Interactions between Loci | 基因座位间的相互作用

📚 Interactions between Loci | 基因座位间的相互作用

In Mendelian genetics, a single locus is often assumed to control a single phenotype. However, many inherited traits depend on interactions between two or more loci. This article explains how alleles at different gene loci can mask, modify, or add to one another, and how these interactions alter the expected 9:3:3:1 dihybrid ratio.

在孟德尔遗传学中，通常假设一个基因座位控制一种表型。但许多遗传性状取决于两个或多个基因座位之间的相互作用。本文解释不同基因座位上的等位基因如何相互掩盖、修饰或叠加，以及这些相互作用如何改变预期的 9:3:3:1 二杂合比率。

1. What Are Loci and Gene Interactions? | 什么是基因座位与基因互作？

A locus is a fixed position on a chromosome where a particular gene is located. When the alleles at one locus affect the phenotypic expression of alleles at another locus, the loci are said to interact. Gene interaction is common in metabolic pathways, where the product of one gene is required before a second gene can produce its effect.

基因座位是染色体上特定基因所在的固定位置。当一个基因座位上的等位基因影响另一个基因座位上等位基因的表型表达时，这两个座位就发生了互作。基因互作在代谢途径中很常见，因为一个基因的产物往往需要先被合成，另一个基因才能发挥作用。

In a standard dihybrid cross involving two unlinked loci, independent assortment predicts a 9:3:3:1 phenotypic ratio. If this ratio is altered, it is strong evidence that the two loci interact. Recognising the modified ratios allows geneticists to infer the type of interaction between the loci.

在涉及两个不连锁座位的标准二杂合杂交中，自由组合定律预测表型比率为 9:3:3:1。如果这个比率发生改变，就强烈表明两个座位发生了互作。识别修饰后的比率可以让遗传学家推断基因座位之间互作的类型。

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2. The Dihybrid Cross Baseline: 9:3:3:1 | 二杂合杂交基准：9:3:3:1

Consider a cross between two heterozygous individuals at two unlinked loci, for example A and B. If the alleles assort independently and neither locus influences the other, the F2 generation shows four phenotypic classes in a 9:3:3:1 ratio.

考虑两个不连锁基因座位上都为杂合子的个体杂交，例如 A 和 B。如果等位基因自由组合，且两个座位互不影响，则 F2 代会出现四种表型，比率为 9:3:3:1。

Expected ratio: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb

The underscore indicates that either the dominant or recessive allele can be present at that locus. This 9:3:3:1 ratio is the starting point for identifying gene interactions, because deviations from it indicate that the two loci are not acting independently.

下划线表示该座位上可以是显性等位基因或隐性等位基因。9:3:3:1 比率是识别基因互作的起点，因为偏离这个比率就表明两个座位并不是独立起作用。

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3. Epistasis: A Locus That Hides Another | 上位性：一个座位掩盖另一个座位

Epistasis is a form of gene interaction in which one locus masks or suppresses the phenotypic effect of another locus. The masking locus is called epistatic, and the hidden locus is called hypostatic. Epistasis should not be confused with dominance, which occurs between alleles at the same locus.

上位性是一种基因互作，其中一个基因座位掩盖或抑制另一个基因座位的表型效应。起掩盖作用的座位称为上位座位，被掩盖的座位称为下位座位。上位性不应与显性混淆，显性发生在同一个基因座位的等位基因之间。

Epistasis alters the 9:3:3:1 ratio by merging two or more phenotypic classes. The exact modified ratio depends on whether the epistatic allele is dominant or recessive, and how the interacting genes control the phenotype. The most common types are recessive epistasis, dominant epistasis, complementary interaction and duplicate interaction.

上位性通过合并两个或多个表型类别来改变 9:3:3:1 比率。具体修饰后的比率取决于上位等位基因是显性还是隐性，以及互作基因如何控制表型。最常见的类型包括隐性上位、显性上位、互补互作和重复互作。

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4. Recessive Epistasis: The 9:3:4 Ratio | 隐性上位：9:3:4 比率

Recessive epistasis occurs when two recessive alleles at one locus mask the expression of a second locus. The classic example is coat colour in mice. At one locus, B produces black pigment and b produces brown pigment. At a second locus, C allows pigment deposition and cc blocks all pigment, producing an albino phenotype regardless of the B locus.

隐性上位是指一个座位上的两个隐性等位基因掩盖了第二个座位的表达。经典例子是小鼠的毛色。在一个座位上，B 产生黑色素，b 产生棕色毛；在另一个座位上，C 允许色素沉积，而 cc 阻断所有色素，无论 B 座位如何都产生白化表型。

In an F2 dihybrid cross, the phenotypic classes become 9 black : 3 brown : 4 albino. The 4 albino class combines the 3 aaB_ and 1 aabb classes from the original 9:3:3:1 ratio, because the homozygous recessive cc masks both black and brown pigmentation.

在 F2 二杂合杂交中，表型类别变为 9 黑色 : 3 棕色 : 4 白化。4 白化类别合并了原始 9:3:3:1 比率中的 3 aaB_ 和 1 aabb 类别，因为纯合隐性 cc 同时掩盖了黑色和棕色色素。

Modification: 9 A_B_ : 3 A_bb : 4 aa__

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5. Dominant Epistasis: The 12:3:1 Ratio | 显性上位：12:3:1 比率

Dominant epistasis occurs when a single dominant allele at one locus masks the expression of another locus. In summer squash fruit colour, the dominant allele W at one locus produces a white fruit, hiding the effects of a second locus where Y produces yellow and yy produces green fruit.

显性上位是指一个座位上的单个显性等位基因掩盖另一个座位的表达。在夏南瓜果实颜色中，一个座位上的显性等位基因 W 产生白色果实，掩盖了第二个座位的作用，该座位上 Y 产生黄色，yy 产生绿色果实。

Thus any genotype with at least one W allele is white. Only when the genotype is ww can the second locus express itself, giving yellow or green fruits. In the F2 generation, the ratio becomes 12 white : 3 yellow : 1 green, because the 9 A_B_ and 3 A_bb classes are both masked by the dominant W allele.

因此任何含有至少一个 W 等位基因的基因型都是白色。只有当基因型为 ww 时，第二个座位才能表达，产生黄色或绿色果实。在 F2 代中，比率变为 12 白色 : 3 黄色 : 1 绿色，因为 9 A_B_ 和 3 A_bb 两个类别都被显性 W 等位基因掩盖。

Modification: 12 A___ : 3 aaB_ : 1 aabb

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6. Complementary Gene Action: The 9:7 Ratio | 互补基因作用：9:7 比率

Complementary gene interaction occurs when two loci must both carry at least one dominant allele to produce a particular phenotype. If either locus is homozygous recessive, the same recessive phenotype is produced. This often occurs when two enzymes are needed in the same biochemical pathway.

互补基因互作是指两个基因座位都必须至少含有一个显性等位基因才能产生特定表型。如果任何一个座位为纯合隐性，则会产生相同的隐性表型。当同一条生化途径需要两种酶时，常出现这种互作。

In sweet pea flower colour, two genes C and P must both be dominant for purple pigment to form. If either cc or pp is present, no pigment is produced and the flower is white. The F2 dihybrid ratio becomes 9 purple : 7 white, formed by combining the 3 A_bb, 3 aaB_ and 1 aabb classes into one non-purple class.

在香豌豆花色中，C 和 P 两个基因都必须为显性才能形成紫色色素。如果存在 cc 或 pp，则不产生色素，花为白色。F2 二杂合比率变为 9 紫色 : 7 白色，这是将 3 A_bb、3 aaB_ 和 1 aabb 类别合并为一个非紫色类别。

Modification: 9 A_B_ : 7 aa__ or __bb

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7. Duplicate Gene Action: The 15:1 Ratio | 重复基因作用：15:1 比率

Duplicate gene action occurs when two loci produce the same effect, and only one dominant allele at either locus is sufficient to give the dominant phenotype. The recessive phenotype appears only when both loci are homozygous recessive. This gives a 15:1 F2 ratio.

重复基因作用是指两个座位产生相同效应，任一座位上只要有一个显性等位基因就足以产生显性表型。只有当两个座位都是纯合隐性时，隐性表型才会出现。这导致 F2 比率为 15:1。

An example is the shape of shepherd’s purse seed capsules, where two genes A and B each contribute to the triangular shape. Only the double recessive aabb produces the alternate narrow shape. In the F2 generation, the 15 triangular : 1 narrow ratio arises because the 9 A_B_, 3 A_bb and 3 aaB_ classes all have the dominant phenotype.

一个例子是荠菜种子蒴果的形状，其中 A 和 B 两个基因各自都促进三角形果形。只有双隐性 aabb 产生另一种窄形蒴果。在 F2 代中，15 三角形 : 1 窄形的比率出现，是因为 9 A_B_、3 A_bb 和 3 aaB_ 类别都具有显性表型。

Modification: 15 A___ or __B_ : 1 aabb

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8. Modifier Genes, Penetrance and Expressivity | 修饰基因、外显率与表现度

Not all locus interactions change the standard dihybrid ratio in a simple way. A modifier gene at one locus may enhance or reduce the expression of another locus without completely masking it. This can lead to intermediate phenotypes and a wider range of variation.

并非所有座位互作都会以简单方式改变标准二杂合比率。一个座位上的修饰基因可以增强或减弱另一个座位的表达，但不完全掩盖它。这可能导致中间表型和更广泛的变异范围。

Penetrance is the proportion of individuals with a particular genotype that actually show the expected phenotype. If a modifier locus affects penetrance, some individuals carrying a disease-causing allele may not express the condition. Expressivity refers to the degree or intensity of the phenotype in individuals who do express it.

外显率是指具有特定基因型的个体中真正表现出预期表型的比例。如果修饰座位影响外显率，一些携带致病等位基因的个体可能不会表现出该病症。表现度则指已表达该表型的个体中表型的程度或强度。

These concepts are important in human genetics, where many traits are influenced by multiple loci and environmental factors. Exam questions may ask you to distinguish between epistasis and incomplete penetrance, or to explain why a single gene mutation may not always produce the same phenotype.

这些概念在人类遗传学中很重要，因为许多性状受到多个座位和环境因素的影响。考试题目可能会要求你区分上位性和不完全外显率，或解释为什么单个基因突变并不总是产生相同的表型。

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9. Polygenic Inheritance and Additive Effects | 多基因遗传与加性效应

When several loci each contribute a small additive effect to a quantitative trait, the inheritance is described as polygenic. Unlike epistasis, where one locus can mask another, polygenic loci act additively. Each dominant allele may add a fixed quantity to the phenotype, producing continuous variation.

当多个基因座位各自对数量性状贡献一个小的加性效应时，这种遗传称为多基因遗传。与上位性中一个座位可以掩盖另一个座位不同，多基因座位是以加性方式起作用。每个显性等位基因可能为表型增加一个固定量，从而产生连续变异。

For two loci with additive effects, the F2 phenotypic ratio for a quantitative trait can be 1:4:6:4:1, representing the number of dominant alleles present, from 0 to 4. This produces a bell-shaped distribution rather than discrete classes, and helps explain traits such as height and skin colour in humans.

对于两个具有加性效应的座位，数量性状的 F2 表型比率可以是 1:4:6:4:1，表示所含显性等位基因数目从 0 到 4。这会产生钟形分布而不是离散类别，有助于解释人类身高和肤色等性状。

Additive ratio: 1 : 4 : 6 : 4 : 1 (0-4 dominant alleles)

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10. Worked Example: Diagnosing a Modified Dihybrid Ratio | 实例分析：判断修饰的二杂合比率

Suppose a cross between two heterozygous plants produces 90 purple flowers and 70 white flowers in the F2 generation. The total is 160 offspring, and the ratio is 90:70, which approximates to 9:7. This indicates complementary gene interaction, because two dominant genes are required for purple pigment.

假设两个杂合植株杂交，F2 代产生 90 株紫花和 70 株白花。总共有 160 个后代，比率约为 90:70，接近 9:7。这表明发生了互补基因互作，因为紫色色素需要两个显性基因共同存在。

To test the ratio, divide each number by the total and compare with the expected proportion. For a 9:7 ratio, expected purple = 160 × 9/16 = 90, and expected white = 160 × 7/16 = 70. The match confirms that the two loci interact in a complementary manner, and that both genes act in the same pathway.

为了检验比率，可以将每个数值除以总数并与预期比例比较。对于 9:7 比率，预期紫色 = 160 × 9/16 = 90，预期白色 = 160 × 7/16 = 70。数据吻合，确认两个座位以互补方式互作，并且两个基因在同一途径中起作用。

Interaction type | 互作类型	Modified ratio | 修饰比率	Explanation | 解释
Recessive epistasis | 隐性上位	9:3:4	Homozygous recessive at one locus masks the second locus | 一个座位纯合隐性掩盖第二个座位
Dominant epistasis | 显性上位	12:3:1	One dominant allele masks the second locus | 一个显性等位基因掩盖第二个座位
Complementary | 互补	9:7	Both dominant alleles needed | 两个显性等位基因缺一不可
Duplicate | 重复	15:1	Either dominant allele gives the same phenotype | 任一显性等位基因产生相同表型

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11. Summary and Exam Tips | 总结与考试提示

Interactions between loci are detected when F2 dihybrid ratios differ from the expected 9:3:3:1. The four main modified ratios are 9:3:4 for recessive epistasis, 12:3:1 for dominant epistasis, 9:7 for complementary interaction, and 15:1 for duplicate interaction.

当 F2 二杂合比率偏离预期的 9:3:3:1 时，就可以检测到基因座位之间的互作。四种主要的修饰比率分别是：隐性上位 9:3:4、显性上位 12:3:1、互补互作 9:7 和重复互作 15:1。

In exam answers, always state the expected 9:3:3:1 ratio first, then show how the observed ratio is a modification of it. Link the ratio to the biological explanation, such as a blocked enzyme in a metabolic pathway, rather than just memorising the numbers. Use Punnett squares or probability calculations where the question asks for a full genetic diagram.

在考试答题时，应先写出预期的 9:3:3:1 比率，然后说明观察到的比率是如何由它修饰而来的。要把比率与生物学解释联系起来，例如代谢途径中某个酶被阻断，而不要只记住数字。如果题目要求完整的遗传图解，应使用庞纳特方格或概率计算。

Finally, remember that gene interaction does not mean the genes are physically linked. The loci may be on different chromosomes, but their products interact in the same biochemical pathway or developmental process. Understanding this distinction is often tested in Cambridge A-Level Biology.

最后，请记住基因互作并不意味着基因在物理上连锁。这些座位可能位于不同染色体上，但它们的产物在同一条生化途径或发育过程中相互作用。理解这一区别是剑桥 A-Level 生物常考的内容。

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