Hereditary Diseases

How are hereditary diseases transmitted? All the information on genetic conditions and their types.

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Symptoms and Causes

Hereditary diseases refer to genetic alterations that are passed down from parents to children across generations. A genetic disease, in turn, is a condition caused by a change in the genome. It will be hereditary if the altered gene is present in the germline, that is, in the cells responsible for transmitting hereditary genetic material, the egg and sperm cells.

Genetic diseases are classified according to the factor that causes them:

  • Monogenic diseases: Caused by a mutation or alteration in the DNA sequence of a single gene. Examples include cystic fibrosis and Huntington’s disease.
  • Multifactorial or polygenic diseases: Caused by a combination of multiple environmental agents and mutations in several genes. These genetic alterations do not directly cause the disease but predispose to a certain susceptibility to manifest in the presence of specific environmental factors. Examples include cancer, type 2 diabetes, and heart disease.
  • Chromosomal diseases: Result from a variation in the number or structure of chromosomes. This includes Down syndrome or Turner syndrome.
  • Mitochondrial diseases: Mutations appear in mitochondrial DNA, not in chromosomal DNA.

Symptoms

There is no common symptomatology for all hereditary diseases, as each one has its specific symptoms. However, certain circumstances may point to the existence of a genetic disorder:

  • Multiple spontaneous miscarriages, fetal death, or infant death in more than one family member.
  • Congenital malformations.
  • Delays in neurological or physical development.
  • Family history of genetic or hereditary diseases.

Causes

Hereditary diseases are transmitted through different mechanisms, depending on their type:

  • Monogenic diseases: Follow Mendelian inheritance laws.
  • Autosomal recessive diseases: For the disease to manifest, two copies of the mutated gene in the person's genome are required, meaning one copy from each parent. If only one copy is inherited, the person is a carrier and may pass the gene to their descendants, but they will not show the disease.
  • Autosomal dominant diseases: The disease develops with only one copy of the mutated gene inherited from one parent.
  • X-linked recessive diseases: The mutated gene is on the X chromosome. Women may be carriers of the disease if they inherit one copy of the gene or develop the condition if they inherit two copies, while men only need one copy for the disease to manifest.
  • X-linked dominant diseases: These develop in both men and women when they have one copy of the causative gene.
  • Y-linked diseases: The defective gene is on the Y chromosome, so only men with the mutation can develop and transmit the disease.
  • Multifactorial diseases: These do not follow a Mendelian inheritance pattern. Environmental factors that may affect these diseases include age, gender, diet, level of physical activity, obesity, smoking, alcohol consumption, exposure to toxic substances, or the use of certain medications.
  • Chromosomal diseases: Not all of these are hereditary; some result from a random error during the formation of reproductive cells or fetal development. These errors may be caused by external factors such as the mother’s age, drug use, or cancer treatments.
  • Mitochondrial diseases: These are inherited exclusively from the mother, as mitochondria come only from the egg.

Risk Factors

Conditions that increase the chances of inheriting genetic diseases or causing mutations during fetal development include:

  • Parental age: The chances of fetal genetic anomalies increase after the mother is 35 and the father is 50.
  • Family history of chromosomal abnormalities.
  • Congenital malformation in a previous offspring.
  • Previous spontaneous abortions.
  • Consanguinity between parents.
  • Use of toxic substances during pregnancy.

Complications

Genetic anomalies that manifest during pregnancy can lead to severe situations such as multiple miscarriages, fetal death, or neonatal death. Additionally, there are hereditary degenerative diseases with poor prognosis, such as Huntington’s disease, cystic fibrosis, or spinal muscular atrophy. Early detection and proper management are essential for improving the quality of life of affected individuals, which is why understanding genetic history is crucial when planning for a pregnancy.

Prevention

The most effective way to prevent the transmission of a hereditary disease is for parents to be informed about the presence of mutations in their own genes and to understand the possibilities of transmission. For this, both should undergo carrier screening or genetic compatibility testing: through a blood sample, each person’s DNA is analyzed to study the probability that the baby will inherit a disease based on whether they are carriers of the same recessive genes. Based on the result, the most appropriate reproductive method can be chosen.

What doctor treats hereditary diseases?

Hereditary diseases are evaluated and treated by specialists in clinical genetics and assisted reproduction.

Diagnosis

There are various tests that allow the early detection of hereditary diseases:

  • Prenatal tests: Conducted on the pregnant woman or the fetus before birth.
  • Non-invasive screening tests:
    • Fetal DNA analysis: The fetal DNA present in the mother’s blood is analyzed to identify chromosomal abnormalities.
    • Serum marker analysis: Tumor markers are searched for in the mother’s blood.
    • Ultrasound to measure fetal nuchal translucency: The thickening of the nuchal fold is measured to evaluate the risk of genetic issues in the baby.
  • Invasive tests:
    • Amniocentesis: A sample of amniotic fluid, containing cells shed from the fetus, is analyzed. The cells are cultured in a lab to examine their chromosomes. This is performed from the 15th week of pregnancy.
    • Chorionic villus sampling: A sample of small projections from the placenta is taken, as these cells have the same genetic makeup as the fetus. It is performed between the 10th and 14th weeks.
    • Cordocentesis or percutaneous umbilical cord blood sampling: A blood sample is extracted from the fetus via the umbilical cord. This is performed from the 18th week, especially if fetal anemia is suspected.
  • Neonatal tests:
    • Heel prick test: When the baby is 2 or 3 days old, a blood sample is taken from the heel to identify metabolic and genetic diseases.

Treatment

Whether a risk of hereditary disease transmission is detected in carrier screening or sperm or egg donation is chosen, two complementary assisted reproduction treatments may be used:

  • In vitro fertilization (IVF): The procedure involves extracting eggs from the woman’s ovaries through follicular aspiration, fertilizing them in the lab with the man’s sperm, and then placing the resulting embryos in the mother’s uterus. There is a variant of IVF, intracytoplasmic sperm injection, in which the most viable sperm is selected and injected directly into the egg.
  • Preimplantation genetic diagnosis (PGD): This technique ensures that embryos transferred to the uterus are free of hereditary diseases and chromosomal anomalies. It involves performing a biopsy on embryos cultured for 3-5 days and extracting several cells to obtain and analyze their DNA. This way, genetically healthy embryos are identified and those with abnormalities are discarded.