📚 Autosomal Linkage | 常染色体连锁
Autosomal linkage is a core A-Level genetics topic. It explains why some alleles do not follow Mendel’s law of independent assortment and how crossing over generates new allele combinations.
常染色体连锁是 A-Level 遗传学的核心专题。它解释了为什么一些等位基因不遵循孟德尔的自由组合定律，以及交叉互换如何产生新的等位基因组合。
1. Definition of Autosomal Linkage | 常染色体连锁的定义
In diploid organisms, autosomes are chromosomes that are not involved in sex determination. Humans have 22 pairs of autosomes. If two gene loci are located on the same autosome, they are described as autosomal linked genes.
在二倍体生物中，常染色体是不参与性别决定的染色体。人类有 22 对常染色体。如果两个基因座位位于同一条常染色体上，它们就被称为常染色体连锁基因。
The physical proximity of these loci means that their alleles tend to be inherited together. The combination of alleles on one chromosome is called a linkage group.
这些基因座在物理上彼此靠近，意味着它们的等位基因倾向于一起遗传。位于一条染色体上的等位基因组合称为一个连锁群。
2. Independent Assortment vs Linkage | 自由组合与连锁的对比
Mendel’s second law states that alleles of different genes assort independently during gamete formation. This is true for genes on different chromosomes. For such unlinked genes, a heterozygous AaBb individual produces four gamete types AB, Ab, aB and ab in equal proportions.
孟德尔第二定律指出，不同基因的等位基因在配子形成过程中自由组合。这对于位于不同染色体上的基因是成立的。对于这类不连锁的基因，杂合子 AaBb 会产生四种配子 AB、Ab、aB 和 ab，比例相等。
However, if genes A and B are on the same autosome, the alleles on each homologue stay together unless crossing over occurs. This departure from independent assortment is the basis of autosomal linkage.
然而，如果基因 A 和 B 位于同一条常染色体上，每条同源染色体上的等位基因会保持在一起，除非发生交叉互换。这种对自由组合的偏离就是常染色体连锁的基础。
| Arrangement | Test cross ratio |
|---|---|
| Unlinked genes | 1 : 1 : 1 : 1 |
| Complete linkage | 1 : 1 parental only |
| Linked with crossing over | Parental types > recombinants |
The table summarises how the arrangement of genes affects the offspring ratios in a test cross. Notice that linkage always increases the proportion of parental type offspring.
该表总结了基因排列方式如何影响测交后代的比例。注意连锁总是增加亲本型后代的比例。
3. Notation for Linked Genes | 连锁基因的书写方式
When writing genotypes for linked genes, the slash notation shows which alleles are on the same chromosome. AB/ab indicates a cis (coupling) heterozygote; Ab/aB indicates a trans (repulsion) heterozygote.
书写连锁基因的基因型时，斜线表示法表明哪些等位基因位于同一条染色体上。AB/ab 表示顺式（相引）杂合子；Ab/aB 表示反式（相斥）杂合子。
This notation is important because it distinguishes between the two possible double heterozygotes. In a test cross, the cis and trans arrangements give the same phenotypic ratio only if no recombination occurs, but the parental classes are reversed.
这种表示法很重要，因为它区分了两种可能的双杂合子。在测交中，如果不发生重组，顺式和反式排列给出的表型比例相同，但亲本类型互换了。
4. Complete Linkage | 完全连锁
Complete linkage is rare. It occurs when two loci are so close together that crossing over between them is effectively zero. A cis heterozygote AB/ab forms only AB and ab gametes in equal numbers.
完全连锁很少见。当两个基因座非常接近，以至于它们之间的交叉互换实际上为零时才会发生。顺式杂合子 AB/ab 只产生数量相等的 AB 和 ab 配子。
When this individual is test-crossed with ab/ab, the offspring show a 1 : 1 ratio of the two parental phenotypes. No recombinant phenotypes appear.
当这个个体与 ab/ab 测交时，后代表现出两种亲本表型的 1 : 1 比例。不会出现重组表型。
5. Crossing Over and Recombination | 交叉与重组
During prophase I, homologous chromosomes synapse and form bivalents. Non-sister chromatids may break and rejoin at chiasmata, exchanging corresponding segments. If the chiasma forms between the two linked loci, recombinant chromatids are produced.
在减数第一次分裂前期，同源染色体联会并形成二价体。非姐妹染色单体可能在交叉点发生断裂和重接，交换相应片段。如果交叉形成在两个连锁基因座之间，就会产生重组染色单体。
The frequency of crossing over depends on the physical distance between loci. Greater distance means more opportunity for a chiasma to form between them.
交叉互换的频率取决于基因座之间的物理距离。距离越大，它们之间形成交叉的机会越多。
6. Recombinant Frequency and Map Distance | 重组频率与图距
Recombinant frequency is a measure of how often new allele combinations arise. It is estimated from test cross data using the equation below.
重组频率是衡量新等位基因组合出现频率的指标。它通过下面的公式由测交数据估算。
Recombinant frequency = (number of recombinant offspring ÷ total offspring) × 100%
A recombinant frequency of 1% corresponds to one map unit or one centimorgan. This allows geneticists to construct linkage maps showing the relative positions of genes along a chromosome.
1% 的重组频率对应一个图距单位或一个厘摩。这使遗传学家能够构建连锁图谱，显示基因在染色体上的相对位置。
7. Why 50% Is the Maximum | 为什么上限是 50%
If two genes are far apart on the same chromosome, many meioses will have at least one crossover between them. Multiple crossovers can also occur. The net result is that the four gamete types approach equal numbers.
如果两个基因在同一条染色体上相距很远，许多减数分裂中它们之间至少会发生一次交叉。还可能发生多次交叉。最终结果是四种配子的数量趋于相等。
This produces a test cross ratio close to 1 : 1 : 1 : 1, which is identical to the ratio for unlinked genes. Therefore linkage can only be detected easily when the genes are relatively close, giving a recombinant frequency clearly below 50%.
这产生接近 1 : 1 : 1 : 1 的测交比例，与不连锁基因的比例完全相同。因此，只有当基因相对靠近、重组频率明显低于 50% 时，连锁才容易被检测到。
8. Detecting Linkage from Data | 从数据中检测连锁
To detect autosomal linkage, a test cross or a suitable breeding experiment is carried out. The offspring are classified into four phenotypic classes. If the two classes corresponding to parental allele combinations are significantly more frequent than the two recombinant classes, linkage is indicated.
为了检测常染色体连锁，需要进行测交或合适的杂交实验。将后代分为四种表型类别。如果对应亲本等位基因组合的两类比两类重组型显著更多，则表明存在连锁。
A chi-squared test can be used to compare observed numbers with the 1 : 1 : 1 : 1 expected ratio for independent assortment. A significant result suggests the genes are not assorting independently.
可以用卡方检验将观察值与自由组合的 1 : 1 : 1 : 1 预期比例进行比较。显著的结果表明基因没有自由组合
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