| Multifactorial Inheritance | 多基因遺傳 |
| 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. | 近親之間（包括三級親屬關(guān)系）在很多可定量或測量的性狀上彼此相似（例如身高,體重,鼻子大小和形狀,面部特征,血壓,智力）.很多性狀均按鐘形曲線分布,這和由幾個基因決定的性狀分布情況相似.每一基因?qū)υ撔誀罨蛟鰪?或削弱,并以外加方式發(fā)揮作用而與其他基因無關(guān).在分布曲線兩個極端的人是很少的,很多人處于曲線的中間,因為一個人不大會遺傳到很多同方向作用的因子.環(huán)境因素對最后結(jié)果會產(chǎn)生增強或削弱作用,也產(chǎn)生一正態(tài)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. | 許多相對常見的先天性異常和家族性疾病并不遵循單基因遺傳規(guī)律（孟德爾遺傳）.它們更接近于多基因遺傳,受累者和非受累者之間有一閾值分開.受累者易患病癥,代表著遺傳和環(huán)境影響的總和.因此,在分享受累者50%基因的一級親屬中（同胞和子女）出現(xiàn)病癥的風險是較高的.這種風險在較遠的親屬中,由于他們只獲得幾個高易患性基因,則要小得多. |
| 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%. | 例如神經(jīng)管缺陷（NTD,包括無腦,脊柱裂,腦膨出,脊髓脊膜膨出）通常有多基因造成,雖然也有許多已知的單基因,染色體和環(huán)境原因.在北美人群中,NTD的綜合發(fā)病率約為每1000個活胎中1.5例.受累兒童的未受累父母攜帶著許多高易患性基因,并有1/30（3%）的機會再生一個受累的子女.同樣,父母一方有多基因NTD者會有3%~4%的機會生下一個受累的小孩.罕見情況下,一對夫婦已有兩個受累的孩子時,產(chǎn)生第3個受累兒的風險增加到7%~8%.
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| 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. | 環(huán)境因素在多基因遺傳中有影響.在英國西部地區(qū)NTD的發(fā)病率接近1/100,然而當人們從高風險區(qū)移居到北美后,風險減少,但仍維持于比北美人群為高的水平.同樣,50%~70%的NTD是可以預防的；只要母親在懷孕前一個月到懷孕后三個月期間補充攝入葉酸（400μg/d）即可.雖然母親缺乏葉酸起著主要的作用,但并不是NTD的唯一環(huán)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. | 其他的多基因遺傳的例子,受累者的同胞和子女有相同風險的,還有先天性心臟病,原發(fā)性癲癇（小發(fā)作或大發(fā)作）和大部分伴有或不伴有腭裂的唇裂.先天性幽門狹窄的男女比例是5:1,說明女性的閾值較高.因此,女性需要更強的易患性基因才發(fā)病,同胞中受累者更多,而生出受累子女的風險也更大 |
| 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見的成人疾病上（例如高血壓,動脈硬化性心臟病,糖尿病,癌,關(guān)節(jié)炎）.許多特殊基因正被發(fā)現(xiàn).遺傳學上確定的易感因素,包括家族史和生化及分子生物學參數(shù)可以確定有風險者,以便其采取預防措施而得益. |
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