Notes on Genetic Diseases (1)

A hereditary disease, or genetic disease for short, is a disease caused by a change in the genetic material within cells (chromosomal aberration or gene mutation). It involves changes in the structure and function of genetic material in somatic cells, germ cells or fertilized eggs. Genetic diseases have the following three features:

1. Vertical transmission: transmission from parent to offspring. What is passed on is not the disease itself but the altered genetic material.
2. Congenitality: A disease or developmental abnormality present at birth is called a congenital disease. Most genetic diseases are congenital; for example, an albino shows "albinism" at birth. But not all genetic diseases are congenital—many give no sign at birth and only manifest at a certain age.
3. Familiality: Genetic diseases often run in families, but not all genetic diseases are familial, and not all familial diseases are genetic.

  Roles of genetic and environmental factors in disease: The vast majority of diseases result from the interaction of heredity and environment. The relationship between disease, heredity and environment is as follows:

1. Disease determined entirely by genetic factors: some diseases are entirely determined by heredity, with no visible environmental influence.
2. Disease determined mainly by genetic factors but requiring an environmental trigger.
3. Disease to which both genetic and environmental factors contribute, but to different degrees.
4. Disease determined entirely by environmental factors, essentially unrelated to heredity.

    Classification of genetic diseases. By the mode of alteration of genetic material and its transmission, modern medical genetics divides human genetic disease into five major categories.

1. Monogenic disorders: Diseases caused by mutation of one gene or one allele pair, following Mendelian inheritance, also called Mendelian disorders. By whether the determining gene is dominant or recessive and whether it lies on an autosome or a sex chromosome, monogenic disorders are further divided into five types:

• Autosomal dominant (AD)
• Autosomal recessive (AR)
• X-linked dominant (XD)
• X-linked recessive (XR)
• Y-linked (YL)
  2. Polygenic (multifactorial) disorders: Diseases caused jointly by genetic factors (multiple pairs of minor-effect genes) and environmental factors. They include congenital developmental abnormalities and some common diseases, with familial clustering and relatively high incidence, but the family-line feature is less obvious than in monogenic disorders.
  3. Chromosomal disorders: Diseases caused by numerical or structural chromosomal abnormalities. Clinically they are divided into autosomal and sex-chromosome disorders. Because chromosomal aberrations involve many genes, they are generally more harmful to the individual than monogenic or polygenic disorders.
  4. Somatic-cell genetic diseases: Diseases caused by changes in genetic material in somatic cells; generally not transmitted to offspring.
  5. Mitochondrial genetic diseases: Diseases caused by mutation of mitochondrial DNA; they show maternal inheritance.

  Harm of genetic diseases

1. Genetic diseases are numerous and have high incidence.
2. They are a leading cause of childhood death.
3. They are a leading cause of intellectual disability.
4. They are a leading cause of infertility, miscarriage and spontaneous abortion.

Carrying a disease-causing gene is a potential threat to human health.