Autosomal Linkage: Key Features | 常染色体连锁遗传的特点

📚 Autosomal Linkage: Key Features | 常染色体连锁遗传的特点

Autosomal linkage occurs when two or more genes are located on the same autosome, a non-sex chromosome. During meiosis, these genes remain physically connected on the same chromosome and therefore tend to be inherited together, rather than following Mendel’s law of independent assortment.

常染色体连锁是指两个或多个基因位于同一条常染色体（即非性染色体）上的现象。减数分裂时，这些基因由于位于同一条染色体上而联合传递，因此通常一起遗传，而不遵循孟德尔独立分配定律。

1. Definition of Autosomal Linkage | 常染色体连锁的定义

Any gene found on an autosome is said to be autosomal. When several genes are on the same autosome, they form a linkage group and are described as linked genes. In a diploid cell, the number of autosomal linkage groups equals the number of pairs of autosomes in that species.

位于常染色体上的任何基因都称为常染色体基因。当多个基因位于同一条常染色体上时，它们组成一个连锁群，称为连锁基因。在二倍体细胞中，常染色体连锁群的数目等于该物种常染色体对数。

Linked genes do not show the independent assortment expected from a dihybrid cross; instead, they are inherited as a unit unless crossing over separates them.

连锁基因不会表现出双因子杂交中预期的独立分配，而是作为一个整体遗传，除非交叉互换将它们分开。

2. Linkage vs Independent Assortment | 连锁与独立分配的比较

If two genes are on different chromosomes, a double heterozygote AaBb produces four gametes AB, Ab, aB and ab in equal proportions. When the genes are linked on the same autosome, the parental gametes are produced more often than the recombinant gametes.

如果两对基因位于不同染色体上，双杂合子AaBb会产生AB、Ab、aB和ab四种配子，比例相等。当基因连锁在同一条常染色体上时，亲本型配子产生的频率高于重组型配子。

3. Complete Linkage and No Crossing Over | 完全连锁与无交叉互换

When linked genes are extremely close together on the same autosome, crossing over may not occur between them. This is called complete linkage. A double heterozygote with genotype AB/ab then produces only two types of gametes, AB and ab, in a 1 : 1 ratio.

当连锁基因在同一条常染色体上距离非常近时，它们之间可能不发生交叉互换，这称为完全连锁。基因型为AB/ab的双杂合子此时只产生两种配子：AB和ab，比例为1 : 1。

AB/ab diploid → gametes AB : ab = 1 : 1

Complete linkage is uncommon in natural populations. Most linked gene pairs show at least some crossing over, giving rise to incomplete linkage.

完全连锁在自然群体中很少见。大多数连锁基因对都会发生一定程度的交叉互换，从而表现为不完全连锁。

4. Incomplete Linkage and Crossing Over | 不完全连锁与交叉互换

During prophase I of meiosis, non-sister chromatids of homologous chromosomes exchange corresponding segments in a process called crossing over. This produces recombinant gametes with new combinations of alleles. In incomplete linkage, the frequency of recombinant gametes is less than 50%.

在减数分裂前期I，同源染色体的非姐妹染色单体之间交换对应片段，这一过程称为交叉互换。这会产生具有新等位基因组合的重组型配子。在不完全连锁中，重组型配子的频率总是小于50%。

For a double heterozygote of the form AB/ab, the four gametes are produced in the order AB, ab (parental types) > Ab, aB (recombinant types).

以AB/ab型双杂合子为例，四种配子的产生频率为：AB、ab（亲本型）多于Ab、aB（重组型）。

5. Recombinant Frequency and Gene Distance | 重组频率与基因距离

The recombination frequency is the proportion of recombinant offspring produced in a test cross, calculated using the formula below. It is usually expressed as a percentage, and 1% recombination is equal to 1 centimorgan (cM) on a genetic map.

重组频率是测交后代中重组型子代所占的比例，可用下面的公式计算。它通常以百分比表示，1%的重组率等于遗传图谱上的1厘摩（cM）。

Recombination frequency = (recombinants / total offspring) × 100%

重组频率 = （重组型子代数 / 子代总数） × 100%

The greater the physical distance between two genes on a chromosome, the greater the chance of crossing over, and hence the higher the recombination frequency.

两个基因在染色体上的物理距离越大，发生交叉互换的机会越大，因此重组频率越高。

6. Test Cross for Linked Genes | 连锁基因的测交

A test cross is made by crossing a double heterozygote with a homozygous recessive individual. If the genes are independently assorting, four phenotypes appear in a 1 : 1 : 1 : 1 ratio. If autosomal linkage is present, parental phenotypes are much more common than recombinant phenotypes.

测交是将双杂合子与隐性纯合子杂交。如果基因独立分配，四种表型比例为1 : 1 : 1 : 1。如果存在常染色体连锁，则亲本表型明显多于重组表型。

Example: In a plant, T = tall, t = short; R = round seed, r = wrinkled

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