📚 Sex-linked Inheritance: WJEC IGCSE Biology Key Points | 伴性遗传：WJEC IGCSE 生物考点精讲
Sex-linked inheritance is a crucial topic in the WJEC IGCSE Biology specification. It explains how genes located on sex chromosomes (X and Y) are passed from parents to offspring, leading to unique patterns of inheritance. Unlike autosomal genes, sex-linked traits often appear more frequently in one sex, particularly males, because of the differences in the sex chromosomes. Understanding this concept is essential for solving genetic problems, interpreting pedigree charts, and explaining real-life conditions such as red-green colour blindness and haemophilia.
伴性遗传是WJEC IGCSE生物考试大纲中的一个关键主题。它解释了位于性染色体（X和Y）上的基因如何从亲代传递给子代，从而导致独特的遗传模式。与常染色体基因不同，伴性性状往往在某一种性别中更常见，特别是男性，这是因为性染色体存在差异。理解这一概念对于解决遗传学问题、解读家族谱系图以及解释红绿色盲和血友病等现实疾病至关重要。
1. What is Sex-linked Inheritance? | 什么是伴性遗传？
Sex-linked inheritance refers to the transmission of genes that are located on the sex chromosomes (X or Y). In humans and many other organisms, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Because the X chromosome is much larger and carries many more genes than the Y chromosome, most sex-linked traits are X-linked. Genes present only on the Y chromosome are called Y-linked and are rare.
伴性遗传指的是位于性染色体（X或Y）上基因的传递。在人类和许多其他生物中，雌性有两条X染色体（XX），而雄性有一条X和一条Y染色体（XY）。由于X染色体比Y染色体大得多，携带的基因也多得多，因此大多数伴性性状是X连锁的。仅存在于Y染色体上的基因称为Y连锁，较为罕见。
A key feature of X-linked genes is that males have only one allele for such genes, as they possess a single X chromosome. This means that any recessive allele on the X chromosome will be expressed in males, even if it is harmful. In contrast, females have two X chromosomes, so they can be carriers of a recessive allele without showing the trait.
X连锁基因的一个关键特征是，由于男性只有一条X染色体，他们对于这类基因只有一个等位基因。这意味着X染色体上的任何隐性等位基因都会在男性身上表达，即使它是有害的。相比之下，女性有两条X染色体，因此她们可以是隐性等位基因的携带者而不表现出该性状。
2. The X and Y Chromosomes | X和Y染色体
The X chromosome contains around 800–900 genes, many of which are essential for normal development and are not related to sex determination. Examples of X-linked genes include those involved in colour vision and blood clotting. The Y chromosome is much smaller, containing only about 50–60 genes, most of which are associated with male sex determination and sperm production.
X染色体包含大约800-900个基因，其中许多对正常发育至关重要，且与性别决定无关。X连锁基因的例子包括与色觉和血液凝固有关的基因。Y染色体则小得多，仅包含大约50-60个基因，其中大多数与男性性别决定和精子产生有关。
Because the Y chromosome lacks homologous alleles for most X-linked genes, a male's phenotype for these traits is determined solely by the allele on his single X chromosome. This phenomenon is described as hemizygosity. Hemizygous males are thus more vulnerable to X-linked recessive disorders.
由于Y染色体上缺乏大多数X连锁基因的同源等位基因，男性这些性状的表现型完全由他单条X染色体上的等位基因决定。这种现象称为半合子性。因此，半合子男性更容易患X连锁隐性遗传病。
During meiosis in females, the two X chromosomes undergo crossing over and segregate equally into eggs. In males, the X and Y chromosomes pair only in small pseudoautosomal regions, and they segregate to produce sperm carrying either an X or a Y chromosome, determining the sex of the offspring.
在女性的减数分裂过程中，两条X染色体会发生交叉并均等地分离到卵细胞中。在男性中，X和Y染色体仅在小的拟常染色体区域配对，它们分离后产生携带X或Y染色体的精子，从而决定后代的性别。
3. X-linked Recessive Inheritance | X连锁隐性遗传
X-linked recessive inheritance is the most common pattern of sex-linked inheritance studied at IGCSE level. In this pattern, the allele causing the disorder is recessive and located on the X chromosome. A male needs only one copy of the recessive allele to be affected, while a female must inherit two copies (one from each parent) to express the trait. A female with one normal and one recessive allele is a carrier – she does not show symptoms but can pass the allele to her children.
X连锁隐性遗传是IGCSE阶段研究的最常见的伴性遗传模式。在这种模式下，引起疾病的等位基因是隐性的，并且位于X染色体上。男性只需一个隐性等位基因就会患病，而女性必须遗传两个隐性等位基因（分别来自父母）才会表现出该性状。带有一个正常和一个隐性等位基因的女性是携带者——她不表现症状，但可以将该等位基因传递给子女。
The characteristic features of X-linked recessive inheritance include: more males than females are affected; affected males cannot pass the trait to their sons (since sons inherit the Y chromosome from their father); all daughters of an affected male will be carriers if the mother is homozygous normal; and carrier females have a 50% chance of passing the recessive allele to each child.
X连锁隐性遗传的典型特征包括：男性患者多于女性；患病父亲不会将性状传给儿子（因为儿子从父亲那里继承Y染色体）；如果母亲是纯合正常，患病父亲的所有女儿都将是携带者；携带者女性每次生育有50%的几率将隐性等位基因传给每个孩子。
4. Genetic Notation for Sex Linkage | 伴性遗传的遗传学表示
When writing genotypes for sex-linked traits, the alleles are written as superscripts on the X chromosome symbol. The Y chromosome is written without a corresponding allele because it does not carry that gene. For example, using the allele symbols for colour vision: Xᴮ represents the normal allele, Xᵇ represents the recessive colour-blind allele. A normal male is written as XᴮY, a colour-blind male as XᵇY. A normal female could be XᴮXᴮ (homozygous normal) or XᴮXᵇ (carrier), while a colour-blind female must be XᵇXᵇ.
在书写伴性性状的基因型时，等位基因被写成X染色体符号的上标。Y染色体不写对应等位基因，因为它不携带该基因。例如，使用色觉的等位基因符号：Xᴮ代表正常等位基因，Xᵇ代表隐性色盲等位基因。正常男性写作XᴮY，色盲男性写作XᵇY。正常女性可以是XᴮXᴮ（纯合正常）或XᴮXᵇ（携带者），而色盲女性必须是XᵇXᵇ。
For haemophilia, a similar notation is used: Xᴴ for the normal clotting allele and Xʰ for the haemophilia allele. So a haemophiliac male is XʰY, a normal male XᴴY, a carrier female XᴴXʰ, and a haemophiliac female (very rare) XʰXʰ.
对于血友病，使用类似的符号：Xᴴ表示正常凝血等位基因，Xʰ表示血友病等位基因。因此，血友病男性为XʰY，正常男性为XᴴY，携带者女性为XᴴXʰ，血友病女性（非常罕见）为XʰXʰ。
| Genotype | Phenotype |
|---|---|
| XᴮXᴮ | Normal female |
| XᴮXᵇ | Carrier female (normal vision) |
| XᵇXᵇ | Colour-blind female |
| XᴮY | Normal male |
| XᵇY | Colour-blind male |
5. Example: Red-Green Colour Blindness | 例：红绿色盲
Red-green colour blindness is a common X-linked recessive disorder that affects about 8% of males and 0.5% of females of Northern European descent. It is caused by a recessive allele on the X chromosome that disrupts the function of photoreceptor proteins in the retina, making it difficult to distinguish between red and green hues.
红绿色盲是一种常见的X连锁隐性遗传病，在北欧血统人群中大约8%的男性和0.5%的女性受到影响。它是由X染色体上的一个隐性等位基因引起的，该基因破坏了视网膜中感光蛋白的功能，使人难以区分红色和绿色。
Consider a cross between a normal male (XᴮY) and a carrier female (XᴮXᵇ). A Punnett square is drawn with the male gametes (Xᴮ and Y) and the female gametes (Xᴮ and Xᵇ). The possible offspring genotypes are: XᴮXᴮ (normal female), XᴮXᵇ (carrier female), XᴮY (normal male), and XᵇY (colour-blind male). This yields a phenotypic ratio of 1 normal female : 1 carrier female : 1 normal male : 1 colour-blind male.
考虑一个正常男性（XᴮY）与一个携带者女性（XᴮXᵇ）的杂交。绘制庞奈特方格，包含雄性配子（Xᴮ和Y）和雌性配子（Xᴮ和Xᵇ）。可能的后代基因型为：XᴮXᴮ（正常女性），XᴮXᵇ（携带者女性），XᴮY（正常男性）和XᵇY（色盲男性）。这产生的表现型比例为1正常女性:1携带者女性:1正常男性:1色盲男性。
| Xᴮ (sperm) | Y (sperm) | |
| Xᴮ (egg) | XᴮXᴮ normal female | XᴮY normal male |
| Xᵇ (egg) | XᴮXᵇ carrier female | XᵇY colour-blind male |
The risk of having an affected child increases if the mother is a carrier. A colour-blind male (XᵇY) and a homozygous normal female (XᴮXᴮ) will have all daughters as carriers and all sons normal, since sons always receive the Y chromosome from their father and an X from their mother.
如果母亲是携带者，生出患病孩子的风险会增加。一个色盲男性（XᵇY）和一个纯合正常女性（XᴮXᴮ）所生的所有女儿都是携带者，所有儿子都正常，因为儿子总是从父亲那里继承Y染色体，从母亲那里继承X染色体。
6. Example: Haemophilia | 例：血友病
Haemophilia is another well-known X-linked recessive disorder in which the blood fails to clot properly due to a deficiency in clotting factor VIII (haemophilia A) or IX (haemophilia B). A person with haemophilia bleeds for longer after an injury and may suffer internal bleeding. The condition is managed with regular injections of the missing clotting factor.
血友病是另一种众所周知的X连锁隐性遗传病，由于缺乏凝血因子VIII（血友病A）或IX（血友病B），血液无法正常凝固。血友病患者受伤后会出血更长时间，并可能出现内出血。通过定期注射缺失的凝血因子来控制病情。
Using the symbols Xᴴ (normal) and Xʰ (haemophilia), a cross between a carrier female (XᴴXʰ) and a normal male (XᴴY) can be analysed. The gametes are Xᴴ and Xʰ from the mother, and Xᴴ and Y from the father. The Punnett square shows the possible combinations: XᴴXᴴ (normal female), XᴴXʰ (carrier female), XᴴY (normal male), and XʰY (haemophiliac male).
使用符号Xᴴ（正常）和Xʰ（血友病），可以分析携带者女性（XᴴXʰ）与正常男性（XᴴY）的杂交。母亲的配子是Xᴴ和Xʰ，父亲的配子是Xᴴ和Y。庞奈特方格显示可能的组合：XᴴXᴴ（正常女性）、XᴴXʰ（携带者女性）、XᴴY（正常男性）和XʰY（血友病男性）。
If a haemophiliac male (XʰY) and a carrier female (XᴴXʰ) have children, there is a chance of having a haemophiliac daughter (XʰXʰ), although such females are extremely rare because two recessive alleles are required. More commonly, a haemophiliac male and a normal homozygous female will produce carrier daughters and normal sons.
如果血友病男性（XʰY）和携带者女性（XᴴXʰ）生育孩子，则有可能生出患血友病的女儿（XʰXʰ），尽管这样的女性极为罕见，因为需要两个隐性等位基因。更常见的是，血友病男性和正常纯合女性将生出携带者女儿和正常儿子。
7. Pedigree Analysis of Sex-linked Traits | 伴性性状的谱系分析
A pedigree chart is used to track the inheritance of a trait through generations. For X-linked recessive disorders, the pedigree typically shows more affected males than females. Affected males often appear in alternating generations, passed from a carrier mother to her son. Crippled or half-shaded symbols may be used to denote carriers.
谱系图用于跟踪某一性状在世代间的遗传。对于X连锁隐性遗传病，谱系通常显示男性患者多于女性。患病男性通常隔代出现，由携带者母亲传给儿子。可以使用点状或半阴影符号来表示携带者。
When reading a pedigree, check the following clues: if an affected male has an affected daughter, the disorder cannot be X-linked recessive because the father would pass his X chromosome to all daughters, making them at least carriers; if all sons of an affected male are normal, it strongly suggests X-linkage because sons receive the father's Y, not his X.
在读取谱系图时，请检查以下线索：如果患病父亲的女儿也患病，则该病不可能是X连锁隐性，因为父亲会将他的X染色体传给所有女儿，使她们至少成为携带者；如果患病父亲的所有儿子都正常，则强烈提示X连锁，因为儿子接受的是父亲的Y染色体，而不是X。
An example: a pedigree shows a woman who is a carrier (half-shaded circle) has a colour-blind son (filled square) and a normal daughter. The carrier woman's mother was also a carrier, and her father was normal. This pattern fits X-linked recessive inheritance perfectly.
例如：一个谱系图显示一位携带者女性（半阴影圆圈）有一个患色盲的儿子（实心方块）和一个正常女儿。这位携带者女性的母亲也是携带者，父亲正常。这种模式完美符合X连锁隐性遗传。
8. Comparison with Autosomal Inheritance | 与常染色体遗传的比较
Autosomal genes are located on chromosomes 1–22 and are inherited equally by both males and females. In autosomal recessive conditions, an affected individual must inherit two recessive alleles, and males and females are affected in roughly equal proportions. However, for X-linked recessive conditions, males are predominantly affected because they have only one X chromosome.
常染色体基因位于1-22号染色体上，男性和女性均等遗传。对于常染色体隐性遗传病，患病个体必须遗传两个隐性等位基因，男女患病的比例大致相等。然而，对于X连锁隐性遗传病，由于男性只有一条X染色体，因此主要影响男性。
A key difference is seen in the offspring of an affected male. In an autosomal recessive trait, an affected father (aa) and a homozygous normal mother (AA) will have all offspring as carriers (Aa) but none affected. In an X-linked recessive trait, an affected father (XᵇY) and a homozygous normal mother (XᴮXᴮ) produce carrier daughters (XᴮXᵇ) and normal sons (XᴮY), with no affected children.
一个关键区别体现在患病父亲的后代中。对于常染色体隐性性状，患病父亲（aa）和纯合正常母亲（AA）的所有后代都是携带者（Aa），但没有人患病。对于X连锁隐性性状，患病父亲（XᵇY）和纯合正常母亲（XᴮXᴮ）生出的女儿都是携带者（XᴮXᵇ），儿子都正常（XᴮY），没有患病孩子。
| Feature | Autosomal Recessive | X-linked Recessive |
|---|---|---|
| Affected sexes | Males and females equally | Mainly males |
| Carrier status | Both sexes can be carriers | Females can be carriers; males cannot |
| Affected father to son | Possible | Not possible (son gets Y from father) |
9. Common Exam Tips and Pitfalls | 常见考点与易错点
A very common mistake in exams is forgetting that the Y chromosome does not carry an allele for X-linked traits. Students sometimes write a genotype for a sex-linked trait on the Y chromosome, like Yᵇ, which is incorrect under standard IGCSE WJEC notation. Always attach the allele to the X chromosome only.
考试中一个非常常见的错误是忘记Y染色体不携带X连锁性状的等位基因。学生有时会在Y染色体上书写伴性性状的基因型，如Yᵇ，这在标准的IGCSE WJEC符号中是不正确的。务必将等位基因只附加在X染色体上。
Another pitfall is misidentifying carriers in a pedigree. A female carrier is phenotypically normal; therefore, in a pedigree she will be represented by a circle with no shading or with a dot/half-shading if specified. Do not assume that a normal-appearing female is homozygous normal unless the genetic evidence suggests so.
另一个易错点是错误识别谱系图中的携带者。女性携带者表现型正常；因此，在谱系图中她会被表示为无阴影的圆圈，或者如果标注明确则带点或半阴影。除非有遗传证据支持，否则不要假定一个表型正常的女性就是纯合正常。
Be precise with genetic crosses: always define the allele symbols clearly at the start. Use Punnett squares correctly, showing gametes and offspring genotypes. Many marks are awarded for setting out the cross logically, even if a final ratio is slightly misstated. Also, remember that the probability of having an affected child is independent for each birth.
处理遗传杂交题时要精确：一开始就要清晰地定义等位基因符号。正确使用庞奈特方格，标明配子和后代基因型。即使最终比例略有表述错误，只要杂交步骤逻辑清晰，通常能获得大部分分数。同时要记住，每次生育出现患病孩子的概率是独立的。
10. Step-by-step Problem Solving | 分步解题法
To solve a sex-linked genetics problem, follow these steps: (1) identify whether the trait is X-linked recessive from the description or pedigree pattern; (2) assign allele symbols, e.g., Xᴺ for normal and Xⁿ for the recessive allele; (3) determine the genotypes of the parents using the information given; (4) determine the gametes each parent can produce – remember that a male produces two types of sperm: carrying X or carrying Y; (5) construct a Punnett square; (6) list the genotypes and phenotypes of offspring; (7) calculate the probability or ratio asked for in the question.
要解决伴性遗传问题，可遵循以下步骤：（1）根据描述或谱系模式判断该性状是否为X连锁隐性；（2）分配等位基因符号，例如Xᴺ代表正常，Xⁿ代表隐性等位基因；（3）根据所给信息确定亲本的基因型；（4）确定每个亲本可以产生的配子——记住雄性产生两种类型的精子：携带X或携带Y；（5）绘制庞奈特方格；（6）列出后代的基因型和表现型；（7）计算问题要求的概率或比例。
For instance, a typical question might state: 'A woman whose father was colour-blind marries a man with normal vision. What is the probability that their son will be colour-blind?' Since the woman's father was colour-blind (XᵇY), she must have inherited her father's Xᵇ and is therefore a carrier (XᴮXᵇ). The normal-vision man is XᴮY. The cross gives a 25% chance of a colour-blind son (XᵇY).
例如
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