Sex Inheritance | 性别遗传

📚 Sex Inheritance | 性别遗传

In A-level Biology, sex inheritance refers to how biological sex is determined by sex chromosomes and how genes located on these chromosomes are passed from parents to offspring. Understanding sex-linked inheritance is essential for interpreting genetic crosses, predicting disease risk, and analysing pedigrees.

在 A-level 生物中，性别遗传指的是生物性别如何由性染色体决定，以及位于这些染色体上的基因如何从亲代传递给子代。理解性连锁遗传对于解释遗传杂交、预测疾病风险和分析家系至关重要。

1. Sex chromosomes and autosomes | 性染色体与常染色体

Human body cells contain 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes. Females typically have two X chromosomes (XX), while males have one X and one much smaller Y chromosome (XY).

人类体细胞含有 46 条染色体：22 对常染色体和 1 对性染色体。女性通常有两条 X 染色体（XX），男性有一条 X 染色体和一条小得多的 Y 染色体（XY）。

During meiosis in females, every egg receives one X chromosome. In males, half of the sperm cells carry an X chromosome and half carry a Y chromosome. Fertilisation therefore restores the diploid chromosome number and determines the sex of the zygote.

在女性减数分裂过程中，每个卵细胞都获得一条 X 染色体。在男性中，一半精子携带 X 染色体，一半携带 Y 染色体。因此，受精恢复二倍体染色体数目，并决定合子的性别。

2. Sex determination in humans | 人类的性别决定

Human sex determination depends primarily on the presence or absence of the Y chromosome, not on the number of X chromosomes. An individual with a Y chromosome usually develops as male, while an individual without a Y chromosome develops as female.

人类性别决定主要取决于 Y 染色体的有无，而不是 X 染色体的数目。有 Y 染色体的个体通常发育为男性，没有 Y 染色体的个体发育为女性。

The SRY gene (sex-determining region Y) on the Y chromosome acts as a master switch. If SRY is expressed, the embryonic gonads develop into testes; if it is absent or non-functional, ovaries develop.

Y 染色体上的 SRY 基因（性别决定区 Y）起主开关作用。如果 SRY 表达，胚胎性腺发育为睾丸；如果缺失或无功能，则发育为卵巢。

3. SRY gene and testis development | SRY基因与睾丸发育

The SRY protein is a transcription factor that triggers a cascade of gene activity leading to testis formation. The developing testes secrete testosterone and anti-Müllerian hormone, which masculinise the reproductive ducts and external genitalia.

SRY 蛋白是一种转录因子，能触发一系列基因活动，最终形成睾丸。发育中的睾丸分泌睾酮和抗穆勒氏管激素，使生殖管道和外生殖器男性化。

If the SRY gene is deleted or mutated, an XY embryo may develop as female. Conversely, if SRY is translocated onto an X chromosome during crossing over, an XX individual may develop as male. These rare events show that the Y chromosome’s key male-determining factor is SRY.

如果 SRY 基因缺失或突变，XY 胚胎可能发育为女性。相反，如果 SRY 在交换时易位到 X 染色体上，XX 个体可能发育为男性。这些罕见事件说明 Y 染色体上关键的雄性决定因子是 SRY。

4. X-linked inheritance: why males are hemizygous | X连锁遗传：男性为何半合子

The X chromosome carries many genes that have no equivalent allele on the Y chromosome. A male has only one X chromosome, so he expresses whatever X-linked allele he inherits, even if it is recessive. This condition is called hemizygosity.

X 染色体携带许多在 Y 染色体上没有对应等位基因的基因。男性只有一条 X 染色体，所以无论他遗传到什么 X 连锁等位基因，即使是隐性基因也会表达。这种情况称为半合子。

A female has two X chromosomes, so she must inherit two copies of a recessive X-linked allele to show the recessive phenotype. If she carries one recessive and one dominant allele, she is usually a carrier and may show no symptoms.

女性有两条 X 染色体，因此必须遗传两个隐性 X 连锁等位基因才会表现隐性表型。如果她携带一个

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