Calculation Methods of Genetic Disease Prevalence | 遗传病患病率的计算方法

📚 Calculation Methods of Genetic Disease Prevalence | 遗传病患病率的计算方法

Genetic disease prevalence is a key topic in A-level biology. It connects population genetics with Mendelian inheritance and allows students to calculate the risk of inherited disorders in a population or family. This article summarizes the main calculation methods, using Hardy-Weinberg equilibrium, pedigree analysis, and clinical examples.

遗传病患病率是 A-level 生物学的核心考点之一。它将群体遗传学与孟德尔遗传联系起来，帮助学生在群体或家系中计算遗传病的发生风险。本文总结主要计算方法，结合 Hardy-Weinberg 平衡、系谱分析和临床实例进行讲解。

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1. Basic Concepts: Gene Frequency, Genotype Frequency, and Disease Prevalence | 基本概念：基因频率、基因型频率与患病率

In population genetics, a gene frequency (allele frequency) is the proportion of a specific allele among all alleles at a locus. For a gene with two alleles A and a, if the frequency of A is p and the frequency of a is q, then p + q = 1. Genotype frequencies describe the proportion of each genotype: AA, Aa and aa.

在群体遗传学中，基因频率（等位基因频率）是指某特定等位基因占该位点全部等位基因的比例。对于一对等位基因 A 和 a，若 A 的频率为 p，a 的频率为 q，则 p + q = 1。基因型频率表示各基因型所占比例，即 AA、Aa 和 aa。

The disease prevalence usually refers to the proportion of affected individuals in a population. For a recessive disease, this equals the frequency of aa; for a dominant disease, this equals the combined frequency of AA and Aa. Thus, the calculation begins with identifying the disease gene's mode of inheritance.

患病率通常指人群中患病个体所占比例。对于隐性遗传病，它等于 aa 的频率；对于显性遗传病，它等于 AA 与 Aa 频率之和。因此，计算的第一步是明确遗传病的遗传方式。

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2. The Hardy-Weinberg Equilibrium | Hardy-Weinberg 平衡定律

Hardy-Weinberg equilibrium provides the foundation for most prevalence calculations. It holds true under five conditions: a very large population, random mating, no mutation, no migration, and no natural selection. In such a population, allele frequencies remain constant and genotype frequencies are predictable.

Hardy-Weinberg 平衡为大多数患病率计算提供了基础。它成立需要满足五个条件：群体极大、随机交配、无突变、无迁移、无自然选择。在此条件下，等位基因频率保持不变，基因型频率可以预测。

p² + 2pq + q² = 1

Here p² is the frequency of AA homozygotes, 2pq the frequency of heterozygotes, and q² the frequency of aa homozygotes. When the population is not in equilibrium, this equation cannot be used directly.

其中 p² 是 AA 纯合子频率，2pq 是杂合子频率，q² 是 aa 纯合子频率。若群体不处于平衡状态，则不能直接套用此公式。

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3. Autosomal Recessive Diseases | 常染色体隐性遗传病

For autosomal recessive diseases, an affected individual must be homozygous for the disease allele (aa). If we know the prevalence, that is the frequency of aa, we can calculate the disease allele frequency by taking the square root.

对于常染色体隐性遗传病，患病个体必须是致病基因的纯合子（aa）。若已知患病率，即 aa 的频率，则可通过开平方计算出致病基因频率。

prevalence = q², therefore q = √prevalence

For example, if the prevalence of cystic fibrosis in a population is 1 in 2,500, then q² = 1/2500 = 0.0004, so q = 0.02 and p = 0.98. The carrier frequency is 2pq ≈ 0.0392, roughly 1 in 25.

例如，某群体囊性纤维化的患病率为 1/2500，则 q² = 1/2500 = 0.0004，因此 q = 0.02，p = 0.98。携带者频率为 2pq ≈ 0.0392，约 1/25。

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4. Autosomal Dominant Diseases | 常染色体显性遗传病

For autosomal dominant diseases, individuals with genotypes AA and Aa are affected. Therefore the prevalence equals p² + 2pq. Since dominant disease alleles are rare, p is tiny, so the AA homozygote frequency is negligible and prevalence ≈ 2p. Thus p ≈ half the prevalence.

对于常染色体显性遗传病，基因型为 AA 和 Aa 的个体均患病，因此患病率等于 p² + 2pq。由于显性致病基因通常罕见，p 很小，AA 纯合子频率可忽略，患病率约等于 2p，所以 p ≈ 患病率的一半。

prevalence ≈ 2p, therefore p ≈ prevalence / 2

If a dominant disorder affects 1 in 10,000 people, the disease allele frequency is p ≈ 1/20,000 = 0.00005, and the heterozygous genotype frequency is 2p ≈ 1/10,000.

若某显性遗传病患病率为 1/10000，则致病基因频率 p ≈ 1/20000 = 0.00005，杂合子基因型频率为 2p ≈ 1/10000。

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5. X-Linked Recessive Diseases | X连锁隐性遗传病

X-linked recessive diseases are more common in males because males have only one X chromosome. In a population, the allele frequency q is equal to the male prevalence. Affected females require two copies of the allele and have frequency q².

X连锁隐性遗传病在男性中更常见，因为男性只有一条 X 染色体。在群体中，致病等位基因频率 q 等于男性患病率。患病女性需要两条 X 染色体都携带致病基因，频率为 q²。

If 8% of males are colour blind, then q = 0.08 and p = 0.92. The expected frequency of affected females is q² = 0.0064, about 0.64%, while carrier females

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