Inheriting Genes | 基因的遗传

📚 Inheriting Genes | 基因的遗传

Inheritance is the process by which genetic information is passed from parents to offspring. In eukaryotic organisms, genes are located on chromosomes and are transmitted through sexual reproduction, where gametes carry half the genetic material. Understanding the principles of inheriting genes is fundamental to explaining phenotypic variation, predicting genetic disorders, and applying concepts such as dominance, recessiveness, and sex linkage.

遗传是亲本将遗传信息传递给后代的过程。在真核生物中，基因位于染色体上，并通过有性生殖传递，配子携带一半的遗传物质。理解基因遗传的原理是解释表型变异、预测遗传疾病以及应用显性、隐性和性连锁等概念的基础。

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1. Mendelian Principles | 孟德尔遗传原理

Gregor Mendel established the foundational laws of inheritance through his work on pea plants. The law of segregation states that each individual possesses two alleles for a given gene, and these alleles separate during gamete formation, so that each gamete receives only one allele. This explains how offspring inherit one allele from each parent, preserving the diploid number after fertilisation.

孟德尔通过对豌豆的研究建立了遗传学的基础定律。分离定律指出，每个个体对一个特定基因拥有两个等位基因，这些等位基因在配子形成过程中彼此分离，使每个配子只获得其中一个等位基因。这解释了后代如何从每个亲本继承一个等位基因，从而在受精后保持二倍体数目。

The law of independent assortment describes how alleles of different genes are distributed into gametes independently of one another, provided the genes are located on different chromosomes or are far apart on the same chromosome. This generates genetic variation in the offspring, as novel combinations of alleles arise. Mendel's dihybrid crosses, looking at two traits simultaneously, demonstrated this principle through the classic 9:3:3:1 phenotypic ratio in the F₂ generation.

自由组合定律描述了不同基因的等位基因如何独立地分配到配子当中，前提是这些基因位于不同的染色体上，或者位于同一染色体上且相距甚远。这会在后代中产生遗传变异，因为新的等位基因组合得以出现。孟德尔同时研究两对性状的双因子杂交实验，通过F₂代中经典的9:3:3:1表型比证明了这一原理。

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2. Genetic Terminology | 遗传学术语

A clear grasp of genetic vocabulary is essential. A gene is a sequence of DNA that codes for a polypeptide or functional RNA. An allele is an alternative form of a gene that arises by mutation and occupies the same locus on homologous chromosomes. The genotype refers to the combination of alleles an organism possesses for a particular gene, while the phenotype is the set of observable characteristics resulting from the interaction of the genotype with the environment.

清晰掌握遗传学术语至关重要。基因是一段编码多肽或功能性RNA的DNA序列。等位基因是基因通过突变产生的不同形式，并占据同源染色体上的相同基因座。基因型指个体针对某一特定基因所拥有的等位基因组合，而表现型则是基因型与环境相互作用所产生的一系列可观察的特征。

An individual is described as homozygous if they carry two identical alleles for a gene, and heterozygous if the two alleles are different. Dominant alleles express themselves in the phenotype even when only one copy is present, whereas recessive alleles are only expressed when two copies are present, i.e. in the homozygous state. The terms 'true-breeding' or 'pure-breeding' refer to individuals that are homozygous and therefore produce gametes all carrying the same allele.

如果个体携带两个相同的等位基因，则称为纯合子；如果两个等位基因不同，则称为杂合子。显性等位基因即使仅存在一个拷贝也能在表现型中表现出来，而隐性等位基因只有在存在两个拷贝（即纯合状态下）才会表达。'纯种'或'纯系'指的是纯合个体，因此它们产生的配子全部携带相同的等位基因。

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3. Monohybrid Crosses and Punnett Squares | 单基因杂交与庞纳特方格

A monohybrid cross examines the inheritance of a single gene. The offspring of a cross between two homozygous parents with contrasting traits are all heterozygous and show the dominant phenotype. When these F₁ heterozygotes are interbred, the resulting F₂ generation exhibits a genotypic ratio of 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive, and a phenotypic ratio of 3 dominant : 1 recessive, provided that dominance is complete.

单基因杂交研究单个基因的遗传。两个具有相对性状的纯合亲本杂交所产生的子代全为杂合子，并表现为显性表型。当这些F₁杂合子相互交配时，所产生的F₂代基因型比为1纯合显性 : 2杂合 : 1纯合隐性，表型比为3显性 : 1隐性，前提是显性关系为完全显性。

ozygous plants (Tt × Tt) results in gametes T and t from each parent, giving offspring genotypes TT, Tt, tT, and tt. A test cross, breeding an individual of unknown genotype with a homozygous recessive, reveals the unknown genotype: if any offspring show the recessive trait, the unknown parent must be heterozygous.

庞纳特方格为预测特定杂交后代可能的基因型提供了一种直观的可视化方法。例如，两株杂合子植物杂交（Tt × Tt），每个亲本产生 T 和 t 配子，后代基因型为 TT、Tt、tT 和 tt。测交是将未知基因型的个体与隐性纯合子交配，从而揭示其基因型：如果后代中出现隐性性状，则该未知亲本必定为杂合子。

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4. Codominance and Incomplete Dominance | 共显性与不完全显性

Not all alleles obey simple dominant–recessive relationships. In codominance, both alleles in a heterozygote are fully expressed, so the phenotype shows both traits distinctly. A classic example is the human MN blood group, where alleles L^M and L^N produce different glycoproteins on the red blood cell surface; heterozygotes have both types of glycoproteins.

并非所有等位基因都遵循简单的显性-隐性关系。在共显性中，杂合子中的两个等位基因都能充分表达，其表型可同时明显地表现出两种性状。一个典型例子是人类的MN血型系统，等位基因 L^M 和 L^N 在红细胞表面产生不同的糖蛋白；杂合子同时拥有两种糖蛋白。

In contrast, incomplete dominance creates an intermediate phenotype in heterozygotes. For instance, in snapdragons (Antirrhinum), crossing red-flowered (C^RC^R) and white-flowered (C^WC^W) plants gives pink-flowered (C^RC^W) offspring. The F₂ generation from self-pollinating these pink plants gives a 1 red : 2 pink : 1 white ratio, which mirrors the genotypic ratio exactly.

相比之下，不完全显性会在杂合子中产生中间型表型。例如，在金鱼草（Antirrhinum）中，红花植株（C^RC^R）与白花植株（C^WC^W）杂交，得到粉花植株（C^RC^W）。这些粉花植株自花授粉产生的F₂代表现出1红 : 2粉 : 1白的比例，这与基因型比例完全吻合。

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5. Multiple Alleles and ABO Blood Groups | 多重等位基因与ABO血型

Many genes have more than two allelic forms in a population, although each individual still carries only two alleles. The ABO blood group system in humans is a classic case. Three main alleles exist: I^A, I^B, and i. The I^A and I^B alleles are codominant with each other, while both are dominant over the i allele. This creates four possible blood groups, determined by the combination of alleles present.

许多基因在种群中拥有两种以上的等位基因形式，尽管每个个体仍然只携带其中两个。人类ABO血型系统就是一个经典案例。存在三个主要等位基因：I^A、I^B 和 i。I^A 和 I^B 互为共显性，同时两者都对 i 等位基因呈显性。这就产生了四种可能的血型，由等位基因的组合决定。

The table below summarises the genotypes and corresponding phenotypes for the ABO system.

下表总结了ABO血型系统的基因型及相应表型。

Genotype	Antigens on RBC	Phenotype (Blood Group)
IAIA or IAi	A antigen	Group A
IBIB or IBi	B antigen	Group B
IAIB	Both A and B antigens	Group AB
ii	No A or B antigens	Group O

基因型	红细胞表面抗原	表型（血型）
IAIA 或 IAi	A 抗原	A 型
IBIB 或 IBi	B 抗原	B 型
IAIB	A 和 B 抗原	AB 型
ii	无 A 或 B 抗原	O 型

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6. Sex-linked Inheritance | 性连锁遗传

In mammals and many other organisms, sex is determined by sex chromosomes: females typically have two X chromosomes (XX) while males have one X and one Y (XY). Genes located on the X chromosome but absent from the Y are said to be X-linked. Because males have only one X chromosome, they cannot be heterozygous for X-linked genes; they are hemizygous. As a result, recessive X-linked traits are expressed more frequently in males.

在哺乳动物和许多其他生物中，性别由性染色体决定：雌性通常拥有两条X染色体（XX），而雄性拥有一条X和一条Y染色体（XY）。位于X染色体上但Y染色体上不存在的基因称为X连锁基因。由于雄性只有一条X染色体，他们对X连锁基因不可能成为杂合子，而是半合子。因此，隐性X连锁性状在雄性中表达得更加频繁。

Red-green colour blindness is a common X-linked recessive disorder. The allele for normal colour vision (X^B) is dominant over the allele for colour blindness (X^b). A carrier female (X^BX^b) has normal vision but can pass the recessive allele to her children. If she has children with a colour-blind male (X^bY), the sons have a 50% chance of being colour blind, and the daughters have a 50% chance of being carriers or colour blind depending on the allele inherited from the father. Similar patterns are observed with haemophilia, another X-linked recessive condition where a faulty clotting factor is produced.

红绿色盲是一种常见的X连锁隐性遗传病。正常色觉的等位基因（X^B）对色盲等位基因（X^b）为显性。携带者女性（X^BX^b）具有正常色觉，但能将隐性等位基因传递给子女。如果她与一个色盲男性（X^bY）生育后代，儿子有50%的概率为色盲，女儿则有50%的概率为携带者或色盲，具体取决于从父亲那里继承的等位基因。血友病也遵循类似的遗传模式，这是另一种X连锁隐性遗传病，患者体内产生有缺陷的凝血因子。

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7. Pedigree Analysis | 系谱分析

Pedigree charts trace the inheritance of a trait through generations within a family. By examining the pattern of affected and unaffected individuals, one can deduce the mode of inheritance. Key principles include: if the trait appears in every generation, it is likely dominant; if it skips generations, it is likely recessive. For an autosomal recessive trait, affected individuals can be born to unaffected carrier parents, and males and females are affected in equal proportions.

系谱图通过在一个家族的世代中追踪某一性状的遗传情况。通过分析患病和未患病个体的分布模式，可以推断其遗传方式。关键原则包括：如果某一性状在每一代都出现，则很可能是显性遗传；如果跳跃式出现，则很可能是隐性遗传。对于常染色体隐性遗传，患病个体可由未患病的携带者父母生出，且男性和女性患病的比例相等。

X-linked recessive pedigrees show distinctive features: an affected male does not pass the trait to his sons (since he gives them his Y chromosome), but all his daughters become carriers. More males than females are affected. An affected female must have an affected father and a carrier or affected mother, which is rare. These clues allow genetic counsellors to assess risks for future children.

X连锁隐性遗传的系谱具有一些典型特征：患病男性不会将性状传给儿子（因为他将Y染色体传给儿子），但他的所有女儿都将成为携带者。患者中男性多于女性。患病女性的父亲必定患病，母亲至少是携带者，这种情况十分罕见。这些线索使遗传咨询师能够评估未来子女的患病风险。

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8. Gene Interactions and Epistasis | 基因互作与上位性

Often, the expression of one gene is influenced by another. Epistasis is a form of gene interaction where the alleles of one gene mask the expression of alleles at another locus. The gene that masks is termed epistatic, and the one being masked is hypostatic. Epistatic interactions can alter the classic Mendelian ratios in dihybrid crosses, producing ratios such as 9:3:4 or 12:3:1 instead of 9:3:3:1.

通常，一个基因的表达会受到另一个基因的影响。上位性是基因互作的一种形式，指一个基因的等位基因掩盖了另一基因座上等位基因的表达。起掩盖作用的基因称为上位基因，被掩盖的称为下位基因。上位性相互作用会改变双因子杂交中经典的孟德尔比例，产生如9:3:4或12:3:1而非9:3:3:1的比例。

A well-known example is the coat colour of Labrador retrievers. One gene determines pigment type: allele B (black) is dominant to b (brown). Another gene, with alleles E and e, determines whether pigment is deposited in the hair. The recessive e allele is epistatic: dogs homozygous recessive (ee) have yellow coats regardless of their B or b alleles, because no pigment is deposited. A cross between BbEe dogs thus yields a 9 black :

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