Genetic Disorders, Medical Screening, and Prevention

A genetic disorder is a disease caused by a change in the DNA, either a mutation in a single gene, a change in chromosome number or structure, or a combination of genes and environment; medical screening is the testing of people to find disease or risk before symptoms appear, so that it can be prevented or treated early. In brief: single-gene disorders follow Mendelian patterns (cystic fibrosis is autosomal recessive, Huntington’s disease is autosomal dominant, hemophilia is X-linked recessive), and chromosomal disorders such as Down syndrome come from extra or missing chromosomes. Prenatal tests, newborn screening and carrier testing help families to make decisions.

Genetic disorders affect millions of people, and many can now be detected before or soon after birth. This guide classifies genetic disorders, explains the main screening and diagnostic tests, describes prevention and counseling, compares the types of disorders in a table, works through a carrier risk calculation and discusses ethical questions and common mistakes.

What are the types of genetic disorders?

• Single-gene (Mendelian) disorders:
  • Autosomal recessive: both copies must be faulty, so carriers (with one faulty copy) are healthy. Examples: cystic fibrosis (CFTR gene, thick mucus in lungs and pancreas), sickle cell disease, phenylketonuria (PKU) and Tay-Sachs disease.
  • Autosomal dominant: one faulty copy is enough. Examples: Huntington’s disease (late-onset brain degeneration), familial hypercholesterolemia and Marfan syndrome.
  • X-linked recessive: mostly affects males, who have one X chromosome. Examples: hemophilia A, Duchenne muscular dystrophy and red-green color blindness.
• Chromosomal disorders: a change in the number or structure of chromosomes, usually due to errors in meiosis such as nondisjunction. Examples: Down syndrome (trisomy 21, three copies of chromosome 21), Turner syndrome (one X chromosome, 45,X) and Klinefelter syndrome (47,XXY). The risk of trisomy 21 rises with maternal age.
• Multifactorial disorders: a result of many genes and the environment, such as heart disease, type 2 diabetes, some cancers and neural tube defects (spina bifida), which are reduced by folic acid in the diet before and during early pregnancy.
• Mitochondrial disorders: caused by mutations in mitochondrial DNA, inherited from the mother.

Genetic disorders compared

What is the difference between screening and diagnosis?

Screening is applied to a whole group of apparently healthy people to identify those at higher risk. It is not a final answer, and some results are false positives (the test is positive but the person is healthy) or false negatives. A diagnostic test is used on individuals with a positive screen or symptoms, and is designed to confirm or exclude a condition. A good screening program needs a condition that is serious, an accurate and acceptable test, and an effective treatment or option for those found positive.

What tests are used before and after birth?

• Carrier screening: a blood or saliva test to find out whether a person carries a recessive allele, offered to those with family history or in populations with higher frequency (Tay-Sachs in some communities, cystic fibrosis, sickle cell disease).
• Prenatal screening: ultrasound (for example, nuchal translucency at 11 to 14 weeks), maternal blood tests, and cell-free DNA tests that analyze fetal DNA fragments in the mother’s blood. These estimate the risk of conditions like Down syndrome.
• Prenatal diagnosis: chorionic villus sampling (from about 10 to 13 weeks) and amniocentesis (from about 15 weeks) collect fetal cells for karyotype or DNA analysis. Both have a small risk of miscarriage.
• Preimplantation genetic testing: embryos from in vitro fertilization are tested before transfer to the uterus.
• Newborn screening: a blood spot from the heel of a baby in the first days of life tests for conditions such as PKU, congenital hypothyroidism and cystic fibrosis. Early treatment, for example a special diet in PKU, prevents damage.
• Predictive testing: adults with a family history can test for disease alleles, such as BRCA1/2 or Huntington’s disease.

How are genetic disorders prevented and managed?

• Genetic counseling: helps families to understand the risk, the options and the results of tests, without telling them what to decide.
• Reproductive options: prenatal diagnosis, preimplantation testing, donor gametes, adoption.
• Lifestyle and environmental measures: folic acid supplements, avoiding alcohol in pregnancy, controlling the diet in PKU.
• Treatment: enzyme replacement, drugs that target the faulty protein (for example modulators for cystic fibrosis), bone marrow transplantation, and gene therapy. In 2023, regulators in the US and UK approved the first CRISPR-based therapy for sickle cell disease and beta-thalassemia.

What ethical issues arise?

Genetic information raises questions of privacy and discrimination (by employers or insurers), the right of individuals to know or not to know their risk, the implications for relatives who share the genes, the choice to end a pregnancy, selection of embryos, and fair access to testing. Many countries have laws that restrict genetic discrimination, for example the Genetic Information Nondiscrimination Act (GINA) of 2008 in the United States for health insurance and employment.

Worked example: risk for carrier parents

Two healthy parents are both carriers of the cystic fibrosis allele (Cc, where c is the recessive disease allele). A test shows that their unborn child is not affected (it does not have the genotype cc).

1. Punnett square: Cc × Cc gives 1 CC : 2 Cc : 1 cc.
2. Risk before testing: P(cc) = 1/4 = 25 percent; P(carrier, Cc) = 2/4 = 50 percent; P(CC) = 25 percent.
3. After a result that the child is unaffected: only CC and Cc remain, so there are 3 possible outcomes in total (1 CC + 2 Cc).
4. Probability that the unaffected child is a carrier: 2/3, or about 67 percent.

The 2/3 is a conditional probability: it is the probability of a carrier among the children who are not affected, not 1/2. In a population, the chance that two random people are carriers depends on the allele frequency. In people of European ancestry, about 1 in 25 is a carrier of a cystic fibrosis allele (this is an approximate figure), so the chance that both members of a random couple are carriers is about (1/25) × (1/25) = 1/625, and the risk for an affected child is about 1/625 × 1/4 = 1/2,500. This is close to the observed incidence of about 1 in 2,500 to 3,500 births in that population.

How did medical genetics develop?

Archibald Garrod described “inborn errors of metabolism” in 1902, and linked alkaptonuria with Mendel’s laws. Jérôme Lejeune and colleagues showed in 1959 that Down syndrome is caused by an extra chromosome 21. Linus Pauling identified sickle cell anemia as a “molecular disease” in 1949. Robert Guthrie developed the newborn blood spot test for PKU in the early 1960s. The CFTR gene was identified in 1989, and the Human Genome Project was completed in 2003. Noninvasive prenatal testing with cell-free DNA became available from 2011.

Which mistakes are common?

Mistake 1: Treating screening results as a diagnosis

A positive screening test only indicates a higher risk, and should be confirmed by a diagnostic test.

Mistake 2: Thinking that a recessive disorder needs an affected parent

Two healthy carriers can have an affected child, with a 25 percent chance in each pregnancy.

Mistake 3: Believing that genetic means inherited

Chromosomal disorders such as Down syndrome usually arise from errors in gamete formation and are not inherited from the parents. Cancers involve genetic changes that arise in body cells.

Mistake 4: Assuming that a gene variant always means disease

Many variants have incomplete penetrance, which means that not everyone who has the variant gets the condition, and multifactorial diseases depend on the environment.