Sex-linked Inheritance for WJEC A-Level Biology | A-Level WJEC 生物伴性遗传考点精讲

📚 Sex-linked Inheritance for WJEC A-Level Biology | A-Level WJEC 生物伴性遗传考点精讲

Sex-linked inheritance refers to the transmission of genes located on the sex chromosomes, typically the X chromosome in humans. In WJEC A-Level Biology, understanding sex-linked traits such as red-green colour blindness and haemophilia is essential for analysing genetic crosses and interpreting pedigree charts. This article breaks down the key concepts, worked examples, and exam techniques to help you master this topic.

伴性遗传是指位于性染色体（通常是人类X染色体）上基因的传递。在WJEC A-Level 生物中，理解红绿色盲和血友病等伴性性状对于分析遗传杂交和解读系谱图至关重要。本文将拆解核心概念、详解典型例题和应试技巧，助你精通此专题。

1. Introduction to Sex Determination and Sex Chromosomes | 性别决定与性染色体简介

In humans, sex is determined by the 23rd pair of chromosomes, known as the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome carries the SRY gene, which triggers male development. Because the X chromosome is larger and carries many genes unrelated to sex determination, mutations on the X chromosome can lead to sex-linked conditions.

人类的性别由第23对染色体（即性染色体）决定。女性有两条X染色体（XX），男性有一条X和一条Y染色体（XY）。Y染色体携带SRY基因，该基因启动男性发育过程。由于X染色体较大且携带大量与性别决定无关的基因，X染色体上的突变可能导致伴性遗传病。

2. What is Sex-linked Inheritance? | 什么是伴性遗传？

Sex-linked inheritance is the pattern of inheritance for genes located on the sex chromosomes. Most sex-linked genes are found on the X chromosome; Y-linked genes are rare. Because males have only one X chromosome, they are hemizygous for X-linked genes – meaning they express the allele they inherit, whether dominant or recessive. This makes males more susceptible to X-linked recessive disorders.

伴性遗传是性染色体上基因的遗传模式。绝大多数的伴性基因位于X染色体上；Y连锁基因极为少见。由于男性只有一条X染色体，他们对X连锁基因呈半合子状态——即无论显性还是隐性，他们都会表达所继承的等位基因。这使男性更容易患上X连锁隐性遗传病。

3. X-linked Recessive Inheritance: Basic Principles | X连锁隐性遗传的基本原理

In X-linked recessive inheritance, the disease allele is recessive and located on the X chromosome. A female must inherit two copies of the recessive allele (one from each parent) to show the trait, while a male needs only one copy (from his mother) because he lacks a second X to mask it. Affected fathers cannot pass the trait to their sons (since they give only the Y chromosome to sons), but all their daughters become carriers if the mother is unaffected.

在X连锁隐性遗传中，致病等位基因为隐性并位于X染色体上。女性必须从父母双方各继承一个隐性等位基因才会表现出性状，而男性只需从母亲处继承一个致病基因即可发病，因为他没有第二条X染色体来掩盖该基因。患病父亲不会将性状传给儿子（因为他传给儿子的只是Y染色体），但如果母亲正常，他的全部女儿都将成为携带者。

4. Example: Red-Green Colour Blindness | 实例：红绿色盲

Red-green colour blindness is a classic example of an X-linked recessive condition caused by a mutation in genes coding for photopigments in cone cells. Using the alleles: Xᴮ = normal vision (dominant), Xᵇ = colour blindness (recessive). A heterozygous female (XᴮXᵇ) has normal vision but is a carrier. A colour-blind male is XᵇY. Let’s consider a cross between a carrier female and a normal male (XᴮY). The possible offspring are: XᴮXᴮ (normal female), XᴮXᵇ (carrier female), XᴮY (normal male), XᵇY (colour-blind male). Thus, each son has a 50% chance of being colour-blind from a carrier mother.

红绿色盲是X连锁隐性遗传的典型例子，由视锥细胞中编码感光色素的基因突变引起。使用等位基因符号：Xᴮ = 正常视觉（显性），Xᵇ = 色盲（隐性）。杂合子女性（XᴮXᵇ）视力正常但为携带者。色盲男性为XᵇY。考虑携带者女性和正常男性（XᴮY）的杂交。可能的后代为：XᴮXᴮ（正常女性），XᴮXᵇ（携带者女性），XᴮY（正常男性），XᵇY（色盲男性）。因此，若母亲为携带者，每个儿子患色盲的概率为50%。

5. Example: Haemophilia | 实例：血友病

Haemophilia A is caused by a deficiency of clotting factor VIII, inherited in an X-linked recessive manner. Alleles: Xᴴ = normal clotting (dominant), Xʰ = haemophilia (recessive). A haemophiliac male (XʰY) and a homozygous normal female (XᴴXᴴ) will have all carrier daughters (XᴴXʰ) and all normal sons (XᴴY). If a carrier female (XᴴXʰ) mates with a normal male (XᴴY), their sons have a 50% risk of haemophilia. Unlike colour blindness, haemophilia significantly affects blood clotting, and patients require careful management.

血友病A由凝血因子VIII缺乏引起，以X连锁隐性方式遗传。等位基因符号：Xᴴ = 正常凝血（显性），Xʰ = 血友病（隐性）。一位血友病男性（XʰY）与纯合正常女性（XᴴXᴴ）所生子女：全部女儿为携带者（XᴴXʰ），全部儿子正常（XᴴY）。若携带者女性（XᴴXʰ）与正常男性（XᴴY）结合，他们的儿子患血友病的风险为50%。与色盲不同的是，血友病严重影响凝血功能，患者需要精细的医疗管理。

6. Characteristics of X-linked Recessive Pedigrees | X连锁隐性的系谱特征

When analysing pedigrees, X-linked recessive disorders show distinct patterns: more males are affected than females; affected males are usually born to unaffected parents (the mother is a carrier); no male-to-male transmission; daughters of affected males are all carriers (if the mother is normal); and the trait can skip generations. These features help distinguish X-linked recessive from autosomal recessive inheritance.

在分析系谱时，X连锁隐性遗传病表现出鲜明的特征：患病男性远多于女性；患病男孩通常由表现正常的父母所生（母亲为携带者）；没有男传男现象；患病男性的所有女儿均为携带者（若母亲正常）；性状可能隔代出现。这些特征有助于区分X连锁隐性与常染色体隐性遗传。

Beyond X-linked recessive, it is useful to compare with other sex-linked patterns:

除X连锁隐性外，比较其他伴性遗传模式也很有用：