Multiple cases suggest that the liability is high in that particula family. In contrast, for single-gene traits the risk to the next child remains unchanged even after two three or more affected children have een born 5.The more severe the malformation, the greater therecurrence risk More severely affected patients, and their relatives, have greater 6. If a multifactorial trait is more frequent in one sex than in the other, the risk is higher for relatives of patients of the less susceptible sex An affected person of the less susceptible sex is likely to have a higher liability, and thus any relative has a greater risk of being affected 7. If the concordance rate in dz twins is less than half the rate in mz twins the trait cannot be antosomal dominant and if it is less than a quarter ofthe Mz rate, it cannot be autosomal recessive The concordance rate for non-insulin-dep endent diabetes mellitus(NIDDM) is close to 100 percent in Mz twins but only about 10% in DZ pairs; at first glance the mz data support a single-gene model with complete penetrance, but the low concordance rate in DZ twins contradicts this impression, and further studies have supported the conclusion that NIDDM has complex etiology 8. An increased recurrence risk when the parents are consanguineous suggests that multiple factors with additive effects may be involved For multifactorial traits, the risk to subsequent sibs increased when the parents are consanguineous. This is in contrast to autosomal recessive inher itance; although parental consanguinity usually indicates a high probability of autosomal recessive inheritance, the recurrence risk(174)is the same whether the parents are consanguineous or not 二、遗传度及其估算7 affected. Multiple cases suggest that the liability is high in that particular family. In contrast, for single-gene traits the risk to the next child remains unchanged even after two, three, or more affected children have been born. 5.The more severe the malformation, the greater the recurrence risk. More severely affected patients, and their relatives, have greater liability. 6.If a multifactorial trait is more frequent in one sex than in the other, the risk is higher for relatives of patients of the less susceptible sex. An affected person of the less susceptible sex is likely to have a higher liability, and thus any relative has a greater risk of being affected. 7.If the concordance rate in DZ twins is less than half the rate in MZ twins, the trait cannot be antosomal dominant, and if it is less than a quarter of the MZ rate, it cannot be autosomal recessive. The concordance rate for non-insulin-dependent diabetes mellitus(NIDDM) is close to 100 percent in MZ twins but only about 10% in DZ pairs; at first glance the MZ data support a single-gene model with complete penetrance, but the low concordance rate in DZ twins contradicts this impression, and further studies have supported the conclusion that NIDDM has complex etiology. 8.An increased recurrence risk when the parents are consanguineous suggests that multiple factors with additive effects may be involved. For multifactorial traits, the risk to subsequent sibs increased when the parents are consanguineous. This is in contrast to autosomal recessive inheritance; although parental consanguinity usually indicates a high probability of autosomal recessive inheritance, the recurrence risk (1/4) is the same whether the parents are consanguineous or not. 二、遗传度及其估算