Sex-linked Inheritance in GCSE OCR Biology | GCSE OCR 生物：伴性遗传考点精讲

📚 Sex-linked Inheritance in GCSE OCR Biology | GCSE OCR 生物：伴性遗传考点精讲

In GCSE OCR Biology, sex-linked inheritance refers to the pattern of genetic transmission determined by genes located on the sex chromosomes. Understanding how conditions such as colour blindness and haemophilia are passed from one generation to the next is a key part of the genetics topic. This article breaks down the core concepts, worked examples, and exam techniques you need to master sex-linked traits.

在 GCSE OCR 生物学中，伴性遗传是指由位于性染色体上的基因决定的遗传传递模式。理解色盲和血友病等疾病如何代代相传是遗传学专题的核心内容。本文为你梳理了必须掌握的核心概念、计算示例和考试技巧，帮助你彻底攻克伴性性状。

1. Introduction to Sex-linked Inheritance | 伴性遗传简介

Most traits are controlled by genes located on the 22 pairs of autosomes (non-sex chromosomes). However, some important characteristics are determined by genes found on the 23rd pair – the sex chromosomes X and Y. When a gene is located on a sex chromosome, the pattern of inheritance does not follow the simple dominant/recessive rules of autosomal traits. Instead, the biological sex of an individual influences how the trait appears. This is known as sex-linked inheritance.

大多数性状由位于22对常染色体（非性染色体）上的基因控制。然而，一些重要的特征是由第23对染色体——性染色体 X 和 Y 上的基因决定的。当基因位于性染色体上时，其遗传模式并不遵循常染色体性状简单的显性/隐性规律。相反，个体的生物性别会影响性状的表现。这被称为伴性遗传。

2. Sex Determination in Humans | 人类的性别决定

Human body cells contain 46 chromosomes arranged in 23 pairs. The sex of an individual is controlled by the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome is much smaller and carries fewer genes. During fertilisation, an egg always carries one X chromosome, whereas a sperm can carry either an X or a Y. The combination of the sperm’s sex chromosome with the egg’s X determines the zygote’s sex: an XX combination produces a female, and an XY combination produces a male.

人体细胞含有46条染色体，排列成23对。个体的性别由性染色体控制。女性有两条 X 染色体（XX），而男性有一条 X 和一条 Y 染色体（XY）。Y 染色体小得多，携带的基因也更少。在受精过程中，卵子总是携带一条 X 染色体，而精子可能携带 X 或 Y。精子的性染色体与卵子的 X 结合，决定了受精卵的性别：XX 组合发育为女性，XY 组合发育为男性。

3. The X and Y Chromosomes | X 和 Y 染色体

The X chromosome is large and carries over 1000 genes, many of which are essential for both sexes. The Y chromosome is small and mainly carries genes that trigger male development, such as the SRY gene. Critically, many genes present on the X chromosome have no counterpart on the Y chromosome. This means males have only one copy of these genes – any recessive allele on their single X will be expressed because there is no dominant allele on the Y to mask it. Females, having two X chromosomes, can be heterozygous carriers of a recessive allele without showing the trait.

X 染色体较大，携带超过1000个基因，其中许多对两性都至关重要。Y 染色体较小，主要携带触发男性发育的基因，如 SRY 基因。关键是，X 染色体上的许多基因在 Y 染色体上没有对应的等位基因。这意味着男性这些基因只有一份拷贝——其唯一的 X 染色体上的任何隐性等位基因都会表达，因为 Y 染色体上没有显性等位基因来掩盖它。女性因为有两条 X 染色体，可以是隐性等位基因的杂合子携带者而不表现出该性状。

4. What are Sex-linked Genes? | 什么是伴性基因？

A sex-linked gene is any gene located on a sex chromosome. In GCSE Biology, we focus almost exclusively on genes found on the X chromosome. X-linked recessive disorders are the most commonly examined. Because males are hemizygous for X-linked genes (possessing only one allele), they are far more likely to exhibit a recessive X-linked condition than females. Females must inherit two recessive alleles (one from each parent) to be affected, which is statistically less likely.

伴性基因是位于性染色体上的任何基因。在 GCSE 生物学中，我们几乎只关注 X 染色体上的基因。X 连锁隐性遗传病是最常考查的内容。由于男性是 X 连锁基因的半合子（只拥有一个等位基因），他们表现出隐性 X 连锁疾病的概率远高于女性。女性必须遗传两个隐性等位基因（分别来自父母）才会患病，这在统计上较不可能发生。

5. Red-Green Colour Blindness | 红绿色盲

Red-green colour blindness is an X-linked recessive disorder that affects the ability to distinguish between red and green hues. The gene responsible codes for photoreceptor proteins in the retina. The normal allele is dominant (let’s represent it as X^N), while the recessive allele causing colour blindness is Xⁿ. A colour-blind male has the genotype XⁿY. A female can be unaffected (X^NX^N), a carrier (X^NXⁿ), or affected (XⁿXⁿ). Carrier females have normal colour vision but can pass the recessive allele to their offspring.

红绿色盲是一种 X 连锁隐性遗传病，影响区分红色和绿色色调的能力。相关基因编码视网膜中的感光蛋白。正常等位基因为显性（我们用 X^N 表示），导致色盲的隐性等位基因为 Xⁿ。患色盲的男性基因型为 XⁿY。女性可能为正常（X^NX^N）、携带者（X^NXⁿ）或患病（XⁿXⁿ）。携带者女性拥有正常的色觉，但能将隐性等位基因传递给后代。

6. Haemophilia | 血友病

Haemophilia is another well-known X-linked recessive condition in which the blood fails to clot properly due to a deficiency in clotting factors. The standard notation uses X^H for the normal allele and X^h for the haemophilia allele. The genotypes and patterns of inheritance are directly analogous to colour blindness. A male with haemophilia (X^hY) will suffer from uncontrolled bleeding, while a female would need the genotype X^hX^h to be affected – a much rarer occurrence.

血友病是另一种著名的 X 连锁隐性遗传病，由于凝血因子缺乏，血液无法正常凝固。标准记法中，用 X^H 表示正常等位基因，X^h 表示血友病等位基因。其基因型与遗传模式与色盲直接类似。患血友病的男性（X^hY）会遭受无法控制的出血，而女性必须具有基因型 X^hX^h 才会患病——这是一种罕见得多的情形。

7. Understanding Alleles: Dominant and Recessive | 理解等位基因：显性和隐性

In sex-linked inheritance, the concept of dominance operates differently in males and females. For a female with two X chromosomes, the usual dominance rules apply: one dominant allele can mask a recessive allele. For a male, however, ‘dominant’ and ‘recessive’ refer only to the trait if the allele were present on an X chromosome. Since the Y does not carry an equivalent allele, any allele on the X, whether classed as dominant or recessive in females, will always be expressed in the male phenotype. Therefore, X-linked recessive conditions are much more common in males.

在伴性遗传中，显性概念在男性和女性中运作不同。对于有两条 X 染色体的女性，通常的显性规则适用：一个显性等位基因可以掩盖隐性等位基因。然而，对于男性而言，“显性”和“隐性”仅指该等位基因如果位于 X 染色体上对应性状的术语。由于 Y 染色体不携带对等位基因，X 染色体上的任何等位基因，不论在女性中被归类为显性还是隐性，都会在男性表型中表达。因此，X 连锁隐性病症在男性中要常见得多。

8. Punnett Squares for Sex-linked Traits | 伴性性状的庞纳特方格

When drawing genetic crosses for sex-linked inheritance, you must include the sex chromosomes in gametes. The Punnett square displays both the allele and the chromosome. For example, a cross between a carrier female (X^NXⁿ) and a normal male (X^NY) will show gametes as X^N, Xⁿ from the mother and X^N, Y from the father. The resulting offspring genotypes are X^NX^N, X^NXⁿ, X^NY, and XⁿY. This reveals a 25% chance of a colour-blind male and 0% chance of a colour-blind female in this cross.

在绘制伴性遗传的遗传杂交图时，您必须将性染色体包含在配子中。庞纳特方格同时显示等位基因和染色体。例如，一位携带者女性（X^NXⁿ）和一位正常男性（X^NY）的杂交，其母方配子为 X^N、Xⁿ，父方配子为 X^N、Y。产生的后代基因型为 X^NX^N、X^NXⁿ、X^NY 和 XⁿY。这表明该杂交中有 25% 的概率出生一个色盲男孩，而色盲女孩的概率为 0%。

9. Drawing Genetic Crosses – Example 1: Colour Blindness | 绘制遗传杂交图 – 例1：色盲

Let’s work through a cross between a colour-blind male (XⁿY) and a homozygous normal female (X^NX^N). The male’s gametes are Xⁿ and Y; the female’s gametes are all X^N. The Punnett square yields daughters who are all carriers (X^NXⁿ) and sons who are all normal (X^NY). There is no child with colour blindness in this generation, but all daughters carry the recessive allele, which can reappear in grandsons.

我们来看一个色盲男性（XⁿY）与纯合正常女性（X^NX^N）的杂交。男性配子为 Xⁿ 和 Y；女性配子全部为 X^N。庞纳特方格得出所有女儿都是携带者（X^NXⁿ），所有儿子都正常（X^NY）。这一代没有儿童患有色盲，但所有女儿都携带隐性等位基因，可能在孙辈的男孩身上重新出现。

Now consider a carrier female (X^NXⁿ) and a colour-blind male (XⁿY). The female gametes: X^N and Xⁿ; male gametes: Xⁿ and Y. The Punnett square gives X^NXⁿ (carrier female), XⁿXⁿ (colour-blind female), X^NY (normal male), XⁿY (colour-blind male). Thus, there is a 25% chance for each type, including the relatively rare occurrence of a colour-blind female.

现在考虑一位携带者女性（X^NXⁿ）和一位色盲男性（XⁿY）。女性配子：X^N 和 Xⁿ；男性配子：Xⁿ 和 Y。庞纳特方格产生 X^NXⁿ（携带者女性）、XⁿXⁿ（色盲女性）、X^NY（正常男性）、XⁿY（色盲男性）。因此，每个类型都有 25% 的概率，包括相对罕见的色盲女性。

10. Drawing Genetic Crosses – Example 2: Haemophilia | 绘制遗传杂交图 – 例2：血友病

The cross for haemophilia follows the same pattern. A haemophiliac male (X^hY) and a normal non-carrier female (X^HX^H) produce all carrier daughters and all normal sons. If a carrier daughter (X^HX^h) eventually mates with a normal male (X^HY), the possible offspring are: X^HX^H, X^HX^h, X^HY, X^hY. There is a 50% chance that a son will have haemophilia and a 0% chance that a daughter will be affected, although daughters have a 50% chance of being carriers.

血友病的杂交遵循相同模式。一位患血友病的男性（X^hY）与一位正常的非携带者女性（X^HX^H）生育，会产生全部为携带者的女儿和全部正常的儿子。如果一位携带者女儿（X^HX^h）最终与一位正常男性（X^HY）结合，可能的后代为：X^HX^H、X^HX^h、X^HY、X^hY。儿子患血友病的概率是 50%，女儿患病的概率是 0%，但女儿有 50% 概率成为携带者。

11. Pedigree Charts and Sex-linked Inheritance | 系谱图与伴性遗传

Pedigree charts (family trees) can help deduce the pattern of inheritance. When a condition is X-linked recessive, you will observe that affected individuals are mostly male. An affected father cannot pass the trait to his sons (since he gives his Y to sons), but all his daughters will be carriers. Carrier females pass the condition to about half of their sons. No male-to-male transmission occurs. These clues are vital for interpreting pedigree-based exam questions in OCR Biology.

系谱图（家系图）可帮助推断遗传模式。当某种疾病是 X 连锁隐性遗传时，您会观察到患者大多为男性。患病的父亲不能将该性状传递给儿子（因为他给儿子的是 Y），但他的所有女儿都将是携带者。携带者女性将疾病传递给她大约一半的儿子。不会发生男→男传递。这些线索对于解读 OCR 生物学中基于系谱图的考题至关重要。

12. Key Points & Exam Tips | 关键点与考试技巧

Always include sex chromosomes in genetic diagrams for sex-linked traits. Use superscript notation (e.g., X^N, Xⁿ) to show alleles on the X chromosome. The Y is written without a superscript allele.

务必在伴性性状的遗传图解中包含性染色体。使用上标记法（如 X^N、Xⁿ）表示 X 染色体上的等位基因。Y 染色体写时不带上标等位基因。
Males cannot be carriers of X-linked recessive conditions; they either have the condition or they do not. A single recessive allele on the X causes the trait to appear.

男性不可能是 X 连锁隐性疾病的携带者；他们要么患病，要么不患病。X 染色体上的单个隐性等位基因就会导致性状出现。
When explaining why a condition is more common in males, state that males have only one X chromosome, so a recessive allele on it is not masked by a dominant allele on a second X (or on the Y).

在解释为何某种疾病在男性中更常见时，要表述为：男性只有一条 X 染色体，因此其上的隐性等位基因不会被第二条 X（或 Y）上的显性等位基因所掩盖。
Use Punnett squares accurately. Show both gametes and offspring genotypes clearly. Always convert genotype probabilities into phenotypes and relate them to the question.

准确使用庞纳特方格。清晰地展示配子和后代基因型。始终将基因型概率转化为表型，并与问题对应起来。
In pedigree analysis, look for the pattern: an affected mother will have all affected sons if the condition is X-linked recessive; an affected father has all carrier daughters.

在系谱分析中，寻找以下模式：若疾病为 X 连锁隐性，患病的母亲会生下所有患病的儿子；患病的父亲所有女儿都是携带者。

Mastering sex-linked inheritance requires both conceptual understanding and practice with diagrammatic representations. By following these guidelines and revising the worked examples, you can ensure maximum marks on this topic in your GCSE OCR Biology exam.

掌握伴性遗传既需要概念理解，也需要图解表示法的练习。遵循这些指南并复习所示示例，您可以确保在 GCSE OCR 生物学考试中拿下该主题的最高分。

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Sex-linked Inheritance in GCSE OCR Biology | GCSE OCR 生物：伴性遗传 考点精讲