Sex-linked Inheritance in A-Level Biology | 伴性遗传考点精讲

📚 Sex-linked Inheritance in A-Level Biology | 伴性遗传考点精讲

Sex-linked inheritance refers to the transmission of genes located on the sex chromosomes, primarily the X chromosome in humans. Because males have only one X chromosome, any recessive allele on the X will be expressed in the phenotype, making sex-linked disorders far more common in males. This topic is a staple of A-Level Biology, requiring a clear understanding of genetic crosses, pedigree interpretation, and the molecular basis of traits like haemophilia and red-green colour blindness.

伴性遗传是指位于性染色体（主要是人类的X染色体）上的基因的传递方式。由于男性只有一条X染色体，X染色体上的任何隐性等位基因都会在表现型中表达，这使得伴性遗传疾病在男性中更为常见。这个主题是A-Level生物学的核心考点，需要清晰理解遗传杂交、系谱解读以及血友病和红绿色盲等性状的分子基础。

1. Sex Chromosomes and Sex Determination | 性染色体与性别决定

In humans, sex is determined by a pair of sex chromosomes: females have two X chromosomes (XX), while males have one X and one smaller Y chromosome (XY). The Y chromosome carries the SRY gene that triggers male development, but it carries very few other functional genes compared to the X chromosome. Consequently, males are hemizygous for X-linked genes – they possess only one allele for any gene on the X chromosome.

人类的性别由一对性染色体决定：女性有两条X染色体（XX），而男性有一条X染色体和一条较小的Y染色体（XY）。Y染色体携带SRY基因，触发男性发育，但与X染色体相比，它携带的其他功能基因很少。因此，对于X连锁基因，男性是半合子——对于X染色体上的任何基因，他们只拥有一个等位基因。

• Female genotype: XX
• Male genotype: XY
• Female genotype (中文): 女性基因型：XX
• Male genotype (中文): 男性基因型：XY

2. X-linked Recessive Inheritance | X连锁隐性遗传

X-linked recessive traits are caused by recessive alleles on the X chromosome. Because males have only one X chromosome, a single copy of the recessive allele is sufficient to cause the trait. Females, having two X chromosomes, must inherit two copies of the recessive allele to express the trait; otherwise, they are carriers. Carrier females have one normal dominant allele and one recessive allele, and they do not usually show the trait, though occasionally mild symptoms may appear due to X-inactivation.

X连锁隐性性状由X染色体上的隐性等位基因引起。由于男性只有一条X染色体，一个隐性等位基因就足以导致该性状。女性有两条X染色体，必须遗传两个隐性等位基因才会表达该性状；否则，她们是携带者。携带者女性有一个正常显性等位基因和一个隐性等位基因，她们通常不表现该性状，尽管偶尔由于X失活可能出现轻微症状。

Allele notation example: Xᴴ = normal allele; Xʰ = recessive disease allele
等位基因符号示例：Xᴴ = 正常等位基因；Xʰ = 隐性致病等位基因

Genotype (基因型)	Phenotype (表现型)
XᴴXᴴ	Normal female (正常女性)
XᴴXʰ	Carrier female (携带者女性)
XʰXʰ	Affected female (患病女性)
XᴴY	Normal male (正常男性)
XʰY	Affected male (患病男性)

3. Haemophilia: A Case Study | 血友病案例研究

Haemophilia is a classic example of an X-linked recessive disorder. It results from a mutation in the gene encoding clotting factor VIII (haemophilia A) or factor IX (haemophilia B), leading to impaired blood clotting. Affected individuals bleed for longer after injury and may suffer internal bleeding. The gene is located on the X chromosome, so the condition primarily affects males, while females are typically carriers. Queen Victoria was a carrier of haemophilia, and the disease spread through several European royal families, illustrating the pattern of X-linked recessive inheritance.

血友病是X连锁隐性遗传疾病的经典例子。它由编码凝血因子VIII（血友病A）或因子IX（血友病B）的基因突变引起，导致凝血功能受损。患者受伤后出血时间延长，并可能发生内出血。该基因位于X染色体上，因此该病主要影响男性，而女性通常是携带者。维多利亚女王是血友病携带者，该病在多个欧洲皇室家族中传播，清晰地展示了X连锁隐性遗传的模式。

4. Red-Green Colour Blindness | 红绿色盲

Red-green colour blindness is another common X-linked recessive condition. The genes for the red and green photopigments are located close together on the X chromosome. Mutations or unequal crossing over can disrupt these pigments, making it difficult to distinguish red from green hues. Approximately 8% of males of Northern European ancestry are affected, whereas only about 0.5% of females are affected, because a female would need two defective X chromosomes. The Ishihara colour test is commonly used to diagnose this condition.

红绿色盲是另一种常见的X连锁隐性遗传疾病。红色和绿色感光色素的基因紧密位于X染色体上。突变或不平等交叉会破坏这些色素，使患者难以区分红色和绿色色调。大约8%的北欧血统男性受到影响，而只有约0.5%的女性受影响，因为女性需要两条有缺陷的X染色体。石原色彩测试通常用于诊断此症。

5. Genetic Crosses for X-linked Recessive Traits | X连锁隐性性状的遗传杂交

When solving genetic problems, it is essential to set out the parental genotypes and gametes, then construct a Punnett square. For an X-linked cross, the sex chromosomes must be included in the gametes. A common cross is between a carrier female (XᴴXʰ) and a normal male (XᴴY). The offspring genotypes are: XᴴXᴴ (normal female), XᴴXʰ (carrier female), XᴴY (normal male), XʰY (affected male). Thus, there is a 25% chance of an affected son, a 25% chance of a carrier daughter, and no affected daughters.

在解决遗传问题时，必须列出亲本基因型和配子，然后构建庞纳特方格。对于X连锁杂交，配子中必须包含性染色体。一个常见的杂交是携带者女性（XᴴXʰ）与正常男性（XᴴY）交配。子代基因型为：XᴴXᴴ（正常女性）、XᴴXʰ（携带者女性）、XᴴY（正常男性）、XʰY（患病男性）。因此，有25%的概率生出患病儿子，25%的概率生出携带者女儿，没有患病女儿。

Punnett square: Carrier female × Normal male (庞纳特方格：携带者女性 × 正常男性)

	Xᴴ (from father)	Y (from father)
Xᴴ (from mother)	XᴴXᴴ	XᴴY
Xʰ (from mother)	XᴴXʰ	XʰY

6. X-linked Dominant Inheritance | X连锁显性遗传

X-linked dominant disorders are rarer. In these cases, a single dominant allele on the X chromosome causes the trait in both males and females, though often males are more severely affected. Affected males will pass the trait to all of their daughters (since they give their X chromosome to daughters) but to none of their sons (sons receive the Y chromosome). Affected heterozygous females will pass the trait to 50% of their children, regardless of sex, if they have one dominant allele (Xᴰ). An example is vitamin D-resistant rickets (hypophosphatemic rickets).

X连锁显性遗传疾病比较罕见。在这种情况下，X染色体上的单个显性等位基因会在男性和女性中引起该性状，尽管男性通常更严重。患病的男性会将此性状传递给所有女儿（因为她们从父亲那里获得X染色体），但不会传递给任何儿子（儿子获得Y染色体）。患病的杂合女性（XᴰXᵈ）如果有一个显性等位基因，会将性状传递给50%的子女，无论性别。例如维生素D抵抗性佝偻病（低磷血症性佝偻病）。

Genotype (基因型)	Phenotype (表现型)
XᴰXᴰ	Affected female (患病女性)
XᴰXᵈ	Affected female (milder) (患病女性，较轻)
XᵈXᵈ	Normal female (正常女性)
XᴰY	Affected male (患病男性)
XᵈY	Normal male (正常男性)

7. Y-linked Inheritance | Y连锁遗传

Y-linked traits are determined by genes on the Y chromosome. Because the Y chromosome is present only in males, these traits are passed exclusively from father to son. The Y chromosome is small and carries relatively few genes, most of which are involved in male sex determination and spermatogenesis. Y-linked inheritance is sometimes called holandric inheritance. A well-known example is the SRY gene itself, but other traits like hairy ears (hypertrichosis pinnae) have been suggested to be Y-linked, though the evidence is debated.

Y连锁性状由Y染色体上的基因决定。由于Y染色体只存在于男性中，这些性状只能从父亲传给儿子。Y染色体较小，携带的基因相对较少，其中大多数与男性性别决定和精子发生有关。Y连锁遗传有时被称为全雄遗传。一个著名的例子是SRY基因本身，但其他性状如耳廓多毛症（hypertrichosis pinnae）被认为可能是Y连锁的，尽管证据尚有争议。

8. Pedigree Analysis for Sex-linked Traits | 伴性性状的系谱分析

Pedigree diagrams are used to trace the inheritance of traits through generations. For X-linked recessive traits, classical features include: more males than females are affected; affected males do not pass the trait to their sons; all daughters of an affected male are carriers (if the mother is unaffected); carrier females will have, on average, 50% affected sons. For X-linked dominant traits, an affected male parent produces all affected daughters and no affected sons; an affected female (heterozygous) produces 50% affected offspring of both sexes. These patterns help distinguish sex-linked inheritance from autosomal inheritance.

系谱图用于追踪性状在世代中的遗传。对于X连锁隐性性状，经典特征包括：患病的男性多于女性；患病的男性不会将性状传给儿子；患病男性的所有女儿都是携带者（如果母亲未患病）；携带者女性平均会有50%的儿子患病。对于X连锁显性性状，患病的父亲所有女儿都患病，儿子都不患病；患病的女性（杂合子）会产生50%的患病后代，不分男女。这些模式有助于区分性连锁遗传和常染色体遗传。

9. Key Features of Sex-linked Inheritance | 伴性遗传的关键特征

Summarising the key features for exams: (1) Males are more frequently affected by X-linked recessive disorders. (2) The trait often skips generations via carrier females. (3) Affected males cannot pass the condition to their sons (no male-to-male transmission). (4) Daughters of affected males are obligate carriers for recessive traits. (5) For X-linked dominant traits, affected males pass the trait to all daughters, and affected heterozygous females have a 50% chance of passing the trait to any child. (6) The use of superscript allele notation on the X chromosome is standard.

为考试总结关键特征：（1）男性更常受到X连锁隐性遗传疾病的影响。（2）该性状常通过携带者女性隔代相传。（3）患病男性不能将疾病传给儿子（无男传男现象）。（4）患病男性的女儿必然是隐性性状的携带者。（5）对于X连锁显性性状，患病男性将性状传给所有女儿，患病的杂合女性有50%的概率将性状传给每个孩子。（6）在X染色体上使用上标等位基因符号是标准做法。

Criss-cross inheritance: affected male → carrier daughters → affected grandsons
交叉遗传：患病男性 → 携带者女儿 → 患病外孙

10. Summary and Exam Tips | 总结与考试技巧

When tackling exam questions on sex-linked inheritance, always identify the pattern of transmission: if more males are affected, suspect X-linked recessive; if affected males always produce affected daughters, suspect X-linked dominant. Practice drawing genetic diagrams carefully, using clear symbols (Xᴴ, Xʰ, etc.), and state the probability of affected offspring as a ratio or percentage. Remember that the Y chromosome does not carry the corresponding allele, so it is written as Y without a superscript for X-linked genes. Also, be prepared to explain why females can be carriers but rarely suffer from X-linked recessive disorders – the possession of a second normal X chromosome masks the recessive allele.

在处理关于伴性遗传的考试题目时，首先要识别遗传模式：如果男性患者较多，怀疑X连锁隐性；如果患病男性总是生患病女儿，怀疑X连锁显性。练习仔细绘制遗传图解，使用清晰的符号（Xᴴ, Xʰ等），并将患病后代的概率表示为比率或百分比。记住Y染色体不携带对应的等位基因，因此在X连锁基因中书写为Y，没有上标。同时，准备解释为什么女性可以是携带者却很少患X连锁隐性遗传病——拥有第二条正常的X染色体掩盖了隐性等位基因。

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