| Multifactorial Inheritance | 多基因遗传 |
| Close relatives （up to third-degree） tend to resemble each other with respect to a number of quantitative or measurable characteristics （eg, height, weight, size and shape of nose, facial features, BP, intelligence）. Many traits are distributed along a bell-shaped curve, a phenomenon that is compatible with determination of the trait by several genes. Each gene adds to or subtracts from the trait and acts in an additive manner independent of other genes. Few persons are at the extremes of the distribution and many are in the middle, because a person is unlikely to inherit many factors acting in the same direction. Environmental factors, each adding to or subtracting from the final result, will also produce a normal distribution. | 近亲之间（包括三级亲属关系）在很多可定量或测量的性状上彼此相似（例如身高,体重,鼻子大小和形状,面部特征,血压,智力）.很多性状均按钟形曲线分布,这和由几个基因决定的性状分布情况相似.每一基因对该性状或增强,或削弱,并以外加方式发挥作用而与其他基因无关.在分布曲线两个极端的人是很少的,很多人处于曲线的中间,因为一个人不大会遗传到很多同方向作用的因子.环境因素对最后结果会产生增强或削弱作用,也产生一正态分布. |
| Many relatively common congenital anomalies and familial diseases do not follow expectations for single-gene （Mendelian） inheritance. More likely, these conditions result from multifactorial inheritance; a threshold separates the affected person from the unaffected person. The affected person is predisposed to the condition, representing the sum of genetic and environmental influences. Thus, the risk of expression of such a trait in first-degree relatives （siblings and children）, who share 50% of the affected person's genes, is relatively high. The risk in more distant relatives, who are likely to inherit only a few high-liability genes, is much lower. | 许多相对常见的先天性异常和家族性疾病并不遵循单基因遗传规律（孟德尔遗传）.它们更接近于多基因遗传,受累者和非受累者之间有一阈值分开.受累者易患病症,代表着遗传和环境影响的总和.因此,在分享受累者50%基因的一级亲属中（同胞和子女）出现病症的风险是较高的.这种风险在较远的亲属中,由于他们只获得几个高易患性基因,则要小得多. |
| For example, neural tube defects （NTDs--anencephaly, spina bifida, encephalocele, myelomeningocele） usually have multifactorial causes, although they also have many specific known single-gene, chromosomal, and environmental causes. In the North American white population, NTDs occur with a combined incidence of about 1.5/1000 live births. Unaffected parents of an affected infant are thought to carry many high-liability genes and have about a 1/30 （3%） chance of having a second affected offspring. Similarly, a parent with a multifactorial NTD has a 3 to 4% chance of having an affected child. In the rare instance when a couple has had two affected children, the risk for a third increases to 7 to 8%. | 例如神经管缺陷（NTD,包括无脑,脊柱裂,脑膨出,脊髓脊膜膨出）通常有多基因造成,虽然也有许多已知的单基因,染色体和环境原因.在北美人群中,NTD的综合发病率约为每1000个活胎中1.5例.受累儿童的未受累父母携带着许多高易患性基因,并有1/30（3%）的机会再生一个受累的子女.同样,父母一方有多基因NTD者会有3%~4%的机会生下一个受累的小孩.罕见情况下,一对夫妇已有两个受累的孩子时,产生第3个受累儿的风险增加到7%~8%. |
| Environmental factors play a role in multifactorial inheritance. The incidence of NTDs approaches 1/100 in western parts of the United Kingdom, yet when persons from a high-risk area emigrate to North America, their risk falls but remains higher than that of the North American population. Also, 50 to 70% of NTDs can be prevented by maternal folic acid supplementation （400 µg/day） 1 mo before conception to 3 mo after conception. Although maternal folic acid deficiency plays a major role, it is not the only environmental cause of NTDs. | 环境因素在多基因遗传中有影响.在英国西部地区NTD的发病率接近1/100,然而当人们从高风险区移居到北美后,风险减少,但仍维持于比北美人群为高的水平.同样,50%~70%的NTD是可以预防的；只要母亲在怀孕前一个月到怀孕后三个月期间补充摄入叶酸（400μg/d）即可.虽然母亲缺乏叶酸起着主要的作用,但并不是NTD的唯一环境原因. |
| Other examples of multifactorial inheritance, with similar risks for siblings and offspring of affected persons, are congenital anomalies of the heart, idiopathic epilepsy （petit or grand mal）, and most cases of cleft lip with or without cleft palate. In congenital pyloric stenosis, the male:female ratio is 5:1, suggesting that the threshold for females is higher. Thus, compared with a male, a female requires more potent liability genes to develop the condition, has more affected siblings, and has a greater risk of having affected offspring. | 其他的多基因遗传的例子,受累者的同胞和子女有相同风险的,还有先天性心脏病,原发性癫痫（小发作或大发作）和大部分伴有或不伴有腭裂的唇裂.先天性幽门狭窄的男女比例是5:1,说明女性的阈值较高.因此,女性需要更强的易患性基因才发病,同胞中受累者更多,而生出受累子女的风险也更大 |
| Attention is being focused on common adult disorders with multifactorial causes （eg, hypertension, arteriosclerotic heart disease, diabetes mellitus, cancer, arthritis）. Many specific genes are being found. Genetically determined predisposing factors, including a family history and biochemical and molecular parameters, can identify persons at risk who would most likely benefit from preventive measures. | 目前,注意力集中在多基因造成的常见的成人疾病上（例如高血压,动脉硬化性心脏病,糖尿病,癌,关节炎）.许多特殊基因正被发现.遗传学上确定的易感因素,包括家族史和生化及分子生物学参数可以确定有风险者,以便其采取预防措施而得益. |
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