| Nontraditional Inheritance | 非传统性遗传 |
| In addition to the traditional inheritance patterns, several other mechanisms have been described. They involve genetically determined structures and can produce various disorders. | 除了传统遗传模式外,还有一些其他的机制.它们涉及遗传学上确定的结构并可产生各种病症. |
| Mosaicism: Mosaicism is the presence of >= 2 cell lines differing in genotype or karyotype but derived from one zygote. Mutations are likely to occur during the cell divisions of any large multicellular organism even though the genetic apparatus of cell division is usually accurate and many mechanisms exist to repair mistakes made during replication. It is estimated that each time a cell divides, four or five changes occur in the genome. Thus, any large multicellular organism will have subclones of cells having a slightly different genetic makeup. These somatic mutations （ie, mutations during mitotic cell division） may not lead to disease but to disorders in which patchy changes occur. Molecular genetic techniques have shown mutations in the abnormal cells involved in a patch compared with the normal surrounding supporting tissues. For example, in McCune-Albright syndrome, there are patchy dysplastic changes in the bone, abnormalities of endocrine glands, patchy pigmentary changes, and occasionally abnormalities in the heart or liver. Persons with these abnormalities in all cells would die, so the condition would not be passed on to the next generation, but they survive because normal tissue supports the abnormal tissue. Occasionally, in a single-gene disorder, a parent seems to have a milder form but is actually a mosaic; their more severely affected child would have received a germ cell with the mutant allele and thus would have the abnormality present in every cell. Chromosomal mosaicism occurs in some embryos and can be demonstrated in the placenta on chorionic villus sampling. Most chromosomally abnormal embryos and fetuses abort spontaneously. However, the development of normal cells may support certain chromosomal abnormalities, allowing offspring to be born alive. | 镶嵌性 镶嵌是指体内有从同一个合子衍生出来的两个或更多的不同基因型或核型的细胞系.虽然细胞分裂的遗传系统一般是精确的并且有许多机制可对复制过程中产生的错误进行修复,任何大的多细胞机体在细胞分裂中仍可出现突变.据估计,每次细胞分裂,基因组中会出现4~5个变化.因此,任何大的多细胞机体会有一些基因构造略有不同的细胞亚克隆.这些体细胞突变（即有丝分裂时的突变）可以不引起疾病,而只是一些紊乱,出现一些斑块状变化.分子遗传学技术可以把斑块中的异常细胞的突变从周围正常的支持组织中区别出来.例如在McCune-Albright综合征中,骨中有发育不良斑块,还有内分泌腺异常,色素斑以及偶有心或肝的异常.如果所有细胞均有此类异常必死无疑,所以该病症不会传给第二代.然而这些病人可以存活,因为正常组织支持着异常组织.偶尔,在单基因遗传病中,父母一方是轻型镶嵌体.其更为严重受累的孩子可能得到一个带有突变等位基因的生殖细胞,因而在每一细胞中均有异常.染色体镶嵌性见于某些胚胎并可在胎盘中的绒毛膜标本上被显示.大部分染色体异常的胚胎和胎儿均自发流产.然而,正常细胞的发育可以支持某些染色体异常者而使后代活着出生. |
| Genomic imprinting: Genomic imprinting is the differential expression of genetic material depending on whether it has been inherited from the father or mother. Genomic imprinting is tissue-specific and time-in-development-specific. Bi-allelic or biparental expression of alleles may be present in some tissues and uniparental expression in other tissues. Angelman syndrome and Prader-Willi syndrome can both be produced by deletions of chromosome 15. Groups of specific genes exist in close proximity on chromosome 15 with only paternal or maternal expression. Depending on whether the deleted chromosome is paternally or maternally inherited, a different syndrome will be produced. | 基因组印迹 基因组印迹是遗传物质根据其来自父亲还是母亲而出现的不同表达.基因组印迹是组织特异的和时间-发育特异的.某些组织中可能出现等位基因的双等位基因或双亲表达,其他组织中是单亲表达.Angelman综合征和Prader-Willi综合征都可因第15号染色体缺失而产生.特异基因存在于第15号染色体的近端,只具有父源或母源表达.根据缺失染色体来自父亲还是母亲而出现不同的综合征. |
| Many areas on several chromosomes have this type of parent-of-origin effect. The involved genes seem to be related to growth and behavior in early development. Some of these genes are also involved in tumors and cancers. Genomic imprinting must be considered in disorders that appear to have skipped a generation. | 好几条染色体的许多区域都有这种亲源效应.涉及的基因似与早期发育的生长和行为有关.其中某些基因还与肿瘤和癌的发生有关.当遗传性病症跳越一代下传时,必须考虑作遗传印迹检查. |
| Uniparental disomy: Uniparental disomy occurs when two chromosomes of a pair are inherited from only one parent. This is very rare and is thought to involve trisomy rescue; ie, the zygote started off as a trisomy and one of the three chromosomes was lost, leading to uniparental disomy in 1/3 of cases. Imprinting effects may be seen because genetic information from the other parent is absent. In addition, if the same chromosome is in duplicate （isodisomy） and that chromosome carries an abnormal allele for an autosomal recessive disorder, a person with uniparental disomy can have an autosomal recessive disorder although only one parent is a carrier. Vestigial chromosomal abnormalities in some tissues must be considered in the presence of uniparental disomy. | 单亲二体 当同一对的两条染色体都来自父亲一方时可出现单亲二体.这是很罕见的并被认为还涉及三体挽救,即合子开始时为三体,而三条染色体中的一条丢失后在1/3的情况下导致单亲二体.印迹效应可以显示,因为没有来自父母另一方的遗传信息.此外,假如相同的染色体为复制品（同源二体）,而染色体又带有常染色体隐性遗传病的一个异常等位基因,那么这位带有单亲二体的人可能会出现常染色体隐性遗传病,尽管父母中只有一方是携带者.在有单亲二体时,必须考虑到某些组织中的残余染色体异常. |
| Triplet repeat, unstable mutations: A triplet repeat is an unusual type of mutation in which a triplet of nucleotides increases in number within a gene （a normal gene has relatively few tandem triplet repeats）. This type of mutation has been recognized to occur in several disorders, particularly those involving the CNS. When the gene is transmitted from one generation to the next, or sometimes within the body as cells divide, the triplet repeat can expand and enlarge to a point at which the gene stops functioning normally. Examples include myotonic dystrophy, Huntington's disease, fragile X mental retardation, and several other neurologic disorders. The number of repeats may increase dramatically in the formation of germ cells or in certain tissues as the embryo and fetus develop. Expansion may be greater when transmitted from one parent （eg, the mother in myotonic dystrophy, the father in Huntington's disease）; thus, a parent-of-origin effect and anticipation can be observed. This type of mutation is detected by molecular studies. | 三联体重复序列,不稳定突变 三联体重复序列是一种不常见的突变类型,发生时,在基因内的核苷酸三联体的数目增加（正常基因只有相对很少的串联三联体重复序列）.此种突变曾见于某些病症,特别是涉及中枢神经系统者.当基因从一代传到下一代,或有时在体内当细胞分裂时,三联体重复序列可以扩张增大到使基因不能正常发挥功能的地步.例子有肌强直性营养不良,Huntington病,脆性X智力障碍和其他一些神经科疾病.在生殖细胞形成过程中或胚胎和胎儿发育时的某些组织中,重复序列的数目可以明显增加.如果突变系从父母一方传来（例如母亲有肌强直性营养不良,父亲有Huntington病）,则扩张可以更厉害.因此,可以观察到一种亲源效应和早现现象.此种突变可由分子技术检出. |
| Anticipation: Anticipation occurs when a disorder has an earlier age of onset and severity of expression in each successive generation. It may occur because a parent is a mosaic, and the child has the full mutation in all cells. Triplet repeat expansion may demonstrate anticipation when the number of repeats increases with each generation. | 早现 早现是指病症在逐代传递过程中发病年龄越来越早,病情越来越重.它的出现是因为父母一方是镶嵌体,孩子的所有细胞都有完全突变.当三联体重复序列的数目逐代增加时,它的扩张就可表现出早现之象 |