of either sex. In particular, male-to-male transmission can occur, and males can have unaffected daughters 3. The phenotype appears in every generation, each affected person hav ing an affected parent Exceptions or apparent exceptions to this rule in clinical genetics: ( 1) cases originating by fresh mutation in a gamete of a phenotypically normal parent; (2 )apparent but not true exceptions in which the disorder is not expressed(non-penetrant) or is expressed very mild in a person who has inherited the responsible gene 4. Any child of an affected parent has a 50% risk of inheriting the trait This is true for most families, in which the other parent is phenotypically normal. Because statistically each family member is the result of an"independent event there may in a single family be wid deviation from the expected 1: I ratio 第三节常染色体隐性遗传病的遗传 由于常染色体隐性遗传病的致病基因为隐性基因,所以只有隐性 纯合子才会发病。在杂合子时,隐性致病基因的作用被其显性基因所 掩盖,而不表现相应的疾病,表型与正常人相同,但是却可将致病基 因遗传给后代。这种表型正常而带有致病基因的杂合子,称为携带者 ( carrier)。白化病、先天性聋哑、先天性肌弛缓等都属于此种遗传方 式 、Tay- Sachs病 Tay-Sachs病(OMM#272800)也称为黑朦性痴呆。在北美的 Ashkenazi犹太人(遗传上隔离群体)中很常见。是一种常染色体隐 性遗传病,患者在出生后6个左右即开始发病,表现为神经系统的退 行性变性,随即致盲,智能和体能不断退化,最后在儿童期死亡7 of either sex. In particular, male-to-male transmission can occur, and males can have unaffected daughters. 3. The phenotype appears in every generation, each affected person having an affected parent. Exceptions or apparent exceptions to this rule in clinical genetics:(1) cases originating by fresh mutation in a gamete of a phenotypically normal parent; (2)apparent but not true exceptions in which the disorder is not expressed (non-penetrant) or is expressed very mild in a person who has inherited the responsible gene. 4. Any child of an affected parent has a 50% risk of inheriting the trait. This is true for most families, in which the other parent is phenotypically normal. Because statistically each family member is the result of an “independent event,” there may in a single family be wide deviation from the expected 1:1 ratio. 第三节 常染色体隐性遗传病的遗传 由于常染色体隐性遗传病的致病基因为隐性基因，所以只有隐性 纯合子才会发病。在杂合子时，隐性致病基因的作用被其显性基因所 掩盖，而不表现相应的疾病，表型与正常人相同，但是却可将致病基 因遗传给后代。这种表型正常而带有致病基因的杂合子，称为携带者 （carrier）。白化病、先天性聋哑、先天性肌弛缓等都属于此种遗传方 式。 一、Tay-Sachs 病 Tay-Sachs 病（OMIM#272800）也称为黑朦性痴呆。在北美的 Ashkenazi 犹太人（遗传上隔离群体）中很常见。是一种常染色体隐 性遗传病，患者在出生后 6 个左右即开始发病，表现为神经系统的退 行性变性，随即致盲，智能和体能不断退化，最后在儿童期死亡