📚 Sex Inheritance | 性别遗传
Sex inheritance is a fundamental concept in genetics that explains how biological sex is determined and how certain traits are passed on through sex chromosomes. In many organisms, including humans, sex determination relies on a specific pair of chromosomes — the sex chromosomes — which differ between males and females. These chromosomes not only govern the development of sexual characteristics but also carry genes that can be inherited in unique patterns, known as sex linkage. Understanding sex inheritance involves exploring the mechanisms of sex determination, the roles of key genes like SRY, alternative systems across the animal kingdom, and the consequences of genes located on the X and Y chromosomes. For A-Level Biology students, mastering pedigree analysis for sex-linked traits and the phenomenon of X-inactivation is essential for explaining observed inheritance patterns and genetic disorders.
性别遗传是遗传学中的一个基本概念，它解释了生物性别的决定方式以及某些性状如何通过性染色体进行传递。在包括人类在内的许多生物中，性别决定依赖于一对特定的染色体——性染色体，它们在雄性和雌性之间存在差异。这些染色体不仅控制着性征的发育，而且还携带能够以独特方式遗传的基因，这种现象称为性连锁。理解性别遗传需要探索性别决定的机制、SRY等关键基因的作用、动物界中存在的替代系统，以及位于X和Y染色体上基因的遗传后果。对于A-Level生物学学生而言，掌握性连锁性状的系谱分析以及X失活现象，对于解释观察到的遗传模式和遗传疾病至关重要。
1. Introduction to Sex Determination | 性别决定简介
Sex determination is the biological process that dictates whether an organism develops as male or female. In most mammals, including humans, sex is genetically determined by the presence of specific sex chromosomes. The cells of an individual typically contain 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Females have two X chromosomes (XX), while males possess one X and one Y chromosome (XY). The Y chromosome carries a gene that triggers male development, making the male parent responsible for determining the sex of the offspring because males produce sperm that carry either an X or a Y chromosome.
性别决定是一个生物学过程，它决定了一个生物体发育为雄性还是雌性。在包括人类在内的大多数哺乳动物中，性别是由特定的性染色体的存在而遗传决定的。个体的细胞通常含有23对染色体：22对常染色体和一对性染色体。雌性拥有两条X染色体（XX），而雄性拥有一条X和一条Y染色体（XY）。Y染色体携带一个触发雄性发育的基因，因此雄性亲本负责决定后代的性别，因为雄性产生携带X或Y染色体的精子。
During fertilisation, if an X-carrying sperm fuses with the egg (which always contains an X chromosome), the resulting zygote will be XX and develop into a female. If a Y-carrying sperm fertilises the egg, the zygote will be XY and develop into a male. This means that there is a roughly 50% chance of having a male or female offspring in each pregnancy. Such a system, where the male is the heterogametic sex, is called the XY system.
在受精过程中，如果一个携带X染色体的精子与卵子（始终含有一条X染色体）结合，所产生的合子将是XX并发育为雌性。如果一个携带Y染色体的精子使卵子受精，合子将是XY并发育为雄性。这意味着每次怀孕生男生女的概率大约为50%。这种雄性为异配子性别的系统称为XY系统。
2. The XY System in Mammals | 哺乳动物的XY系统
The XY sex-determination system is typical of mammals, some insects (like Drosophila), and certain plants. In this system, the male is heterogametic because he produces two types of gametes — sperm carrying either an X or a Y chromosome. The female is homogametic, producing only one type of gamete, each with an X chromosome. The sex chromosome pair behaves like a homologous pair during meiosis in females (X-X pairing), but in males, the X and Y chromosomes pair only in a small region of homology called the pseudoautosomal region, allowing segregation during anaphase I.
XY性别决定系统是哺乳动物、某些昆虫（如果蝇）和某些植物的典型特征。在该系统中，雄性是异配性别，因为它产生两种类型的配子——携带X或Y染色体的精子。雌性为同配性别，只产生一种类型的配子，每种配子都含有一条X染色体。在雌性减数分裂过程中，性染色体对像同源染色体一样配对（X-X配对），但在雄性中，X和Y染色体仅在称为拟常染色体区的一小段同源区域配对，从而允许在后期I分离。
Unlike autosomes, the Y chromosome is much smaller than the X chromosome and contains far fewer genes. Most of the genes on the Y chromosome are involved in male sex determination and spermatogenesis. The X chromosome, in contrast, contains over 1,000 genes, many of which are essential for both sexes and are unrelated to sex determination. Consequently, the inheritance patterns of genes located on the X chromosome differ markedly from those on autosomes, giving rise to sex-linked inheritance.
与常染色体不同，Y染色体比X染色体小得多，含有的基因也少得多。Y染色体上的大多数基因参与雄性性别决定和精子发生。相比之下，X染色体含有超过1000个基因，其中许多对两性都至关重要且与性别决定无关。因此，位于X染色体上基因的遗传模式与常染色体基因显著不同，从而产生了性连锁遗传。
3. The SRY Gene and Male Development | SRY基因与雄性发育
The primary switch for male sex determination in mammals is the SRY gene (Sex-determining Region Y), located on the short arm of the Y chromosome. This gene encodes a transcription factor called Testis-Determining Factor (TDF). Early in embryonic development, the gonads are bipotential — they can develop into either ovaries or testes. The expression of SRY initiates a cascade of gene activation that directs the undifferentiated gonads to form testes, which then secrete testosterone and anti-Müllerian hormone. These hormones masculinise the embryo, promoting the development of male internal and external genitalia.
哺乳动物雄性性别决定的主要开关是位于Y染色体短臂上的SRY基因（性别决定区Y）。该基因编码一种称为睾丸决定因子（TDF）的转录因子。在胚胎发育早期，性腺具有双向潜能——它们可以发育为卵巢或睾丸。SRY的表达启动了一系列基因激活级联反应，引导未分化的性腺形成睾丸，随后睾丸分泌睾酮和抗苗勒氏管激素。这些激素使胚胎男性化，促进男性内外生殖器的发育。
In the absence of the SRY gene (as in XX individuals), the bipotential gonads follow the default pathway and differentiate into ovaries. The female reproductive tract develops due to the absence of anti-Müllerian hormone and testosterone. Therefore, the presence of a functional Y chromosome with an intact SRY gene is critical for male development. Rare cases, such as XX males (due to translocation of SRY to an X chromosome) or XY females (due to a mutated or deleted SRY gene), confirm the pivotal role of SRY.
在缺乏SRY基因的情况下（如XX个体），双向潜能的性腺将遵循默认途径分化为卵巢。由于缺乏抗苗勒氏管激素和睾酮，女性生殖道得以发育。因此，拥有功能性Y染色体和完整的SRY基因对雄性发育至关重要。一些罕见病例，如XX男性（由于SRY易位至X染色体）或XY女性（由于SRY基因突变或缺失），证实了SRY的关键作用。
4. Alternative Sex Determination Systems | 其他的性别决定系统
While the XY system is widespread, other mechanisms exist in nature. In birds, some reptiles, and certain fish, the ZW system is prevalent. Here, the female is the heterogametic sex with ZW sex chromosomes, and the male is homogametic (ZZ). This is the opposite of the XY system. The specific gene responsible for female development in birds is DMRT1, located on the Z chromosome; a double dose of DMRT1 directs maleness, while a single dose permits femaleness.
虽然XY系统广泛存在，但自然界中还存在其他机制。在鸟类、部分爬行动物和某些鱼类中，ZW系统占主导。在此系统中，雌性是异配性别，性染色体为ZW，雄性是同配性别（ZZ）。这与XY系统相反。鸟类中负责雌性发育的具体基因是位于Z染色体上的DMRT1；双倍剂量的DMRT1引导雄性发育，而单倍剂量允许雌性发育。
Another system is the XO system, found in some insects like grasshoppers. Females have two X chromosomes (XX), while males have only one X chromosome and no second sex chromosome (XO). The sex is determined by the ratio of X chromosomes to autosomes. In bees and ants, haplodiploidy determines sex: fertilised eggs (diploid) become females, and unfertilised eggs (haploid) become males. Furthermore, many reptiles, such as crocodiles and some turtles, exhibit temperature-dependent sex determination (TSD), where the incubation temperature of the eggs determines the sex of the offspring, independent of sex chromosomes.
另一种系统是XO系统，见于蝗虫等部分昆虫。雌性有两条X染色体（XX），而雄性只有一条X染色体，没有第二条性染色体（XO）。性别由X染色体与常染色体的比值决定。在蜜蜂和蚂蚁中，性别通过单倍二倍体决定：受精卵（二倍体）发育为雌性，未受精卵（单倍体）发育为雄性。此外，许多爬行动物，如鳄鱼和某些龟类，表现出温度依赖型性别决定（TSD），即蛋的孵化温度决定后代性别，与性染色体无关。
5. Sex Linkage: Genes on Sex Chromosomes | 性连锁：性染色体上的基因
Sex linkage refers to the pattern of inheritance of genes located on the sex chromosomes. Because the X chromosome is larger and carries many more genes than the Y, most sex-linked traits are X-linked. Males are hemizygous for most X-linked genes, meaning they have only one allele for each such gene. As a result, if a male inherits a recessive deleterious allele on the X chromosome, he will express the trait because there is no corresponding allele on the Y chromosome to mask it. Females, having two X chromosomes, can be homozygous dominant, homozygous recessive, or heterozygous (carriers) for X-linked alleles.
性连锁是指位于性染色体上基因的遗传模式。由于X染色体比Y染色体大得多，且携带的基因多得多，因此大多数性连锁性状是X连锁的。雄性对于大多数X连锁基因是半合子的，这意味着它们每个这样的基因只有一个等位基因。因此，如果雄性在X染色体上遗传了一个隐性有害等位基因，他就会表现出该性状，因为Y染色体上没有对应的等位基因来掩盖它。雌性有两条X染色体，对于X连锁等位基因可以是显性纯合、隐性纯合或杂合（携带者）。
Y-linked genes, also called holandric genes, are present only on the Y chromosome and are transmitted exclusively from father to son. Because the Y chromosome is largely devoid of functional genes apart from those involved in male development and fertility, Y-linked inheritance is rare and typically involves traits such as hairy ears (hypertrichosis pinnae auris). The key difference between X-linked and autosomal inheritance is that X-linked traits show a different distribution between males and females in pedigrees.
Y连锁基因，也称限雄遗传基因，只存在于Y染色体上，并且仅由父亲传给儿子。因为Y染色体除了与雄性发育和生育相关的基因外，基本没有功能性基因，所以Y连锁遗传很少见，通常涉及如多毛耳（耳廓多毛症）等性状。X连锁和常染色体遗传的关键区别在于，X连锁性状在系谱中的男女分布不同。
6. X-Linked Recessive Inheritance: Color Blindness and Hemophilia | X连锁隐性遗传：色盲和血友病
Red-green color blindness and haemophilia A are classic examples of X-linked recessive disorders. For a female to express an X-linked recessive trait, she must inherit the recessive allele from both parents — one on each X chromosome. A male only needs to inherit one recessive allele from his mother (who is usually a carrier) to be affected, because his Y chromosome does not carry the gene. This explains why X-linked recessive conditions are much more common in males than in females.
红绿色盲和A型血友病是X连锁隐性遗传病的典型例子。女性要表现出X连锁隐性性状，必须从父母双方各继承一个隐性等位基因——即每条X染色体上一个。而男性只需从母亲（通常是携带者）那里继承一个隐性等位基因就会患病，因为他的Y染色体上不携带该基因。这解释了为什么X连锁隐性遗传病在男性中远比女性中常见。
If we denote the normal vision allele as XN and the colour-blind allele as Xn, a carrier female has the genotype XNXn and a colour-blind female would be XnXn. A normal male is XNY, and a colour-blind male is XnY. In a cross between a carrier female (XNXn) and a normal male (XNY), there is a 50% chance that a son will be colour blind and a 50% chance that a daughter will be a carrier. Affected males cannot pass the trait to their sons, but they will pass the allele to all of their daughters, making them carriers. This pattern, where the trait skips generations through carrier females and predominantly affects males, is a hallmark of X-linked recessive inheritance.
如果我们用XN表示正常视觉等位基因，Xn表示色盲等位基因，那么携带者女性的基因型为XNXn，色盲女性为XnXn。正常男性为XNY，色盲男性为XnY。在携带者女性（XNXn）与正常男性（XNY）的杂交中，儿子有50%的概率是色盲，女儿有50%的概率是携带者。患病的父亲不能将该性状传给儿子，但会将等位基因传给所有女儿，使她们成为携带者。这种通过携带者女性隔代相传并主要影响男性的模式，是X连锁隐性遗传的标志。
Similarly, haemophilia A results from a deficiency of clotting factor VIII and shows the same inheritance pattern. Queen Victoria was a famous carrier of haemophilia, passing the allele to several royal families through her daughters. These historical pedigrees demonstrate how X-linked recessive alleles were transmitted across Europe.
类似地，A型血友病是由凝血因子VIII缺乏引起的，并表现出相同的遗传模式。维多利亚女王就是一个著名的血友病携带者，她通过女儿们将该等位基因传给了多个皇室家族。这些历史系谱展示了X连锁隐性等位基因是如何在欧洲传播的。
7. X-Linked Dominant Inheritance | X连锁显性遗传
X-linked dominant traits are less common but show distinct inheritance patterns. In this case, a dominant allele on the X chromosome causes the phenotype in both hemizygous males and heterozygous females. Because females have two X chromosomes, they may show the trait if they inherit just one dominant allele. Affected males tend to be more severely affected, and they will transmit the trait to all of their daughters (who must receive the father’s X chromosome) but to none of their sons. Affected heterozygous females transmit the trait to half of their children, regardless of sex, on average.
X连锁显性性状不太常见，但表现出独特的遗传模式。在这种情况下，X染色体上的一个显性等位基因在雄性和杂合雌性中都会引起相应表型。由于雌性有两条X染色体，她们只要继承一个显性等位基因就可能表现出该性状。患病的男性通常病情更严重，而且他们会将该性状传给所有女儿（女儿必定接受父亲的X染色体），但不会传给任何儿子。患病的杂合女性平均将性状传给一半后代，且与性别无关。
An example is vitamin D-resistant rickets (hypophosphatemic rickets), where the defective gene on the X chromosome leads to impaired phosphate reabsorption in the kidneys. Affected individuals have low blood phosphate and bone deformities. In a pedigree, the condition may appear in every generation, and the daughters of an affected male are all affected, while his sons are normal. Males and females can both be affected, but the condition may be milder in heterozygous females than in hemizygous males.
一个例子是维生素D抵抗性佝偻病（低磷血症性佝偻病），其中X染色体上的缺陷基因导致肾脏磷酸盐重吸收障碍。患者血磷低并伴有骨骼畸形。在系谱中，该病可能每代出现，患病男性的所有女儿都患病，而他的儿子均正常。男性和女性都可能患病，但杂合女性病情可能比半合子男性轻。
8. Y-Linked Inheritance | Y连锁遗传
Y-linked inheritance (holandric inheritance) involves genes located exclusively on the non-recombining region of the Y chromosome. As the Y chromosome is passed intact from father to son, Y-linked traits are observed only in males and are transmitted to all male offspring. There is no female transmission or expression, as females lack a Y chromosome entirely.
Y连锁遗传（限雄遗传）涉及仅位于Y染色体非重组区域的基因。由于Y染色体原封不动地从父亲传给儿子，Y连锁性状仅在男性中观察到，并传给所有男性后代。没有女性传递或表达，因为女性完全缺乏Y染色体。
The most well-known Y-linked trait in humans is hypertrichosis of the ears (hairy ears), though its precise genetic basis is still debated. Another crucial Y-linked gene is SRY itself, although it is not typically described as a visible trait. Because the Y chromosome has very few genes, Y-linked disorders are extremely rare. In pedigree analysis, a Y-linked trait will show up only in males, appearing in every generation if the mutation is fully penetrant, and all sons of affected males will be affected.
人类最著名的Y连锁性状是耳廓多毛症（长毛耳），但其精确的遗传基础仍有争议。另一个关键的Y连锁基因是SRY本身，尽管它通常不被描述为可见性状。由于Y染色体只有极少的基因，Y连锁疾病极为罕见。在系谱分析中，Y连锁性状将仅在男性中出现，如果突变是完全外显的，则会在每一代出现，而且所有患病男性的儿子都将患病。
9. X-Inactivation and Barr Bodies | X失活与巴氏小体
In female mammals, one of the two X chromosomes in each cell is randomly inactivated early in embryonic development. This process, called X-inactivation or lyonisation, ensures dosage compensation so that females and males produce similar amounts of X-linked gene products. The inactivated X chromosome condenses into a compact structure known as a Barr body, which can be seen in the nucleus of female somatic cells. Males (XY) normally have no Barr body, while females (XX) have one Barr body per cell.
在雌性哺乳动物中，每个细胞中两条X染色体中的一条在胚胎发育早期被随机失活。这一过程称为X失活或莱昂化，它确保了剂量补偿效应，使雌性和雄性产生相似数量的X连锁基因产物。失活的X染色体凝聚成一个称为巴氏小体的致密结构，可在雌性体细胞的细胞核中看到。雄性（XY）通常没有巴氏小体，而雌性（XX）每个细胞中有一个巴氏小体。
X-inactivation is random and clonal: once an X chromosome is inactivated in a cell, all descendant cells will have the same X chromosome inactive. This leads to mosaicism in females for any X-linked heterozygous trait. A classic example is the tortoiseshell cat, where coat colour is determined by an X-linked gene with alleles for orange and black fur. Heterozygous female cats have patches of orange and black fur because some cell clones express the orange allele while others express the black allele, depending on which X chromosome remains active. Male tortoiseshell cats are extremely rare and usually sterile, as they arise from an XXY karyotype.
X失活是随机且克隆性的：一旦某细胞中的X染色体失活，所有后代细胞都会保持同一条X染色体失活。这导致雌性对于任何X连锁杂合性状呈现出嵌合现象。一个经典例子就是玳瑁猫，其毛色由一个X连锁基因决定，该基因具有橙色和黑色毛发的等位基因。杂合雌猫具有橙色和黑色毛发的斑块，因为一些细胞克隆表达橙色等位基因，而另一些表达黑色等位基因，这取决于哪条X染色体保持活性。雄性玳瑁猫极为罕见且通常不育，因为它们由XXY核型产生。
The number of Barr bodies in a cell is always one fewer than the total number of X chromosomes. Therefore, individuals with Turner syndrome (XO) have no Barr body; females with triple X syndrome (XXX) have two Barr bodies; and Klinefelter syndrome males (XXY) have one Barr body. X-inactivation explains why individuals with an abnormal number of X chromosomes can survive, albeit with some phenotypic abnormalities.
细胞中巴氏小体的数量总是比X染色体总数少一个。因此，特纳综合征（XO）个体没有巴氏小体；三X综合征（XXX）女性有两个巴氏小体；克氏综合征（XXY）男性有一个巴氏小体。X失活解释了为什么具有异常数量X染色体的个体能够存活，尽管存在一些表型异常。
10. Pedigree Analysis for Sex-Linked Traits | 性连锁性状的系谱分析
Interpreting pedigrees to determine the likely mode of inheritance is a key skill in genetics. For an X-linked recessive trait, key observations include: (i) the trait is more common in males than females; (ii) affected males usually inherit the trait from a carrier mother; (iii) an affected female must have an affected father and a carrier or affected mother; (iv) the trait often skips generations, with carrier females passing it to affected sons; (v) the trait cannot be transmitted from father to son.
分析系谱以确定最可能的遗传方式，是遗传学中的一项关键技能。对于X连锁隐性性状，关键观察点包括：(i) 该性状在男性中比女性中更常见；(ii) 患病男性通常从携带者母亲那里继承该性状；(iii) 患病女性必定有一个患病的父亲及携带者或患病的母亲；(iv) 该性状常常隔代相传，由携带者女性传给患病的儿子；(v) 该性状不能由父亲传给儿子。
For X-linked dominant traits, we expect: (i) both males and females are affected, but the condition may be more severe in males; (ii) affected males pass the trait to all their daughters but none of their sons; (iii) affected heterozygous females pass the trait to half of their children regardless of sex; (iv) the trait appears in every generation (no skipping). No male-to-male transmission rules out Y-linked inheritance only, while it is consistent with X-linked dominant inheritance if daughters are all affected.
对于X连锁显性性状，我们期望：(i) 男性和女性都会受累，但男性可能病情更重；(ii) 患病男性将性状传给所有女儿，但不传给儿子；(iii) 患病的杂合女性将性状传给一半后代，与性别无关；(iv) 该性状在每代中出现（无跳跃）。没有男→男传递只能排除Y连锁遗传，而如果所有女儿都患病，则与X连锁显性遗传一致。
Y-linked pedigrees are distinct: only males are affected, and all sons of an affected male are affected, with no affected females or female transmission. Autosomal traits, by contrast, show equal distribution between sexes (in large samples), unless there is sex-limited or sex-influenced expression. To distinguish X-linked recessive from autosomal recessive, look for evidence that an affected son can be born to unaffected, non-consanguineous parents — if the mother is a carrier, this points to X-linkage. Also, for an X-linked recessive trait, if an affected male mates with an unaffected homozygous female, none of their sons will be affected (since sons receive the Y from the father and an X from the mother).
Y连锁系谱很独特：只有男性受累，且所有患病男性的儿子都患病，没有患病女性，也没有女性传递。相比之下，常染色体性状在性别间分布均等（大样本中），除非存在限性或从性表达。要区分X连锁隐性和常染色体隐性，需寻找一个患病儿子能否由非近亲的正常父母所生——如果母亲是携带者，则指向X连锁。此外，对于X连锁隐性性状，如果一个患病男性与一个未患病的纯合女性婚配，他们的儿子都不会患病（因为儿子从父亲那里接受Y染色体，从母亲那里接受X染色体）。
When solving pedigree problems, always consider the possibility of multiple modes of inheritance and eliminate those that are inconsistent. For example, if we see a daughter who is affected when her father is unaffected, it rules out X-linked recessive because an affected female must have an affected father in X-linked recessive. Practising with a variety of pedigrees solidifies understanding of sex-linked inheritance patterns.
在解决系谱问题时，应始终考虑多种遗传模式的可能性，并排除不一致的模式。例如，如果我们看到一个女儿患病而其父亲正常，就可以排除X连锁隐性遗传，因为在X连锁隐性遗传中，患病女性必定有一个患病的父亲。通过练习各种系谱，可以巩固对性连锁遗传模式的理解。
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