等位基因如何相互作用、性连锁性状如何遗传，以及如何使用卡方检验等统计方法判断观察到的分离比是否符合预期比例。
1. Key Genetic Terms | 关键遗传术语
A gene is a sequence of DNA nucleotides that codes for a polypeptide or functional RNA. An allele is one alternative form of a gene found at the same gene locus on homologous chromosomes. The genotype is the combination of alleles an organism possesses, while the phenotype is the observable or measurable expression of the genotype plus environmental influences. A homozygous individual has two identical alleles at a locus, and a heterozygous individual has two different alleles at that locus.
基因是编码多肽或功能性RNA的一段DNA核苷酸序列。等位基因是位于同源染色体同一基因座上的基因的另一种形式。基因型是个体拥有的等位基因组合，而表现型是基因型在环境影响下表现出的可观察或可测量的特征。纯合子在某一基因座上具有两个相同的等位基因，杂合子则具有两个不同的等位基因。
- Dominant allele: expressed in the phenotype even when only one copy is present.
- Recessive allele: only expressed when two copies are present.
- Codominant allele: both alleles are fully expressed in a heterozygote.
- Locus: the fixed position of a gene on a chromosome.
显性等位基因：只要存在一个拷贝就能在表现型中表达。隐性等位基因：只有存在两个拷贝时才表达。共显性等位基因：杂合子中两个等位基因均充分表达。基因座：基因在染色体上的固定位置。
2. Monohybrid Inheritance | 单基因杂交遗传
A monohybrid cross follows one gene with two alleles. When a homozygous dominant individual is crossed with a homozygous recessive individual, all F1 offspring are heterozygous and show the dominant phenotype. Crossing two F1 heterozygotes gives a 3:1 phenotypic ratio in the F2 generation and a 1:2:1 genotypic ratio. These ratios can be shown with a Punnett square.
单基因杂交追踪一个基因的两个等位基因。当纯合显性个体与纯合隐性个体杂交时，所有F1子代均为杂合子并表现显性性状。将两个F1杂合子杂交，F2代产生3:1的表现型比例和1:2:1的基因型比例。这些比例可用庞纳特方格表示。
| F2 genotypes | TT | Tt | tt |
|---|---|---|---|
| Ratio | 1 | 2 | 1 |
| Phenotype | Tall | Tall | Dwarf |
F2基因型比例：TT:Tt:tt = 1:2:1。F2表现型比例：高茎:矮茎 = 3:1。
3. Codominance and Multiple Alleles | 共显性与复等位基因
In codominance, both alleles are expressed equally in the heterozygote, so the phenotype is not a blend. The ABO blood group system is controlled by three alleles: Iᴬ, Iᴮ and Iᴼ. Iᴬ and Iᴮ are codominant, and both are dominant to Iᴼ. Different combinations of these three alleles produce four blood groups: A, B, AB and O.
在共显性中，杂合子的两个等位基因均等表达，因此表现型不是中间混合型。ABO血型系统由三个等位基因控制：Iᴬ、Iᴮ和Iᴼ。Iᴬ与Iᴮ为共显性，且两者对Iᴼ均显性。这三个等位基因的不同组合产生四种血型：A型、B型、AB型和O型。
| Genotype | Blood group |
|---|---|
| Iᴬ Iᴬ or Iᴬ Iᴼ | A |
| Iᴮ Iᴮ or Iᴮ Iᴼ | B |
| Iᴬ Iᴮ | AB |
| Iᴼ Iᴼ | O |
基因型IᴬIᴬ或IᴬIᴼ为A型；IᴮIᴮ或IᴮIᴼ为B型；IᴬIᴮ为AB型；IᴼIᴼ为O型。
4. Sex Determination and Sex Linkage | 性别决定与性连锁
In humans, sex is determined by the X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Genes located on the X chromosome but absent from the Y chromosome are described as sex-linked. Recessive sex-linked alleles are expressed more frequently in males because a male has only one X chromosome and therefore cannot be a carrier.
人类的性别由X和Y染色体决定，女性有两条X染色体（XX），男性有一条X染色体和一条Y染色体（XY）。位于X染色体上而Y染色体上缺失的基因称为性连锁基因。隐性性连锁等位基因在男性中表达频率更高，因为男性只有一条X染色体，因此不可能是携带者。
For example, if a carrier female (Xᴴ Xʰ) has children with a normal male (Xᴴ Y), each son has a 50% chance of inheriting the recessive Xʰ allele and being affected. Each daughter is either normal or a carrier.
例如，一名携带者女性（Xᴴ Xʰ）与正常男性（Xᴴ Y）生育后代，每个儿子有50%的概率遗传隐性Xʰ等位基因而患病。每个女儿要么正常，要么是携带者。
5. Dihybrid Inheritance | 双基因杂交遗传
A dihybrid cross follows two genes located on different chromosomes. When two individuals heterozygous for both genes are crossed, independent assortment during meiosis leads to a 9:3:3:1 phenotypic ratio in the F2 generation. This ratio is expected only when the two genes are unlinked and there is no gene interaction.
双基因杂交追踪位于不同染色体上的两个基因。当两个对两对基因均杂合的个体杂交时，减数分裂中的自由组合使F2代产生9:3:3:1的表现型比例。只有当两个基因不连锁且不存在基因互作时，才会出现这一比例。
In Mendel’s pea plant example, round (R) is dominant to wrinkled (r), and yellow (Y) is dominant to green (y). A dihybrid cross between RrYy individuals gives 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.
在孟德尔的豌豆实验中，圆形（R）对皱形（r）显性，黄色（Y）对绿色（y）显性。RrYy个体之间的双基因杂交产生9圆形黄色:3圆形绿色:3皱形黄色:1皱形绿色。
6. Test Cross | 测交
A test cross is used to determine the unknown genotype of an individual showing a dominant phenotype. The individual is crossed with a homozygous recessive individual. If the unknown individual is homozygous dominant, all offspring show the dominant phenotype. If it is heterozygous, the offspring show a 1:1 ratio of dominant to recessive phenotypes.
测交用于确定具有显性表现型但基因型未知的个体的基因型。将该个体与纯合隐性个体杂交。如果未知个体是纯合显性，所有子代都表现显性性状；如果是杂合子，子代将出现1:1的显性与隐性表现型比例。
For example, a tall pea plant could be TT or Tt. Crossing it with a dwarf plant (tt) will reveal its genotype: all tall offspring indicate TT, while a 1:1 ratio of tall to dwarf indicates Tt.
例如，一株高茎豌豆可能是TT或Tt。将其与矮茎植株（tt）杂交即可揭示其基因型：若子代全部为高茎，说明亲本为TT；若高茎与矮茎比例为1:1，说明亲本为Tt。
7. Gene Interaction and Epistasis | 基因互作与上位效应
Epistasis occurs when the allele of one gene masks or modifies the expression of another gene. In recessive epistasis, two recessive alleles at one locus prevent the expression of a second locus, often changing the expected 9:3:3:1 dihybrid ratio to 9:3:4. In dominant epistasis, a dominant allele at one locus masks another locus, giving a 12:3:1 ratio.
上位效应指一个基因的等位基因掩盖或改变另一个基因的表达。在隐性上位中，一个基因座的两个隐性等位基因阻止另一个基因座的表达，常将预期的9:3:3:1双基因比例转变为9:3:4。在显性上位中，一个基因座的显性等位基因掩盖另一个基因座，产生12:3:1的比例。
These ratios arise because the two genes still assort independently, but the biochemical pathway or pigment production controlled by the genes alters the final phenotype. Recognising modified dihybrid ratios is an important exam skill.
产生这些比例的原因是两个基因仍然独立分配，但由基因控制的生化途径或色素合成改变了最终表现型。识别修饰后的双基因比例是一项重要的考试技能。
8. Chi-squared Test | 卡方检验
The chi-squared test is used to determine whether observed phenotypic ratios differ significantly from expected ratios. The formula is:
卡方检验用于判断观察到的表现型比例是否与预期比例存在显著差异。公式为：
χ² = Σ (O – E)² ÷ E
In this formula, O is the observed number and E is the expected number in each class. The degrees of freedom are calculated as the number of phenotypic classes minus one. The calculated χ² value is compared with the critical value at p = 0.05. If the calculated value is smaller than the critical value, the null hypothesis is accepted.
式中O为每一类别的观察数，E为期望数。自由度等于表现型类别数减一。将计算得到的χ²值与p=0.05的临界值比较。如果计算值小于临界值，则接受零假设，说明观察值与预期值之间没有显著差异。
9. Mutations and Sources of Inherited Variation | 突变与遗传变异来源
Inherited variation arises from mutation, meiosis and random fertilisation. A gene mutation changes the nucleotide sequence in DNA and can create a new allele. Chromosome mutations affect whole sections of chromosomes through deletion, duplication, inversion or translocation. Only mutations that occur in gametes can be passed on to offspring.
遗传变异来源于突变、减数分裂和随机受精。基因突变改变DNA中的核苷酸序列，并可产生新的等位基因。染色体突变通过缺失、重复、倒位或易位影响整段染色体。只有发生在配子中的突变才能传递给后代。
Sickle cell anaemia is caused by a point mutation in the gene for the beta-globin chain of haemoglobin. This changes one DNA base, leading to the substitution of one amino acid, which alters the shape of red blood cells under low oxygen conditions.
镰刀细胞贫血由血红蛋白β珠蛋白链基因的点突变引起。该突变改变一个DNA碱基，导致一个氨基酸被替换，从而在低氧条件下改变红细胞的形状。
10. Human Genetic Disorders | 人类遗传病
Several well-known human disorders illustrate Mendelian inheritance. Cystic fibrosis is caused by a recessive allele of the CFTR gene; carriers do not show symptoms. Huntington’s disease is caused by a dominant allele, so an affected person normally has one copy of the mutation and there is a 50% chance of passing it to each child.
一些已知的人类遗传病可以说明孟德尔遗传规律。囊性纤维化由CFTR基因的隐性等位基因引起，携带者没有症状。亨廷顿舞蹈症由显性等位基因引起，因此患者通常只有一个突变拷贝，并且每个子女有50%的概率遗传该突变。
In sickle cell anaemia, the heterozygous state produces sickle cell trait, in which some normal and some sickle haemoglobin is made. In areas where malaria is common, heterozygotes have a selective advantage because the trait provides some protection against severe malaria.
在镰刀细胞贫血中，杂合状态产生镰刀细胞性状，体内同时产生部分正常血红蛋白和部分镰刀血红蛋白。在疟疾高发地区，杂合子具有选择优势，因为该性状能在一定程度上防止重症疟疾。
11. Pedigree Analysis | 系谱分析
A pedigree chart shows the inheritance of a trait through several generations. Squares usually represent males and circles represent females; shaded symbols show affected individuals. Recessive disorders often skip generations and can appear in the children of unaffected carriers, while dominant disorders usually appear in every generation.
系谱图显示某一性状在多个世代间的遗传方式。通常方形代表男性，圆形代表女性，涂黑符号表示患病个体。隐性遗传病常隔代出现，并可在无病携带者的后代中出现；显性遗传病通常每一代都有患者。
When analysing a pedigree, first look for whether affected parents can have unaffected children. In a dominant condition, two unaffected parents cannot have an affected child. In a recessive condition, two affected parents will have only affected children unless a new mutation occurs.
分析系谱时，首先要看患病父母能否生育正常子女。在显性遗传病中，两个正常父母不可能生出患病子女。在隐性遗传病中，两个患病父母通常只会生育患病子女，除非发生新的突变。
12. Exam Tips for Inherited Change | 备考要点
Always define genetic terms precisely, set out crosses using conventional symbols, and state phenotypic ratios clearly. When using the chi-squared test, quote your null hypothesis, show the calculation of expected numbers, and compare the calculated value with the critical value at p = 0.05.
始终准确解释遗传术语，使用常规符号写出杂交过程，并清楚写出表现型比例。使用卡方检验时，要写出零假设，展示期望数的计算过程，并将计算值与p=0.05的临界值比较。
Do not confuse codominance with incomplete dominance. In codominance both alleles are expressed fully, whereas in incomplete dominance the heterozygote shows an intermediate phenotype. Pay attention to whether genes are linked or unlinked, because this changes the expected gamete ratios and phenotypic ratios.
不要将共显性与不完全显性混淆。共显性中两个等位基因都完全表达，而不完全显性中杂合子表现为中间表现型。注意基因是否连锁，因为连锁会改变预期的配子比例和表现型比例。
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