📚 Sex-linked Inheritance: Key Points for GCSE WJEC Biology | 伴性遗传考点精讲
In GCSE WJEC Biology, understanding sex-linked inheritance is crucial for explaining why certain genetic disorders such as colour blindness and haemophilia are more common in males. This article breaks down the key concepts, from sex chromosomes to Punnett squares and pedigree analysis, helping you to answer exam questions with confidence.
在 GCSE WJEC 生物学中，理解伴性遗传对于解释为何某些遗传病（如色盲和血友病）在男性中更常见至关重要。本文将从性染色体、庞纳特方格到谱系分析，分解关键概念，助你自信应对考试。
1. Sex Chromosomes and Gender Determination | 性染色体与性别决定
Humans have 23 pairs of chromosomes in each body cell. One of these pairs are the sex chromosomes, which determine an individual’s biological gender. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY).
人类每个体细胞中有23对染色体。其中一对是决定性别的性染色体。女性拥有两条 X 染色体 (XX)，而男性拥有一条 X 和一条 Y 染色体 (XY)。
During the formation of gametes (eggs and sperm), the chromosome number is halved. All egg cells carry a single X chromosome. Sperm cells, however, can carry either an X or a Y chromosome. Therefore, the sex of the offspring is determined by the sperm that fertilises the egg.
在配子（卵子和精子）形成过程中，染色体数目减半。所有卵细胞携带一条 X 染色体。而精子细胞可携带一条 X 或一条 Y 染色体。因此，后代的性别由与卵子结合的精子决定。
A simple Punnett square for gender inheritance shows a 1:1 ratio of female to male offspring. If a mother (XX) and a father (XY) produce children, half are expected to be female (XX) and half male (XY).
一个简单的性别遗传庞纳特方格显示，女性和男性后代的比例为 1:1。如果母亲 (XX) 和父亲 (XY) 生育子女，预计一半是女孩 (XX)，一半是男孩 (XY)。
2. What is Sex-linked Inheritance? | 什么是伴性遗传？
Sex-linked inheritance refers to the pattern of inheritance of genes located on the sex chromosomes. In most cases, these genes are found on the X chromosome because the Y chromosome is much smaller and carries fewer genes. This means that X-linked alleles have a unique pattern of transmission that differs from autosomal inheritance.
伴性遗传是指位于性染色体上的基因的遗传模式。在大多数情况下，这些基因位于 X 染色体上，因为 Y 染色体要小得多且携带的基因很少。这意味着 X 连锁等位基因具有独特的传递模式，与常染色体遗传不同。
Because males have only one X chromosome, they are said to be hemizygous for X-linked genes. Any allele present on their single X chromosome will be expressed in the phenotype, even if it is recessive. Females, with two X chromosomes, must inherit two copies of a recessive allele for it to be expressed in the phenotype.
由于男性只有一条 X 染色体，因此对于 X 连锁基因他们是半合子。存在于其唯一 X 染色体上的任何等位基因都会在表型中表达，即使它是隐性的。而拥有两条 X 染色体的女性，必须继承两个隐性等位基因拷贝，它才会在表型中表达。
3. X-linked Recessive Inheritance | X 连锁隐性遗传
The most common form of sex-linked inheritance is X-linked recessive. The allele causing the condition is located on the X chromosome and is recessive. We use standard notation: the gene is represented by a superscript letter on the X chromosome. For example, the allele for normal colour vision is written as XB, and the allele for colour blindness as Xb.
最常见的伴性遗传形式是 X 连锁隐性遗传。导致该疾病的等位基因位于 X 染色体上，且是隐性的。我们使用标准符号：基因由 X 染色体上的上标字母表示。例如，正常色觉的等位基因写作 XB，色盲的等位基因写作 Xb。
The possible genotypes and corresponding phenotypes are as follows. In females: XBXB (homozygous normal), XBXb (carrier – normal vision), and XbXb (colour blind). In males: XBY (normal vision) and XbY (colour blind). Note that males cannot be carriers for X-linked recessive traits; they either have the condition or do not.
可能的基因型和相应的表型如下。女性：XBXB（纯合正常）、XBXb（携带者——视力正常）和 XbXb（色盲）。男性：XBY（正常视力）和 XbY（色盲）。注意，对于 X 连锁隐性性状，男性不可能是携带者；他们要么患病，要么不患病。
4. Red-Green Colour Blindness: A Classic Example | 红绿色盲：一个经典例子
Red-green colour blindness is the most common X-linked recessive disorder in humans. Affected individuals have difficulty distinguishing between shades of red and green. It is caused by a recessive allele on the X chromosome.
红绿色盲是人类中最常见的 X 连锁隐性遗传病。患者难以区分红色和绿色色调。它由 X 染色体上的隐性等位基因引起。
In the population, approximately 8% of males are red-green colour blind, while only about 0.6% of females are affected. This striking difference is a direct consequence of the hemizygous nature of males, who need only one copy of the recessive allele to express the condition.
在人群中，约8%的男性是红绿色盲，而只有约0.6%的女性受影响。这一显著差异直接源于男性的半合子特性，他们只需要一个隐性等位基因拷贝即可表达该性状。
A typical exam question involves a cross between a carrier female and a normal male. The female’s genotype is XBXb and the male’s is XBY. You must be able to use a Punnett square to show the expected offspring genotypes and phenotypes.
典型的考题涉及一位携带者女性与一位正常男性的杂交。女性的基因型是 XBXb，男性的基因型是 XBY。你必须能够使用庞纳特方格来展示预期的后代基因型和表型。
5. Haemophilia: Another Sex-linked Disorder | 血友病：另一种伴性疾病
Haemophilia is a well-known X-linked recessive condition where the blood fails to clot properly due to a missing or defective clotting factor. This leads to prolonged bleeding after injury and can be life-threatening. The allele causing haemophilia is often designated Xh, while the normal allele is XH.
血友病是一种著名的 X 连锁隐性遗传病，由于缺少或有缺陷的凝血因子，血液无法正常凝固。这会导致受伤后长时间出血，并可能危及生命。引起血友病的等位基因通常写作 Xh，而正常等位基因为 XH。
Similarly, a female must inherit two faulty alleles (XhXh) to suffer from haemophilia, which is extremely rare. A carrier female (XHXh) has normal clotting but can pass the allele to her children. If she has a son with a normal male, there is a 50% chance that the son will have haemophilia.
同样，女性必须遗传两个缺陷等位基因 (XhXh) 才会患血友病，这极为罕见。携带者女性 (XHXh) 凝血功能正常，但可以将该等位基因传给子女。如果她与一位正常男性生育儿子，儿子患有血友病的概率为50%。
Queen Victoria of the United Kingdom is a historical example of a carrier, and the condition was passed through several royal families across Europe, which is why haemophilia is sometimes called the “royal disease”.
英国维多利亚女王是携带者的一个历史实例，该疾病通过她传递到了欧洲多个皇室家族，因此血友病有时被称为“皇室病”。
6. Analysing Sex-linked Crosses Using Punnett Squares | 使用庞纳特方格分析伴性遗传
To predict the outcome of a sex-linked cross, you must construct a Punnett square showing the possible gametes from each parent. Consider the cross between a carrier female (XBXb) and a normal male (XBY).
为了预测伴性杂交的结果，你必须构建一个庞纳特方格，展示每位亲本可能产生的配子。考虑携带者女性 (XBXb) 与正常男性 (XBY) 的杂交。
| Male gametes | |||
|---|---|---|---|
| XB | Y | ||
| Female gametes | XB | XBXB (normal female) | XBY (normal male) |
| Xb | XBXb (carrier female) | XbY (colour-blind male) | |
From the table, we can see the genotypic ratio is 1 XBXB : 1 XBXb : 1 XBY : 1 XbY. The phenotypic ratio shows that 50% of the offspring are normal females (one is a carrier), and among the males, 50% are normal and 50% are colour blind.
从表格中，我们可以看到基因型比例为 1 XBXB : 1 XBXb : 1 XBY : 1 XbY。表型比例显示，后代中50%为正常女性（其中一位是携带者），而在男性中，50%正常，50%为色盲。
Make sure your Punnett square includes the correct gametes, and always label the offspring with their gender if sex chromosomes are involved. This avoids confusion and shows the examiner you understand sex-linked inheritance clearly.
确保你的庞纳特方格包含正确的配子，并且如果涉及性染色体，务必要为后代标注性别。这样可以避免混淆，并向考官表明你清楚地理解了伴性遗传。
7. The Role of Carriers | 携带者的作用
A carrier is a heterozygous female who possesses one normal dominant allele and one recessive disease-causing allele on her X chromosomes. She does not show symptoms of the disorder because the normal allele is sufficient to produce the required protein or trait.
携带者是一位杂合女性，她的 X 染色体上有一个正常的显性等位基因和一个隐性的致病等位基因。她不表现出疾病症状，因为正常的等位基因足以产生所需的蛋白质或性状。
Although carriers are healthy, they can pass the recessive allele to their children. If a carrier female has a son, there is a 50% risk that the son will inherit the X chromosome with the harmful allele and be affected. Thus carriers play a pivotal role in the transmission of X-linked recessive diseases across generations.
尽管携带者身体健康，她们可以将隐性等位基因传给子女。如果一位携带者女性生育儿子，该儿子有50%的风险继承带有有害等位基因的 X 染色体而患病。因此，携带者在 X 连锁隐性疾病的代代相传中起着关键作用。
Identifying carriers in a family is important for genetic counselling. Carriers can be detected through pedigree analysis and, in some cases, through genetic testing.
在家族中识别携带者对遗传咨询非常重要。携带者可通过谱系分析，有时也可通过基因检测来发现。
8. Why Are Males More Frequently Affected? | 为什么男性更常受影响？
Males are more frequently affected by X-linked recessive disorders because they inherit only one X chromosome. If that X chromosome carries a recessive disease allele, there is no corresponding allele on the Y chromosome to mask its effect. The Y chromosome lacks most genes, so the recessive allele is automatically expressed.
男性更常受 X 连锁隐性遗传病影响，因为他们只继承一条 X 染色体。如果那条 X 染色体带有一个隐性的致病等位基因，Y 染色体上没有相应的等位基因来掩盖其作用。Y 染色体缺乏大多数基因，因此隐性等位基因会自然地表达出来。
Females must inherit two copies of the recessive allele, one from each parent, to show the disorder. Because the frequency of the recessive allele is typically low in the population, the chance of a female being homozygous recessive is very small.
女性必须从父母双方各继承一个隐性等位基因才能表现出疾病。由于隐性等位基因在人群中的频率通常较低，女性成为隐性纯合子的几率非常小。
This simple genetic mechanism explains the stark difference in prevalence between males and females for conditions like red-green colour blindness and haemophilia.
这一简单的遗传机制解释了像红绿色盲和血友病这类疾病在男性和女性中患病率的显著差异。
9. Pedigree Charts for Sex-linked Traits | 伴性遗传的谱系图
In a pedigree chart, sex-linked recessive traits show distinctive patterns. The trait appears more often in males than females. An affected father cannot pass the trait to his sons because sons inherit the Y chromosome from the father. However, all daughters of an affected father will be carriers (assuming the mother is normal), as they receive his X chromosome carrying the allele.
在谱系图中，伴性隐性遗传性状显示出独特的模式。该性状在男性中出现的频率高于女性。患病的父亲无法将该性状传给儿子，因为儿子从父亲那里继承的是 Y 染色体。但是，患病父亲的所有女儿都将是携带者（假设母亲正常），因为她们接收了他携带该等位基因的 X 染色体。
Likewise, if a mother is a carrier, her sons have a 50% chance of being affected, and her daughters have a 50% chance of being carriers. A key feature to look for is that in each generation, the trait often appears to “skip” from an affected man through his carrier daughters to his grandsons.
同样，如果母亲是携带者，她的儿子有50%的概率患病，女儿有50%的概率成为携带者。一个需要注意的关键特征是，在每一代中，该性状常常看似从一位患病男性通过其携带者女儿“隔代”传递给他的外孙。
When interpreting pedigree charts, always write the genotypes of each individual where possible, using the lettering system for X-linked genes, to demonstrate your understanding.
在解读谱系图时，尽可能为每个个体写出基因型，使用 X 连锁基因的字母系统，以展示你的理解。
10. Genetic Counselling and Sex-linked Conditions | 遗传咨询与伴性疾病
Genetic counselling uses knowledge of sex-linked inheritance to inform couples about the risks of having a child with an inherited disorder. For example, if a woman knows she is a carrier for haemophilia, a counsellor can explain the probabilities of her children being affected.
遗传咨询利用伴性遗传知识，向夫妇告知生育患有遗传病子女的风险。例如，如果一位女性知道自己是血友病携带者，咨询师可以解释她的子女受影响的概率。
If the male partner is normal, any son has a 50% risk of haemophilia, and any daughter has a 50% chance of being a carrier. If the disorder is severe, parents may consider prenatal diagnosis to determine the genotype of the foetus.
如果男方正常，任何儿子有50%的血友病风险，任何女儿有50%的概率成为携带者。如果疾病很严重，父母可以考虑产前诊断来确定胎儿的基因型。
In the exam, you might be asked to suggest how genetic counselling can help a family or to calculate the probabilities of certain outcomes based on a given pedigree.
在考试中，你可能被要求说明遗传咨询如何帮助
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