1. What is genetic disease?

Genetic disease refers to a disease in which the genetic material is altered or controlled by a pathogenic gene, usually with characteristics of vertical transmission and it is lifelong. Therefore, genetic diseases are characterized by transmission from parents to offsprings.

2. Types of genetic diseases.

Everyone may have chromosomal structural abnormalities. One or both in a couple carrying some genetic disease-causing genes may lead to various abnormalities of the embryo, mainly manifested as spontaneous abortion, fetal abnormalities, birth defects, etc.

According to the genetic materials involved and the law of transmission, genetic diseases can be divided into the following four types:

(1). Chromosomal genetic diseases.

Chromosomal genetic diseases are mainly caused by chromosomal abnormalities, such as Down's syndrome, cri du chat syndrome, etc. Alterations in the genetic material that cause this type of disease are visible at the chromosomal level.

However, chromosomes may also make errors in the process of DNA replication and sufficiency, sometimes the loss or increase of the whole chromosome; sometimes local errors, such as partial loss. Therefore, the causes of chromosomal genetic diseases include changes in chromosome number and variations in chromosome structure.

a. Changes in chromosome number.

Common autosomal diseases include trisomy 21 (Down's syndrome), trisomy 13 and trisomy 18.

Common sex chromosome diseases include Turner's syndrome and Klinefelter syndrome.

b. Chromosome structural variation.

Due to the large number of genes involved in chromosomal diseases, these can be lethal and disabling. At present, there is no effective treatment for chromosomal diseases and only prenatal diagnosis, genetic counseling and other preventive measures can be used to guide and control the birth of children with chromosomal diseases. All pregnant women who are older, have a history of recurrent spontaneous abortion or have given birth to children with chromosomal diseases, should receive prenatal diagnosis to determine whether the fetus has chromosomal abnormalities.

(2). Monogenic diseases.

Monogenic diseases refer to hereditary diseases controlled by a pair of alleles. Among various birth defects, the proportion of monogenic diseases is as high as 22.2 percent.

There are over 8,000 monogenic diseases and the incidence rate is one percent. Most monogenic diseases can be fatal, teratogenic or severely disabling. However, only five percent of the diseases have effective drugs for treatment.

According to Mendel's law of inheritance, monogenic diseases can be divided into the following types:

a. Autosomal dominant diseases, or AD, such as Marfan syndrome, polycystic kidney, etc.

The causative gene is on the autosome and the inheritance method is dominant inheritance. It has the following characteristics:

As long as there is a pathogenic gene in the body, it will develop.

The disease is not related to sex and the incidence is equal between boy and girl.

In a patient's family, it may occur for generations.

If a non-sick child from the family marries a normal person, his or her offspring will generally not suffer from the disease subsequently.

b. Autosomal recessive diseases, or AR, such as albinism, phenylketonuria, etc.

The pathogenic gene of the disease is autosomal and recessive, and has the following characteristics:

Only homozygotes develop the disease.

Parents do not necessarily develop disease, but are carriers of pathogenic genes (heterozygotes).

If about a quarter of the patients' siblings are ill, boys and girls have an equal chance of developing the disease.

There will be no successive generations of inheritance in the family.

When close relatives get married, the incidence among offspring increases significantly.

c. X-linked dominant inheritance, or XD.

Pathogenic genes are on the X chromosome and inherited in a dominant manner.

d. X-linked recessive genetic disease, or XR.

Pathogenic genes are on the X chromosome and inherited in a recessive manner.

e. Y-linked genetic disease.

The pathogenic genes are all on the Y chromosome, which is male inheritance, also known as all-male inheritance.

(3). Polygenic diseases.

Such as anencephaly, spina bifida, cleft lip and palate, etc.

(4). Mitochondrial genetic diseases.

3. Preventive measures for genetic diseases.

At present, only 5 percent of genetic diseases have drug treatment and the price is high, which many families can hardly afford. Therefore, for genetic diseases, the most effective method is prevention, which could be divided into three levels: before marriage/before pregnancy; prenatal; neonatal.

(1). Primary prevention: before marriage/before pregnancy.

Premarital physical examination;

Pre-pregnancy health care;

Genetic counseling;

Carrier screening;

Preimplantation genetic testing (PGT).

(2). Secondary prevention: prenatal.

Prenatal screening;

Prenatal diagnosis.

(3). Tertiary prevention: neonatal.

Neonatal screening;

Diagnosis;

Treatment;

Recovery.

Whether from the prevention and control effect or at the time of prevention and control, primary prevention is better than secondary prevention and tertiary prevention. Also, secondary prevention is better than tertiary prevention, so genetic counseling and genetic diagnosis should be early or not too late.

When screening carriers of genetic diseases before marriage or pregnancy, if carrying pathogenic genes, embryos without pathogenic genes can be screened out by PGT for implantation, which can fundamentally prevent the occurrence of genetic diseases.