Genetic carriers refer to individuals with normal phenotype but with pathogenic genetic material. Generally include:

① recessive genetic heterozygote; (2) No obvious dominant genetic disease; (3) those with normal phenotype; ④ Individuals with balanced chromosome translocation.

The detection of genetic carriers is of positive significance to the prevention of genetic diseases. Because in the population, although the incidence of many recessive genetic diseases is not high, the proportion of heterozygotes is quite high. For example, the homozygote of phenylketonuria is 1: 1000 in the population, and the frequency of carriers (heterozygotes) is 2: 50, which is 200 times that of homozygotes. For genetic diseases with low incidence, generally, we don't do group screening of heterozygotes, but only screen relatives and their objects, which can also get good results. For high-incidence genetic diseases, the effect of screening carriers is remarkable. For example, the incidence of α and β thalassemia in southern provinces of China is particularly high (* * * accounts for 8% of the population-12%, and some provinces or regions are higher), so there are many opportunities to detect heterozygotes of α and β thalassemia at the same time. At this time, marriage and childbirth guidance, combined with prenatal diagnosis, can prevent the birth of seriously ill children from the first birth, thus receiving huge social and economic benefits, not only reducing

Carriers of chromosome balanced translocation have a great chance of giving birth to stillbirths and children with chromosome diseases (see chapter 2), so it is very important to check the relatives of chromosome balanced translocation.

The theoretical basis of detecting heterozygosity of recessive pathogenic genes is the dose effect of genes, that is, the dose of gene products is between homozygous and normal individuals, which is about half of that of normal individuals. However, due to the influence of various factors on gene expression in vivo and in vitro, and the different detection methods (direct determination of gene products or indirect determination of gene products), the measured values of normal people and heterozygotes overlap, which is difficult to judge.

The detection methods of heterozygote carriers can be roughly divided into clinical level, cellular level, enzyme protein level and molecular level. From the clinical level, it can only provide clues and cannot be accurately detected, so it has been basically abandoned. At the cellular level, chromosome examination is mainly used to detect carriers of balanced translocation. At present, the determination of enzyme and protein level (including the determination of metabolic intermediates) is still of some significance to the detection of heterozygosity in some molecular metabolic diseases, but it is gradually being replaced by gene level method. That is, with the development of molecular genetics, heterozygotes can be directly detected at the molecular level, that is, using DNA or RNA analysis technology, and it is accurate, especially for genetic diseases whose nature and abnormal gene products are not clear, or genetic diseases that cannot be accurately detected by general biochemical methods, such as chronic chorea, hemophilia A and B, DMD, ketonuria and so on. Finally, some late-onset dominant carriers can also be diagnosed before symptoms appear, so it is possible to take early preventive measures, such as adult polycystic kidney disease (see chapter 13). At present, there are more and more methods to detect heterozygotes by gene analysis, and they are gradually developing in the direction of simplicity, rapidity and accuracy, so as to expand the screening of high-risk groups.