📚 Sex-linked Inheritance | 伴性遗传 考点精讲
Sex-linked inheritance refers to the pattern of inheritance for genes located on the sex chromosomes. In humans and many other organisms, sex is determined by the X and Y chromosome system, where females are XX and males are XY. Because the X chromosome is larger and carries many genes not present on the Y, traits controlled by these genes show characteristic patterns of expression that differ between sexes. Understanding sex linkage is essential for IB and OCR biology examinations, as questions frequently test students’ ability to predict offspring genotypes and phenotypes, interpret pedigree charts, and explain the underlying molecular mechanisms.
伴性遗传是指位于性染色体上的基因所决定的性状在遗传上的表现规律。在人类和许多其他生物中，性别由X和Y染色体系统决定，女性为XX，男性为XY。由于X染色体较大，携带着许多Y染色体上没有的基因，由这些基因控制的性状会表现出雌雄性别中不同的遗传模式。理解伴性遗传是IB和OCR生物考试的关键，考题常常要求学生预测子代基因型和表现型、分析系谱图，并解释背后的分子机制。
1. Sex Determination and the Sex Chromosomes | 性别决定与性染色体
In mammals, sex is determined by the presence of the SRY gene on the Y chromosome. Individuals with a Y chromosome typically develop testes and male characteristics, while those without it develop ovaries and female characteristics. The X chromosome contains over 1,000 genes, many of which are essential for both sexes, such as genes involved in blood clotting and colour vision. The Y chromosome is much smaller and carries only about 70 to 200 genes, most of which are related to male sex determination and spermatogenesis. Because males have only one X chromosome, they are hemizygous for X-linked genes, meaning any recessive allele on their single X will be expressed, as there is no corresponding allele on the Y to mask it.
在哺乳动物中，性别由Y染色体上的SRY基因决定。拥有Y染色体的个体通常会发育出睾丸和男性特征，而没有Y染色体的个体则发育出卵巢和女性特征。X染色体包含1000多个基因，其中许多对两性都至关重要，比如与凝血和色觉相关的基因。Y染色体要小得多，只携带约70到200个基因，大部分与男性性别决定和精子发生有关。由于男性只有一条X染色体，他们对X连锁基因是半合子，这意味着他们唯一的X染色体上的任何隐性等位基因都会表现出来，因为Y染色体上没有对应的等位基因来掩盖它。
2. X-linked Recessive Inheritance: Haemophilia | X连锁隐性遗传：血友病
Haemophilia is a classic example of an X-linked recessive disorder. It is caused by a mutation in the gene for factor VIII (haemophilia A) or factor IX (haemophilia B), both located on the X chromosome. The normal allele is dominant (XH), and the mutant allele is recessive (Xh). A female would need two copies of the recessive allele (XhXh) to express the disease, which is rare because an affected father must pass his Xh to all his daughters, and the mother must be at least a carrier. Males, having only one X, will have the disorder if they inherit the Xh allele (XhY). Carrier females (XHXh) usually do not show symptoms because the normal allele produces sufficient clotting factor, though some may have mildly reduced levels.
血友病是X连锁隐性遗传病的经典例子。它由位于X染色体上的凝血因子VIII（血友病A）或凝血因子IX（血友病B）基因突变引起。正常等位基因为显性（XH），突变等位基因为隐性（Xh）。女性需要两个隐性等位基因（XhXh）才会患病，这种情况很罕见，因为患病的父亲必须将他的Xh传给所有女儿，而母亲至少也得是携带者。男性只有一条X染色体，只要遗传到Xh等位基因（XhY）就会患病。女性携带者（XHXh）通常不表现症状，因为正常等位基因能产生足够的凝血因子，尽管有些携带者凝血因子水平可能轻度降低。
In a cross between a carrier female (XHXh) and a normal male (XHY), each son has a 50% chance of being affected, and each daughter has a 50% chance of being a carrier. An affected male (XhY) and a normal female (XHXH) will produce all carrier daughters and all normal sons, because the father passes his Xh only to daughters.
在携带者女性（XHXh）与正常男性（XHY）的杂交中，每个儿子有50%的概率患病，每个女儿有50%的概率成为携带者。患病男性（XhY）与正常女性（XHXH）则会产生全部为携带者的女儿和全部正常的儿子，因为父亲只将Xh传给女儿。
3. X-linked Recessive Inheritance: Red-Green Colour Blindness | X连锁隐性遗传：红绿色盲
Red-green colour blindness is another common X-linked recessive condition, affecting approximately 8% of males of Northern European descent. It results from mutations in the opsin genes on the X chromosome that code for photopigments in cone cells. The most common forms are deuteranopia (green blindness) and protanopia (red blindness). Because the trait is recessive and X-linked, it shows the same inheritance pattern as haemophilia: affected males are far more common than affected females, and the condition can skip generations through carrier females.
红绿色盲是另一种常见的X连锁隐性遗传病，影响大约8%的北欧血统男性。它由X染色体上编码视锥细胞感光色素的视蛋白基因突变引起。最常见的形式是绿色盲（deuteranopia）和红色盲（protanopia）。由于该性状是隐性且X连锁的，它表现出与血友病相同的遗传模式：患病男性远多于患病女性，且该病可以通过女性携带者隔代遗传。
If a colour-blind male (XcY) has children with a homozygous normal female (XCXC), all sons will be normal (XCY) and all daughters will be carriers (XCXc). If a carrier female (XCXc) mates with a normal male (XCY), 50% of sons will be colour-blind and 50% of daughters will be carriers.
如果一位色盲男性（XcY）与纯合正常女性（XCXC）生育后代，所有儿子正常（XCY），所有女儿为携带者（XCXc）。如果一位携带者女性（XCXc）与正常男性（XCY）婚配，50%的儿子会色盲，50%的女儿会成为携带者。
4. X-linked Dominant Inheritance | X连锁显性遗传
X-linked dominant traits are less common but appear in both males and females, though often more severely in males. A single copy of the mutant allele on the X chromosome is sufficient to cause the disorder. Examples include hypophosphatemic rickets (vitamin D-resistant rickets) and Rett syndrome (usually lethal in males before birth or in early infancy). Affected males pass the trait to all their daughters but none of their sons, because sons inherit the Y chromosome from the father. Affected heterozygous females pass the trait to 50% of their offspring, regardless of sex.
X连锁显性性状较少见，但在男性和女性中都会出现，通常在男性中表现更为严重。X染色体上的一个突变等位基因拷贝就足以引起疾病。实例包括低磷酸盐血症性佝偻病（抗维生素D佝偻病）和Rett综合征（通常在男性中于出生前或婴儿早期致死）。患病男性将该性状传给他们所有的女儿而不会传给儿子，因为儿子从父亲那里继承的是Y染色体。患病的杂合女性将该性状传给50%的后代，不论性别。
In pedigrees, X-linked dominant inheritance shows no male-to-male transmission, and all daughters of an affected male are affected. When the mother is affected and heterozygous, both male and female children can be affected in each generation, resembling autosomal dominant inheritance but with the key absence of father-to-son transmission.
在系谱图中，X连锁显性遗传表现为没有男传男的现象，且患病男性的所有女儿都患病。当母亲患病且为杂合时，每一代中的男性和女性后代都可能患病，类似于常染色体显性遗传，但关键区别在于不存在父亲到儿子的传递。
5. Y-linked Inheritance | Y连锁遗传
Y-linked traits are passed exclusively from father to son, as only males possess a Y chromosome. The Y chromosome carries few genes, primarily those involved in male sex determination and spermatogenesis. A well-known Y-linked gene is SRY (sex-determining region Y), which triggers testis development. Other Y-linked genes can cause conditions such as Y-linked deafness or hairy ears (hypertrichosis pinnae auris), though these are rare. Y-linked inheritance is straightforward: all male descendants of an affected male will display the trait, and females are never affected or carriers.
Y连锁性状只从父亲传给儿子，因为只有男性拥有Y染色体。Y染色体携带的基因很少，主要是那些参与男性性别决定和精子发生的基因。一个著名的Y连锁基因是SRY（Y染色体性别决定区），它触发睾丸发育。其他Y连锁基因可能导致Y连锁耳聋或多毛耳（耳廓多毛症），尽管这些很罕见。Y连锁遗传模式简单明了：患病男性的所有男性后代都会表现出该性状，而女性从来不会患病也不是携带者。
6. Genetic Diagrams for Sex-linked Traits | 伴性性状的遗传图解
When constructing genetic diagrams for sex-linked crosses, it is crucial to represent the sex chromosomes with their alleles as superscripts. For example, a female carrier for haemophilia is written as XHXh, and a normal male as XHY. A Punnett square for a cross between a carrier female and a normal male will have gametes XH and Xh from the mother, and XH and Y from the father. The resulting genotypes: XHXH (normal female), XHXh (carrier female), XHY (normal male), XhY (affected male). The phenotypic ratio is 1 normal female : 1 carrier female : 1 normal male : 1 affected male. Always state the sex and phenotype separately to avoid confusion.
在绘制伴性杂交遗传图解时，用上标等位基因的形式表示性染色体至关重要。例如，血友病女性携带者写作XHXh，正常男性写作XHY。一个携带者女性与正常男性杂交的旁纳特方格，母亲产生的配子为XH和Xh，父亲产生的配子为XH和Y。子代基因型为：XHXH（正常女性），XHXh（女性携带者），XHY（正常男性），XhY（患病男性）。表现型的比例为1正常女性 : 1携带者女性 : 1正常男性 : 1患病男性。务必分别陈述性别和表现型以避免混淆。
For examination success, always include a legend defining the symbols used, draw the gametes correctly, and present the offspring genotypes and phenotypes clearly. IB and OCR mark schemes reward including the probability for each category and stating the sex alongside the condition.
为在考试中取得好成绩，务必添加图例说明所用符号，正确写出配子，并清晰列出子代基因型与表现型。IB和OCR的评分方案对给出每一类的概率、以及在说明性状的同时注明性别给予加分。
7. Pedigree Analysis of Sex-linked Disorders | 伴性遗传的家系图分析
Pedigree charts are a common tool for tracing the inheritance of traits through generations. For X-linked recessive traits, key features include: more males than females affected; affected males usually have unaffected parents (the mother is often a carrier); the trait can skip generations via carrier females; no male-to-male transmission is observed. For X-linked dominant traits, affected males pass the condition to all daughters but no sons; affected heterozygous females have a 50% chance of passing it to each child.
系谱图是追踪性状在世代中遗传的常用工具。对于X连锁隐性性状，关键特征包括：患病男性多于患病女性；患病男性的父母通常不患病（母亲往往是携带者）；该性状可以通过女性携带者隔代遗传；没有男传男的现象。对于X连锁显性性状，患病男性将疾病传给所有女儿而不传给儿子；患病的杂合女性传给每个孩子的概率为50%。
When analyzing a pedigree showing an X-linked recessive disorder, look for an affected male whose parents are unaffected – the mother must be a carrier. If two affected parents produce an unaffected son, the condition cannot be X-linked dominant. Practice with sample pedigrees and always exclude autosomal patterns before concluding sex linkage.
当分析一个显示X连锁隐性遗传病的系谱图时，寻找父母不患病但儿子患病的个体——母亲一定是携带者。如果一对患病的父母生出一个不患病的儿子，则该病不可能是X连锁显性遗传。通过样本系谱进行练习，并在确定伴性遗传之前务必排除常染色体遗传模式。
8. X-inactivation and Dosage Compensation | X染色体失活与剂量补偿
Females have two X chromosomes, while males have one. To balance the dosage of X-linked gene products between the sexes, one of the two X chromosomes in each female cell is randomly inactivated during early embryonic development. This process, called X-inactivation or lyonization, results in the formation of a Barr body. The inactivated X chromosome is transcriptionally silent and appears as a condensed structure at the nuclear periphery. The choice of which X is inactivated (maternal or paternal) is random and clonally inherited by descendant cells.
女性有两条X染色体，而男性只有一条。为了平衡两性之间X连锁基因产物的剂量，在早期胚胎发育过程中，女性每个细胞中的两条X染色体会随机失活一条。这一过程称为X染色体失活或莱昂化，结果形成一个巴氏小体。失活的X染色体在转录上是沉默的，表现为核周围的凝缩结构。哪一条X染色体（母源或父源）被失活是随机的，并且由子代细胞克隆式地继承这种失活模式。
This phenomenon explains why female carriers of X-linked recessive disorders sometimes show mild symptoms: if, by chance, a large proportion of cells in a particular tissue have inactivated the normal X chromosome, the mutant allele may be expressed at a level that affects function. It also accounts for the mosaic pattern in female mammals heterozygous for X-linked coat colour genes, such as calico cats.
这一现象解释了为什么X连锁隐性遗传病的女性携带者有时会表现出轻微症状：如果在某一特定组织中，很大比例的细胞碰巧失活了正常的X染色体，突变等位基因的表达水平就可能影响功能。它也解释了杂合X连锁毛色基因的雌性哺乳动物（如三花猫）镶嵌模式的形成。
9. Sex-limited and Sex-influenced Traits | 限性性状和从性性状
Not all traits that differ between sexes are sex-linked. Sex-limited traits are those expressed in only one sex, although the genes are present in both sexes. For example, genes for milk production in mammals are present in both males and females, but only females lactate. Sex-influenced traits are those where an allele is dominant in one sex but recessive in the other, often due to hormonal differences. Pattern baldness in humans is a classic example: the allele is dominant in males but recessive in females, influenced by testosterone levels. These patterns should not be confused with X-linked inheritance, as they often show autosomal loci.
并非所有在性别间存在差异的性状都是伴性遗传。限性性状是指只在一种性别中表达，尽管基因在两性中都存在。例如，哺乳动物中调控产奶的基因在雄性和雌性中都存在，但只有雌性会泌乳。从性性状是指等位基因在一种性别中为显性，在另一种性别中为隐性的性状，通常由激素差异引起。人类的斑秃就是一个典型例子：受睾酮水平影响，该等位基因在男性中为显性，在女性中为隐性。这些遗传模式不应与X连锁遗传混淆，因为它们往往表现出常染色体位点特征。
10. Common Exam Mistakes and How to Avoid Them | 常见考试错误与应对策略
Many students incorrectly use genotypes like XX and XY with a superscript indicating the allele, but forget that the Y chromosome does not carry the allele. Avoid writing ‘carrier male’ for X-linked recessive traits, as males are either affected or normal. In genetic diagrams, always list gametes separately for each parent and never use a hybrid gamete like XhY from a male; it should be Xh or Y. When calculating probabilities, distinguish between the probability of a child being affected and the probability of a son being affected. Exam questions often ask specifically for ‘the probability that a son will be colour-blind’ – in which case only the male offspring are considered.
许多学生错误地使用XX和XY并在上标标明等位基因，却忘了Y染色体不携带该等位基因。对于X连锁隐性性状，不要使用“男性携带者”这样的表述，男性要么患病要么正常。在遗传图解中，要为每个亲本分别列出配子，绝不要组合出像XhY这样的杂合配子；男性产生的配子应当为Xh或Y。在计算概率时，要区分孩子患病的概率与儿子患病的概率。考题常常明确要求“儿子是色盲的概率”——此时只考虑男性后代。
Other pitfalls include misreading pedigree symbols and overlooking the possibility of de novo mutations. Always check that your predicted ratios match the given data and that you have not confused X-linked dominant with autosomal dominant patterns. Writing a clear key for symbols in your answer can earn marks even if the diagram is slightly imperfect.
其他容易出错的地方包括误读系谱图符号以及忽略新发突变（de novo mutation）的可能性。务必核对预测比例是否与给定数据相符，并确保没有将X连锁显性与常染色体显性模式混淆。在答案中写明符号图例，即便图解稍有瑕疵，也能获得相应分值。
11. Real-world Applications and Genetic Counseling | 实际应用与遗传咨询
Understanding sex-linked inheritance has direct applications in genetic counseling. Families with a history of haemophilia or Duchenne muscular dystrophy can be informed about the risks to future children. Carrier testing can identify females heterozygous for the mutation, and prenatal diagnosis via chorionic villus sampling or amniocentesis can detect affected male fetuses. Preimplantation genetic diagnosis (PGD) is also an option for at-risk couples. Moreover, gene therapy trials for some X-linked disorders, such as haemophilia B, have shown promising results by delivering functional copies of the defective gene using viral vectors.
理解伴性遗传在遗传咨询中有着直接应用。有血友病或杜氏肌营养不良症家族史的家庭可以了解到未来孩子的患病风险。携带者检测可以识别出杂合突变的女性，而通过绒毛膜取样或羊膜穿刺进行的产前诊断可以检测出患病的男性胎儿。胚胎植入前遗传学诊断（PGD）也是高风险夫妇的一个选择。此外，针对某些X连锁疾病（如血友病B）的基因疗法试验已通过病毒载体递送功能性的缺陷基因拷贝，显示出令人鼓舞的结果。
12. Summary of Key Concepts | 关键概念总结
Sex-linked inheritance is determined by genes on the X or Y chromosomes, with X-linked recessive being the most commonly tested pattern. Males are more frequently affected by X-linked recessive disorders due to hemizygosity. Carrier females can pass the trait to their sons and carrier status to their daughters. X-linked dominant traits affect both sexes but show no male-to-male transmission. Y-linked traits pass exclusively from father to son. Pedigree analysis, genetic diagram construction, and probability calculations are essential skills. The phenomenon of X-inactivation explains dosage compensation and variable expression in female carriers. Always check for sex linkage by scanning for patterns of affected individuals and confirming the lack of male-to-male transmission for X-linked conditions.
伴性遗传由位于X或Y染色体上的基因决定，其中X连锁隐性是最常考查的类型。由于半合子状态，男性更易受X连锁隐性遗传病影响。女性携带者可将性状传给其儿子，并将携带者身份传给女儿。X连锁显性性状在两性中均会出现，但没有男传男现象。Y连锁性状只从父亲传给儿子。系谱分析、遗传图解绘制和概率计算是必备技能。X染色体失活现象解释了剂量补偿和女性携带者的可变表达。检查伴性遗传时，务必通过扫描患病个体模式并确认X连锁遗传中不存在男传男现象来加以判断。
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