📚 Sex Linkage | 性连锁遗传
Sex linkage describes the inheritance of genes located on the sex chromosomes, typically the X chromosome in mammals and Drosophila. It produces characteristic patterns in which males and females are affected at different frequencies.
性连锁描述位于性染色体（通常是哺乳动物和果蝇的X染色体）上的基因的遗传方式。它产生典型的遗传模式，使男性和女性受影响频率不同。
1. Sex Chromosomes and Sex Determination | 性染色体与性别决定
In humans, sex is determined by the X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y (XY). The Y chromosome carries the SRY gene, which triggers male development.
人类性别由X和Y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（XY）。Y染色体携带SRY基因，启动男性发育。
The X chromosome is much larger and carries many genes unrelated to sex, whereas the Y chromosome is small and contains few functional genes. This size difference means males have only one copy of most X-linked genes.
X染色体大得多，携带许多与性别无关的基因，而Y染色体小，只含少数功能基因。这种大小差异意味着男性大多数X连锁基因只有一份。
2. What Is Sex Linkage? | 什么是性连锁
A gene is sex-linked if its locus is on a sex chromosome. In most A-Level examples, sex linkage refers to X-linked genes because the Y chromosome has very few equivalent loci.
如果一个基因的位点在性染色体上，则该基因是性连锁的。在大多数A-Level例子中，性连锁指X连锁基因，因为Y染色体上几乎没有对等位点。
Because males are hemizygous for X-linked genes, they express whichever allele is present on their single X chromosome. There is no second allele to mask a recessive allele.
由于男性对X连锁基因是半合子，他们表达其唯一X染色体上存在的任何等位基因。没有第二个等位基因来掩盖隐性等位基因。
This means recessive X-linked traits appear far more often in males than in females, even when the allele frequency is the same in both sexes.
这意味着即使等位基因频率在男女中相同，隐性X连锁性状在男性中的出现频率也远高于女性。
3. X-Linked Recessive Inheritance | X连锁隐性遗传
X-linked recessive alleles are expressed in all males who carry them, but in females they are usually expressed only in homozygous recessive individuals. Heterozygous females are carriers and are typically unaffected.
X连锁隐性等位基因在所有携带它们的男性中都会表达，但在女性中通常只在隐性纯合子中表达。杂合女性是携带者，通常不受影响。
Affected males cannot pass the allele to their sons because sons receive the Y chromosome from the father. However, affected males pass the X-linked recessive allele to all daughters, who become carriers if the mother contributes a normal allele.
受影响男性不能把该等位基因传给儿子，因为儿子从父亲获得Y染色体。然而，受影响男性将X连锁隐性等位基因传给所有女儿，如果母亲提供正常等位基因，女儿就成为携带者。
Consider a cross between a carrier female and a normal male:
考虑携带者女性和正常男性之间的杂交：
Carrier female XNXn × normal male XNY
| Female gametes / Male gametes | XN | Y |
|---|---|---|
| XN | XNXN (normal female) | XNY (normal male) |
| Xn | XNXn (carrier female) | XnY (affected male) |
In this cross, 50% of the sons are affected, while none of the daughters are affected. However, 50% of the daughters are carriers.
在这个杂交中，50%的儿子患病，而女儿都不患病。然而，50%的女儿是携带者。
4. Red-Green Colour Blindness | 红绿色盲
Red-green colour blindness is a classic X-linked recessive condition affecting the perception of red and green light. It is much more common in males, with about 8% of males affected in some populations compared with fewer than 1% of females.
红绿色盲是典型的X连锁隐性性状，影响红绿色光的感知。它在男性中更常见，某些人群中约8%的男性受影响，女性不到1%。
A carrier female has normal colour vision but can pass the recessive allele to half of her sons, who will be colour blind, and to half of her daughters, who will be carriers.
携带者女性有正常色觉，但可以将隐性等位基因传给一半儿子（色盲）和一半女儿（携带者）。
For a female to be colour blind, she must inherit two recessive alleles, one from an affected or carrier father and one from an affected or carrier mother.
女性要患色盲，必须遗传两个隐性等位基因，一个来自患病或携带者父亲，另一个来自患病或携带者母亲。
5. Haemophilia | 血友病
Haemophilia A is caused by a recessive allele of a gene on the X chromosome that codes for clotting factor VIII. Affected individuals bleed for longer because blood clotting is impaired.
血友病A由X染色体上编码凝血因子VIII的基因的隐性等位基因引起。患者出血时间延长，因为凝血功能受损。
Haemophilia shows the same inheritance pattern as red-green colour blindness. A carrier female (XHXh) and an unaffected male (XHY) can have affected sons but no affected daughters.
血友病的遗传模式与红绿色盲相同。携带者女性（XHXh）与正常男性（XHY）可能生下患病的儿子，但不会有患病的女儿。
In a pedigree, affected males often have unaffected parents, because the mother is usually a carrier. This is an important clue for identifying X-linked recessive conditions.
在系谱中，患病男性的父母通常不患病，因为母亲通常是携带者。这是识别X连锁隐性疾病的重要线索。
6. X-Linked Dominant Traits | X连锁显性性状
Some X-linked alleles are dominant. In this case, both males and females with one dominant allele are affected, but affected males pass the condition to all daughters and none of their sons.
有些X连锁等位基因是显性的。在这种情况下，携带一个显性等位基因的男性和女性都会患病，但患病男性会把性状传给所有女儿，不传给儿子。
A common A-Level example is hypophosphatemic rickets, although it is less frequently examined than recessive examples. Affected females are often less severely affected than affected males because they have a normal allele on the other X chromosome.
A-Level中常见的例子是低磷性佝偻病，尽管它不如隐性例子常考。患病女性通常比患病男性症状轻，因为她们另一条X染色体上有正常等位基因。
7. Y-Linked Inheritance | Y连锁遗传
Y-linked genes occur only on the Y chromosome and are passed from father to son. They are never expressed in females because females lack a Y chromosome.
Y连锁基因只存在于Y染色体上，由父亲传给儿子。它们从不在女性中表达，因为女性没有Y染色体。
In humans, very few functional genes are Y-linked; the main example is the SRY gene and some genes involved in sperm production. Y-linked pedigrees show transmission only along the male line.
人类中Y连锁的功能基因很少；主要例子是SRY基因和一些与精子发生有关的基因。Y连锁系谱显示只在男性谱系中传递。
8. Pedigree Analysis for Sex Linkage | 性连锁系谱分析
Pedigree charts can be used to identify sex-linked inheritance. Key clues for X-linked recessive traits include: more affected males than females, affected males born to unaffected carrier mothers, and no father-to-son transmission.
系谱图可用于识别性连锁遗传。X连锁隐性性状的关键线索包括：受影响男性多于女性，患病男性由未患病的携带者母亲所生，以及没有父传子传递。
For X-linked dominant traits, affected males have all affected daughters but no affected sons, and affected females may pass the condition to both sexes.
对于X连锁显性遗传，患病男性的所有女儿都患病而儿子不患病；患病女性可能将疾病传给两种性别。
It is important to compare the observed sex distribution with the expected autosomal pattern, in which males and females are usually affected equally.
重要的是将观察到的性别分布与预期的常染色体模式进行比较，在常染色体模式中，男性和女性通常受影响程度相同。
9. Genetic Crosses and Punnett Squares | 遗传杂交与庞纳特方格
When solving X-linked crosses, write alleles as superscripts on the X chromosome, for example XR for red eye colour and Xr for white eye colour in Drosophila. The Y chromosome is written without a matching allele.
在解X连锁杂交时，将等位基因写在X染色体上标，例如果蝇红眼为XR，白眼为Xr。Y染色体不写对应等位基因。
Carrier female XRXr × white-eyed male XrY
| Female gametes / Male gametes | Xr | Y |
|---|---|---|
| XR | XRXr (red-eyed female) | XRY (red-eyed male) |
| Xr | XrXr (white-eyed female) | XrY (white-eyed male) |
In this cross, a carrier female mated with a white-eyed male produces equal proportions of red-eyed and white-eyed male and female offspring.
在这个杂交中，携带者雌性与白眼雄性交配，产生红眼和白眼的雌雄后代比例相等。
10. X-Inactivation and Dosage Compensation | X染色体失活与剂量补偿
In female mammals, one X chromosome in each cell is randomly inactivated during early development. This forms a Barr body and balances the dose of X-linked genes between XX females and XY males.
在雌性哺乳动物中，每个细胞中的一条X染色体在早期发育中随机失活。这形成巴氏小体，使XX雌性和XY雄性之间X连锁基因的剂量达到平衡。
X-inactivation means that females are mosaics for some X-linked traits. In carrier females of X-linked recessive conditions, patches of cells may express the normal allele while other patches express the mutant allele.
X染色体
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