Sex-linked Inheritance | 伴性遗传考点精讲

📚 Sex-linked Inheritance | 伴性遗传考点精讲

Sex-linked inheritance refers to the inheritance of traits that are determined by genes located on the sex chromosomes, typically the X chromosome. In GCSE AQA Biology, understanding how these traits are passed on is crucial, especially for disorders like haemophilia and red-green colour blindness. Because males have only one X chromosome, they are more likely to express recessive sex-linked traits, while females can be carriers without showing the condition.

伴性遗传是指由位于性染色体（通常是X染色体）上的基因所决定的性状遗传方式。在GCSE AQA生物课程中，理解这些性状如何传递至关重要，尤其是血友病和红绿色盲等疾病。由于男性只有一条X染色体，他们更有可能表现出隐性伴性性状，而女性则可以成为携带者而不表现出症状。

1. What are Sex Chromosomes? | 什么是性染色体？

Humans have 23 pairs of chromosomes in each body cell. One of these pairs determines the sex of the individual and is called the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The other 22 pairs are called autosomes.

人体每个体细胞中有23对染色体。其中一对决定个体的性别，称为性染色体。女性有两条X染色体（XX），而男性有一条X染色体和一条Y染色体（XY）。其余22对染色体称为常染色体。

2. The Genetic Basis of Sex Determination | 性别决定的遗传基础

Sex is determined at fertilisation. All egg cells carry a single X chromosome. Sperm cells can carry either an X chromosome or a Y chromosome. If an X-carrying sperm fertilises the egg, the offspring is female (XX). If a Y-carrying sperm fertilises the egg, the offspring is male (XY). The chance of having a boy or a girl is always 50%.

性别在受精时决定。所有卵细胞都携带一条X染色体。精子细胞可以携带一条X染色体或一条Y染色体。如果携带X染色体的精子使卵子受精，后代就是女性（XX）。如果携带Y染色体的精子使卵子受精，后代就是男性（XY）。生男孩或女孩的概率始终是50%。

3. Why are Sex-linked Traits More Common in Males? | 为什么伴性性状在男性中更常见？

Most sex-linked traits are controlled by recessive alleles on the X chromosome. Because males have only one X chromosome, they cannot mask a recessive allele with a dominant one on another X chromosome. Therefore, if a male inherits the recessive allele on his single X, he will express the trait. Females, with two X chromosomes, need two copies of the recessive allele to show the trait.

大多数伴性性状由X染色体上的隐性等位基因控制。因为男性只有一条X染色体，他们无法用另一条X染色体上的显性等位基因掩盖隐性等位基因。因此，如果男性在他的唯一一条X染色体上遗传了隐性等位基因，他就会表现出该性状。女性有两条X染色体，需要两个隐性等位基因的拷贝才会表现出该性状。

4. Common Examples of Sex-linked Disorders | 常见的伴性遗传病实例

The two classic examples examined in GCSE AQA Biology are haemophilia and red-green colour blindness. Both are caused by recessive alleles on the X chromosome. Haemophilia affects the blood's ability to clot, leading to excessive bleeding. Red-green colour blindness means affected individuals cannot distinguish between red and green shades.

GCSE AQA生物考试中常见的两个例子是血友病和红绿色盲。两者都由X染色体上的隐性等位基因引起。血友病影响血液的凝血能力，导致出血过多。红绿色盲意味着患者无法区分红色和绿色调。

5. Genetic Notation for Sex-linked Alleles | 伴性等位基因的遗传学符号表示

In the exam, you need to represent alleles correctly. Use superscript letters on the X chromosome. For example, for haemophilia, Xᴴ represents the normal dominant allele (healthy), and Xʰ represents the recessive allele (haemophiliac). The Y chromosome does not carry this allele, so it is written as Y without a superscript. A carrier female is Xᴴ Xʰ, a normal female is Xᴴ Xᴴ, and a haemophiliac female is Xʰ Xʰ. A normal male is Xᴴ Y, and a haemophiliac male is Xʰ Y.

在考试中，你需要正确地表示等位基因。在X染色体上使用上标字母。例如，对于血友病，Xᴴ 代表正常的显性等位基因（健康），Xʰ 代表隐性等位基因（血友病）。Y染色体不携带这个等位基因，因此写作 Y，没有上标。女性携带者用 Xᴴ Xʰ 表示，正常女性是 Xᴴ Xᴴ，血友病女性是 Xʰ Xʰ。正常男性是 Xᴴ Y，血友病男性是 Xʰ Y。

6. Punnett Squares for Sex-linked Inheritance | 伴性遗传的庞尼特方格

Punnett squares allow you to predict the probability of inheriting sex-linked disorders. When setting up the square, you must include the sex chromosomes with the superscript alleles. For example, a cross between a normal male (Xᴴ Y) and a carrier female (Xᴴ Xʰ) will produce offspring with different genotypes. The gametes from the mother are Xᴴ and Xʰ, while the father's gametes are Xᴴ and Y.

庞尼特方格能让你预测遗传伴性遗传病的概率。在设定方格时，必须包含带有上标等位基因的性染色体。例如，正常男性（Xᴴ Y）和携带者女性（Xᴴ Xʰ）之间的杂交会产生不同基因型的后代。母亲的配子是 Xᴴ 和 Xʰ，父亲的配子是 Xᴴ 和 Y。

	Xᴴ (from mother)	Xʰ (from mother)
Xᴴ (from father)	Xᴴ Xᴴ (normal female)	Xᴴ Xʰ (carrier female)
Y (from father)	Xᴴ Y (normal male)	Xʰ Y (haemophiliac male)

In this cross, there is a 25% chance of having a haemophiliac son, a 25% chance of having a normal son, a 25% chance of a normal homozyous daughter, and a 25% chance of a carrier daughter. No daughter will have haemophilia because she will always inherit at least one normal Xᴴ from her father.

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在这种杂交中，有25%的几率生出血友病儿子，25%的几率生出正常儿子，25%的几率生出正常纯合子女儿，25%的几率生出携带者女儿。没有女儿会患血友病，因为她至少会从父亲那里遗传一条正常的 Xᴴ。

7. Carrier Females and Their Role | 女性携带者及其作用

A carrier female has one normal allele and one recessive disease allele on her X chromosomes. She does not show the disorder because the normal allele masks the recessive one. However, she can pass the recessive allele to her children. If a carrier woman has a son, there is a 50% chance he will be affected, because he gets one of her two X chromosomes at random.

女性携带者在她的X染色体上有一个正常等位基因和一个隐性致病等位基因。她不表现出该疾病，因为正常等位基因掩盖了隐性基因。然而，她可以将隐性等位基因传给她的孩子。如果一个携带者女性生了一个儿子，有50%的几率儿子会患病，因为儿子会随机获得她的两条X染色体中的一条。

8. Why Can't Males Be Carriers? | 为什么男性不能成为携带者？

Males cannot be carriers of X-linked recessive disorders because they only have one X chromosome. If they inherit the recessive allele, they will express the disorder. The concept of a carrier applies only to individuals who have a hidden recessive allele that does not affect their phenotype—this requires two X chromosomes.

男性不能成为X连锁隐性遗传病的携带者，因为他们只有一条X染色体。如果他们遗传了隐性等位基因，他们就会表现出该疾病。携带者的概念仅适用于拥有一个隐藏的隐性等位基因而不影响其表型的个体——这需要两条X染色体。

9. Red-green Colour Blindness: Another Key Example | 红绿色盲：另一个关键例子

Red-green colour blindness is far more common in males than females. The allele for normal colour vision is dominant (Xᴺ), and the allele for colour blindness is recessive (Xⁿ). A colour-blind male has the genotype Xⁿ Y, while a colour-blind female must be Xⁿ Xⁿ. A carrier female has Xᴺ Xⁿ and has normal vision but can pass the condition to her sons.

红绿色盲在男性中远比女性常见。正常色觉的等位基因是显性的（Xᴺ），色盲的等位基因是隐性的（Xⁿ）。色盲男性的基因型是 Xⁿ Y，而色盲女性必须是 Xⁿ Xⁿ。携带者女性的基因型是 Xᴺ Xⁿ，她视力正常，但可以将该病传给她的儿子。

10. Interpreting Family Pedigrees | 解读家族系谱图

Exam questions may provide a family tree showing the occurrence of a sex-linked disorder. Key patterns include: more males are affected than females; affected males cannot pass the trait to their sons (since they give the Y chromosome, not the X); affected males pass the allele to all their daughters, making them carriers if the mother is normal. Understanding these patterns helps you deduce genotypes.

考试题目可能提供显示伴性遗传病发生的家族谱系图。关键模式包括：受影响的男性多于女性；患病的男性不能将该性状传给他们的儿子（因为他们给的是Y染色体，而不是X染色体）；患病的男性将等位基因传给他们所有的女儿，如果母亲正常，女儿就会成为携带者。理解这些模式有助于你推断基因型。

11. Exam Tips for GCSE AQA | AQA考试技巧

Always use the correct notation with superscripts for alleles on the X chromosome. Show your Punnett squares clearly, labelling gametes and genotypes. When explaining why a son is colour-blind, state that he inherited an Xⁿ chromosome from his carrier mother and a Y chromosome from his father. Use precise terms like 'carrier', 'heterozygous', and 'homozyous'. Remember: there is no male carrier in X-linked recessive traits.

在X染色体上始终使用正确的上标等位基因记号。清晰地画出庞尼特方格，标注配子和基因型。在解释为什么儿子是色盲时，要说明他从携带者母亲那里遗传了一条Xⁿ染色体，从父亲那里遗传了一条Y染色体。使用准确的术语，如“携带者”、“杂合子”和“纯合子”。记住：在X连锁隐性性状中，没有男性携带者。

12. Key Equations and Probability Summary | 关键方程式与概率总结

When working out probability, remember that each child is an independent event. The chance of a carrier mother and normal father having a colour-blind son remains 25% for each pregnancy. The overall ratio of genotypes in offspring can be summarised as:

在计算概率时，记住每个孩子都是独立事件。携带者母亲和正常父亲每次怀孕生出患有色盲的儿子的几率仍然是25%。后代基因型的总比例可总结如下：

Carrier mother (Xᴴ Xʰ) × Normal father (Xᴴ Y) → ¼ Xᴴ Xᴴ : ¼ Xᴴ Xʰ : ¼ Xᴴ Y : ¼ Xʰ Y

This shows a 50% chance of a normal child (male or female), 25% chance of a carrier daughter, and 25% chance of an affected son. Always check your working by adding the probabilities to 1.

这表明正常孩子（男性或女性）的几率为50%，携带者女儿的几率为25%，患病儿子的几率为25%。始终通过将概率相加等于1来检查你的计算。

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