Major genes may have a dominant-recessive relationship, but the distinction between minor-effect genes is not clear, and most of them are co-dominant.

The accumulation of the effects of multiple pairs of small-effect genes can form an obvious effect. This phenomenon is called additive effect. Therefore this gene is also called an additive gene

Multiple pairs of minimally effective genes

random combination of genes

The correlation between gene networks is very strong. Any genetic change may affect the core genes related to specific traits. In addition, coupled with the influence of environmental factors, the complexity of quantitative traits is even higher.

During the inheritance process of quantitative traits, the offspring will move closer to the average value of the population. This is the regression phenomenon. This phenomenon is also manifested in other similar quantitative shapes.

The regression phenomenon has important guiding significance for understanding the genetic characteristics of polygenic genetic diseases.

Susceptibility Environmental factors = Susceptibility

Under the same environment, the differences between different individuals can be considered to be caused by different susceptibility, that is to say, caused by genetic differences, and the susceptibility variation in the population is also normally distributed.

The threshold divides the continuously distributed medical-patient variation into two parts, the normal group and the diseased group.

Polygenic genetic diseases also belong to threshold-related diseases. The threshold is a certain point of medical-patient variation. Under certain conditions, the threshold represents the minimum number of susceptible genes necessary for the disease.

①The area within the range of μ 18 (taking the mean μ as 0 and 1 standard deviation on the left and right) accounts for 68.28% of the total area under the normal distribution curve, the area outside this range accounts for 31.72%, and the left and right sides each account for about 16% ②The area within the μ -2δ range accounts for 95.46% of the total area under the normal distribution curve, the area outside this range accounts for 4.54%, and the left and right sides each account for approximately 2.3% ③The area within the μ -3δ range accounts for 99.74% of the total area under the normal distribution curve, the area outside this range accounts for 0.26%, and the left and right sides each account for approximately 0.13%

The role of genetic factors can be measured by heritability

It is established based on the relationship between the prevalence of the proband's relatives and heritability, with the higher the prevalence of the disease in relatives. The greater the heritability

Xg is the number of standard deviations between the average susceptibility and the threshold in the general population; Xc is the number of standard deviations between the average susceptibility and the threshold in relatives of the control group; The number of standard deviations between the mean and the threshold value; Ar is the number of standard deviations between the average susceptibility of the general population and the average susceptibility of patients in the general population; ar is the susceptibility of the proband's relatives The number of standard deviations between the sex mean and the patient's susceptibility mean among the proband's relatives; Qg is the prevalence of the general population; Qc is the prevalence of control relatives, Pc=1-Qc, Qr is the proband's relatives Prevalence

The relatedness coefficient refers to the overall probability that two individuals received a specific allele from a common ancestor.

It is established based on the fact that the higher the heritability of a disease, the greater the difference between the concordance rate of monozygotic twins and the concordance rate of dizygotic twins.

Please pay attention to the following issues when calculating

The incidence of polygenic genetic diseases has an obvious tendency of familial aggregation. The prevalence of the patient's relatives is higher than the population prevalence, and the prevalence decreases sharply as the relationship between the patient and the patient increases, moving closer to the population prevalence.

The risk of recurrence of polygenic genetic diseases is related to the heritability of the disease

Some polygenic diseases have different population prevalence rates and different risk rates even if their heritability is the same.

In polygenic genetic diseases, the more affected people in a family, the higher the risk of recurrence in relatives.

Because the genetic composition of the parents in single-gene genetic diseases has been fixed and is inherited strictly according to Mendelian inheritance rules, the probability of the disease in their offspring changes by 1/2 or 1/4 of the original risk because several patients have been born.

The genetic basis for the onset of polygenic genetic diseases is small-effect genes, which have co-significant additive effects. Therefore, if the patient's condition is severe in polygenic genetic diseases, it proves that his susceptibility far exceeds the incidence threshold. Compared with patients with milder disease, whose parents have more susceptibility genes and have more susceptibility genes, their susceptibility is closer to the threshold.

In single-gene genetic diseases, regardless of the severity of the disease, the risk of recurrence is generally 1/2 or 1/4.

When there is a gender difference in the incidence of a certain polygenic genetic disease, it means that the incidence thresholds for different genders are simply different.