📚 A-Level OCR Biology: Sex-linked Inheritance | A-Level OCR 生物：伴性遗传 考点精讲
Sex-linked inheritance describes the pattern of inheritance for genes located on the sex chromosomes, typically the X chromosome in humans. Understanding how these genes are passed from one generation to the next is essential for explaining why certain genetic disorders, such as red-green colour blindness and haemophilia, appear much more frequently in males. In A-Level OCR Biology, you are expected to interpret genetic diagrams, calculate probabilities, and analyse pedigree charts to trace sex-linked traits.
伴性遗传是指位于性染色体（通常是人类的 X 染色体）上的基因所呈现的遗传规律。理解这些基因如何在代际之间传递，对于解释为什么某些遗传病（如红绿色盲和血友病）在男性中更为常见至关重要。在 A-Level OCR 生物学中，你需要能够解读遗传图解、计算概率，并分析系谱图来追踪伴性性状。
1. Introduction to Sex-linked Inheritance | 伴性遗传简介
Genes carried on the sex chromosomes exhibit distinct inheritance patterns compared to autosomal genes because the sex chromosomes differ between males and females. In mammals, females typically possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). This fundamental difference leads to unique outcomes for alleles located on the X chromosome, as males are hemizygous for most X-linked genes.
位于性染色体上的基因表现出的遗传模式与常染色体基因截然不同，因为雄性和雌性的性染色体不同。在哺乳动物中，雌性通常有两条 X 染色体（XX），而雄性有一条 X 和一条 Y 染色体（XY）。这种根本差异导致位于 X 染色体上的等位基因产生独特的结果，因为雄性对于大多数 X 连锁基因是半合子。
2. Sex Chromosomes in Humans | 人类性染色体
Human females have a homologous pair of X chromosomes, while males have one X and a much smaller Y chromosome. The Y chromosome carries very few functional genes compared to the X, and the two chromosomes are not fully homologous except for small pseudoautosomal regions at the tips. As a result, any recessive allele on the X chromosome will be expressed in males even if it is present in only a single copy, because there is no corresponding allele on the Y chromosome to mask its effect.
人类女性拥有一对同源的 X 染色体，而男性有一条 X 染色体和一条小得多的 Y 染色体。与 X 染色体相比，Y 染色体携带的功能基因非常少，而且除两端短小的拟常染色体区外，这两条染色体并非完全同源。因此，X 染色体上的任何隐性等位基因即使只存在单个拷贝，在男性中也会表达，因为 Y 染色体上没有对应的等位基因来掩盖其效应。
3. X-linked Recessive Inheritance | 伴 X 隐性遗传
X-linked recessive traits are caused by alleles on the X chromosome that only show their effect when the dominant allele is absent. A female with one normal dominant allele and one recessive mutant allele is a carrier and typically does not display the trait, whereas a male inheriting the same recessive allele will express the condition because he has only one X chromosome. This is the most common form of sex-linked inheritance examined at A-Level.
伴 X 隐性性状由 X 染色体上的等位基因引起，仅当显性等位基因缺失时才会显现出效应。拥有一条正常显性等位基因和一条隐性突变等位基因的女性是携带者，通常不表现该性状；而遗传了相同隐性等位基因的男性则会表现出病症，因为他只有一条 X 染色体。这是 A-Level 考试中最常见的伴性遗传形式。
For a female to express an X-linked recessive disorder, she must inherit two copies of the recessive allele – one from each parent. This is much rarer, as it requires an affected or carrier father and at least a carrier mother. Consequently, such conditions are far more prevalent in males in the population.
女性若要表现出伴 X 隐性遗传病，就必须从父母双方各继承一个隐性等位基因。这种情况要罕见得多，因为它要求父亲是患者或携带者，且母亲至少是携带者。因此，这类疾病在人群中的男性发病率远高于女性。
4. Example: Red-Green Colour Blindness | 红绿色盲
Red-green colour blindness is a classic example of an X-linked recessive disorder. It results from a mutation in the genes coding for photopigments in the cone cells of the retina, making it difficult to distinguish between red and green hues. The allele for normal colour vision is dominant (often denoted as XN), while the allele for colour blindness is recessive (Xn).
红绿色盲是伴 X 隐性遗传病的一个经典例子。它由视网膜视锥细胞中与光敏色素相关的基因突变引起，导致难以区分红色和绿色色调。正常色觉的等位基因为显性（常表示为 XN），而色盲的等位基因为隐性（Xn）。
Because males have only one X chromosome, a single Xn allele will cause colour blindness. Females require the genotype XnXn to be affected; a heterozygous female (XNXn) is a carrier with normal vision. This explains why about 8% of males of European descent are affected, compared to only about 0.5% of females.
由于男性只有一条 X 染色体，单个 Xn 等位基因就会导致色盲。女性则需要基因型为 XnXn 才会患病；杂合女性（XNXn）是携带者，拥有正常视觉。这就解释了为何大约 8% 的欧洲裔男性受此影响，而女性仅约 0.5%。
5. Example: Haemophilia | 血友病
Haemophilia is another well-known X-linked recessive disorder, characterised by a deficiency in clotting factors – most commonly Factor VIII (haemophilia A) or Factor IX (haemophilia B). This leads to an inability for blood to clot properly, causing prolonged bleeding even from minor injuries. The normal allele (XH) produces functional clotting factor, while the mutant allele (Xh) does not.
血友病是另一种著名的伴 X 隐性遗传病，其特征是凝血因子缺乏——最常见的是因子 VIII（血友病 A）或因子 IX（血友病 B）。这会导致血液无法正常凝固，即使微小伤口也会引起长时间出血。正常等位基因（XH）能产生功能性凝血因子，而突变等位基因（Xh）则不能。
Again, males with XhY are haemophiliacs, while females with XHXh are carriers who do not show symptoms. A female haemophiliac (XhXh) is extremely rare. The famous pedigree of the British royal family, tracing back to Queen Victoria (a carrier), illustrates how the allele can be passed through unaffected carrier females and manifest in male descendants.
同样，XhY 的男性是血友病患者，而 XHXh 的女性是不表现症状的携带者。女性血友病患者（XhXh）极为罕见。著名的英国王室系谱可追溯至维多利亚女王（一位携带者），展示了该等位基因如何通过不患病的女性携带者传递，并在男性后代中显现出来。
6. Genetic Diagrams for X-linked Recessive Traits | 伴 X 隐性性状的遗传图解
When constructing genetic diagrams for sex-linked traits, it is important to represent the sex chromosomes and the alleles correctly. For instance, a cross between a carrier female (XNXn) and a normal male (XNY) can be shown as:
在构建伴性性状的遗传图解时，正确表示性染色体和等位基因非常重要。例如，一个携带者女性（XNXn）与正常男性（XNY）的杂交可以表示如下：
| Gametes | XN | Y |
| XN | XNXN (normal female) | XNY (normal male) |
| Xn | XNXn (carrier female) | XnY (affected male) |
From this, the expected offspring ratios are 25% normal female, 25% carrier female, 25% normal male, and 25% affected male. Notice that all daughters will have normal phenotype (though half are carriers), whereas half of the sons will be affected.
由此，预期子代比例为：25% 正常女性，25% 携带者女性，25% 正常男性，以及 25% 患病男性。请注意，所有女儿都会表现出正常表型（但一半是携带者），而半数的儿子会患病。
A cross between an affected male (XnY) and a homozygous normal female (XNXN) produces all normal offspring, but all daughters become carriers (XNXn). This pattern where the condition skips generations and appears predominantly in males through carrier females is a hallmark of X-linked recessive inheritance.
患病男性（XnY）与纯合正常女性（XNXN）杂交，产生的所有后代均表现正常，但所有女儿都会成为携带者（XNXn）。这种病症隔代出现、通过女性携带者传给男性后代的现象，是伴 X 隐性遗传的标志性特征。
7. X-linked Dominant Inheritance | 伴 X 显性遗传
Although less common, X-linked dominant disorders also exist. In this pattern, a single copy of the mutant allele is sufficient to cause the disorder in both males and females. However, the condition tends to be more severe in males because females possess a second, normal X chromosome that may partially moderate the effect through X-inactivation. Affected males transmit the condition to all of their daughters but none of their sons, since sons inherit the Y chromosome, not the X.
尽管较为少见，但也存在伴 X 显性遗传病。在这种模式下，单个突变等位基因就足以在男性和女性中引起疾病。不过，男性的病症往往更为严重，因为女性拥有一条正常的 X 染色体，可以通过 X 失活在某种程度上缓和效应。患病的男性会将此病症传给所有女儿，而不会传给儿子，因为儿子继承的是 Y 染色体而非 X 染色体。
In contrast, affected heterozygous females have a 50% chance of passing the mutant allele to each child, regardless of sex. Therefore, X-linked dominant traits appear in every generation, and females are affected more frequently than males in the population.
相比之下，患病的杂合女性每次生育都有 50% 的概率将突变等位基因传递给子女，与子女性别无关。因此，伴 X 显性性状会代代出现，且在人群中女性的发病率高于男性。
8. Example: Vitamin D-resistant Rickets | 抗维生素 D 佝偻病
Vitamin D-resistant rickets (hypophosphataemic rickets) is a classical example of an X-linked dominant condition. It is caused by a mutation in the PHEX gene on the X chromosome, leading to phosphate wasting in the kidneys, which results in rickets that cannot be cured by normal vitamin D intake. An affected father (XRY) will pass the allele to all his daughters (who will all be affected) and to none of his sons. An affected mother (XRXr) has a 50% chance of having an affected child, regardless of sex.
抗维生素 D 佝偻病（低磷血症性佝偻病）是伴 X 显性遗传病的经典实例。它由 X 染色体上 PHEX 基因的突变引起，导致肾脏中磷酸盐大量流失，进而引发无法通过正常维生素 D 摄入治愈的佝偻病。患病的父亲（XRY）会将这个等位基因传给所有女儿（她们都将患病），而不会传给任何儿子。患病的母亲（XRXr）每次怀孕均有 50% 的概率生出患病子女，与性别无关。
9. Y-linked Inheritance | Y 连锁遗传
Genes located on the Y chromosome are passed directly from father to son, a pattern known as holandric inheritance. Since females do not inherit a Y chromosome, Y-linked traits are never expressed in women. In human genetics, very few functional genes are found on the Y chromosome outside the sex-determining region (SRY). The gene for hairy ears (hypertrichosis pinnae auris) has been claimed to be Y-linked, but its genetic basis remains uncertain and it is often cited in textbooks as a putative Y-linked trait.
位于 Y 染色体上的基因直接从父亲传递给儿子，这种模式称为限雄遗传。由于女性不继承 Y 染色体，因此 Y 连锁性状绝不会在女性中表达。在人类遗传学中，除性别决定区（SRY）外，Y 染色体上的功能性基因极少。耳郭多毛基因曾被认为属于 Y 连锁，但其遗传基础尚不确定，不过教科书常将其作为假定的 Y 连锁性状引用。
From an exam perspective, Y-linked inheritance is easily recognised: only males are affected, and every affected male must have an affected father (as long as paternity is assured). All sons of an affected male will inevitably inherit the trait.
从考试角度来看，Y 连锁遗传极易辨认：只有男性患病，且每一位患病男性必然有一位患病的父亲（在确认亲子关系的前提下）。患病男性的所有儿子都必定继承该性状。
10. Differences between Sex-linked and Autosomal Inheritance | 伴性遗传与常染色体遗传的区别
Autosomal genes are inherited equally by both sexes, and recessive conditions appear only when two copies of the mutant allele are present, often with no significant sex bias. Sex-linked traits, particularly X-linked recessive ones, show a pronounced sex disparity, with males being predominantly affected. In pedigree analysis, an unaffected couple having an affected son strongly suggests X-linked recessive inheritance, whereas an affected father transmitting the trait to all his daughters points to X-linked dominance.
常染色体基因由两性均等遗传，隐性病症仅当存在两个突变等位基因拷贝时才显现，通常没有显著的性别偏向。伴性性状，特别是伴 X 隐性性状，则呈现出明显的性别差异，男性为主要受累群体。在系谱分析中，未患病的夫妇生出一个患病的儿子，强烈提示伴 X 隐性遗传；而患病的父亲将性状传给所有女儿，则指向伴 X 显性遗传。
Additionally, in autosomal inheritance, both males and females can be carriers of recessive alleles, and an affected son does not necessarily indicate a sex-linked condition. The key is to look for patterns where expression correlates with the inheritance of sex chromosomes.
此外，在常染色体遗传中，男性和女性都可以成为隐性等位基因的携带者，且一个患病的儿子并不一定意味着伴性遗传。关键在于寻找性状表达与性染色体遗传相关联的模式。
11. Importance of Sex-linked Inheritance in Genetics | 伴性遗传在遗传学中的重要性
Understanding sex-linked inheritance has profound implications for genetic counselling and medicine. Carrier testing and prenatal diagnosis can help families at risk of X-linked disorders make informed reproductive decisions. Moreover, studies of sex-linked traits have historically contributed to the discovery of chromosome theory of inheritance, beginning with T.H. Morgan's fruit fly experiments demonstrating the white-eye locus on the X chromosome.
理解伴性遗传对遗传咨询和医学具有深远意义。携带者检测和产前诊断可以帮助存在伴 X 遗传病风险的家庭做出知情的生育决策。此外，对伴性性状的研究在历史上促成了染色体遗传理论的发现，始于摩尔根（T.H. Morgan）的果蝇实验，证实了白眼基因座位于 X 染色体上。
In evolutionary terms, the hemizygosity of males for X-linked loci exposes deleterious recessive alleles to natural selection more directly, which can influence allele frequencies in populations. This concept also explains why many X-linked disorders are maintained in the gene pool despite their harmful effects.
从进化角度来看，雄性在 X 连锁基因座上的半合子状态使得有害的隐性等位基因更直接地暴露于自然选择，这会影响群体中的等位基因频率。这一概念也解释了为什么许多伴 X 遗传病尽管有害，却依然保留在基因库中。
12. Summary and Key Points | 总结与关键考点
- X-linked recessive disorders (e.g. colour blindness, haemophilia) predominantly affect males. Females are carriers unless homozygous recessive.
伴 X 隐性遗传病（例如色盲、血友病）主要影响男性。女性除非为隐性纯合子，否则是携带者。 - X-linked dominant disorders (e.g. vitamin D-resistant rickets) affect heterozygous females and are often more severe in males. Affected fathers pass the trait to all daughters.
伴 X 显性遗传病（例如抗维生素 D 佝偻病）会影响杂合女性，且在男性中往往更为严重。患病的父亲会将性状传给所有女儿。 - Y-linked genes are transmitted only from father to son.
Y 连锁基因只由父亲传给儿子。 - Use superscript notation to label alleles on sex chromosomes (e.g. XAXa) and clearly distinguish gametes in genetic crosses.
使用上标标注性染色体上的等位基因（如 XAXa），并在遗传杂交中清楚地区分配子。 - Pedigree clues for sex linkage: more affected males for recessive; no father-to-son transmission for X-linked traits.
伴性遗传系谱线索：隐性性状更多表现为男性患者；伴 X 性状不会出现父亲传给儿子的情况。
Be prepared to calculate expected offspring ratios, assign genotypes from pedigree information, and explain why certain disorders show skewed sex ratios. Mastering these principles will give you a strong foundation for tackling both straightforward and application-based questions in the OCR examination.
准备好计算预期的子代比率，根据系谱信息确定基因型，并解释为什么某些疾病表现出偏斜的性别比。掌握这些原理将为你奠定坚实的基础，应对 OCR 考试中既包含直接考查也包含应用类的题目。
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