16.3 Genetic Disorders and Screening

Genetic Disorders and Inheritance

Genetic disorders arise from changes in DNA that disrupt normal body function and development. They range from single-gene mutations to large-scale chromosome rearrangements, and understanding their inheritance patterns is essential for predicting risk and interpreting screening results.

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Common Genetic Disorders and Inheritance Patterns

Genetic disorders result from mutations in single genes, chromosomal abnormalities, or a combination of genetic and environmental factors. The classic Mendelian inheritance patterns describe how many single-gene disorders are passed from parent to offspring.

• Autosomal dominant disorders (Huntington's disease, Marfan syndrome) manifest when only one copy of the mutated allele is present. An affected parent has a 50% chance of passing the allele to each child.
• Autosomal recessive disorders (cystic fibrosis, sickle cell anemia) require two copies of the mutated allele for the trait to be expressed. Both parents must be carriers (or affected) for a child to show the disorder.
• X-linked dominant disorders (Rett syndrome) result from mutations on the X chromosome. They primarily affect females because males with the mutation on their single X chromosome often do not survive to birth.
• X-linked recessive disorders (Duchenne muscular dystrophy, hemophilia) also involve X chromosome mutations but mainly affect males. Males need only one copy of the mutated allele (since they have one X), while females are typically carriers.

Beyond Mendelian patterns, non-Mendelian inheritance includes mitochondrial inheritance (passed exclusively through the mother), genomic imprinting (where expression depends on which parent contributed the allele), and trinucleotide repeat expansions (where repeated DNA sequences grow longer across generations, often increasing disease severity).

Classification and Causes of Genetic Disorders

Genetic disorders fall into three broad categories:

• Single-gene disorders are caused by a mutation in one specific gene and typically follow Mendelian inheritance. Examples include cystic fibrosis (a mutation in the CFTR gene disrupts chloride ion transport, leading to thick mucus in the lungs) and sickle cell anemia (a point mutation in the hemoglobin gene causes red blood cells to become rigid and sickle-shaped).
• Chromosomal disorders result from numerical or structural changes in chromosomes. Down syndrome (trisomy 21) and Turner syndrome (45,X) are common examples.
• Complex (multifactorial) disorders involve interactions between multiple genes and environmental factors. Diabetes, heart disease, and many cancers fall into this category, which is why they don't follow simple inheritance patterns.

Mutations can be inherited from one or both parents, or they can arise spontaneously (de novo) during DNA replication, meiosis, or from environmental mutagens like radiation and certain chemicals.

The severity of a genetic disorder can vary between individuals carrying the same mutation. This variability is influenced by:

• Penetrance: the proportion of individuals with a given genotype who actually show the phenotype. Incomplete penetrance means some carriers never develop symptoms.
• Expressivity: the degree to which a trait is expressed. Two people with the same mutation may have very different symptom severity.
• Genetic background: other genes in the genome can modify how a disorder presents.

Chromosomal Abnormalities and Their Effects

Types and Causes of Chromosomal Abnormalities

Chromosomal abnormalities are classified as either numerical (wrong number of chromosomes) or structural (altered chromosome architecture). Most originate from errors during meiosis, though some occur during early mitotic divisions of the embryo.

Numerical abnormalities involve the gain or loss of whole chromosomes, a condition called aneuploidy. The most common cause is nondisjunction, where homologous chromosomes or sister chromatids fail to separate properly during meiosis.

• Trisomy: an extra copy of a chromosome is present.
  • Down syndrome (trisomy 21) is the most common viable trisomy.
  • Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13) are more severe and often fatal in the first year of life.
• Monosomy: one chromosome is missing.
  • Turner syndrome (45,X) is the only monosomy compatible with life. Monosomy of autosomes is almost always lethal.

Structural abnormalities involve changes to chromosome shape or content:

• Deletions: a segment of a chromosome is lost (e.g., Cri-du-chat syndrome from a deletion on chromosome 5)
• Duplications: a segment is copied, resulting in extra genetic material
• Inversions: a segment is flipped in orientation within the chromosome
• Translocations: a segment moves to a different chromosome (e.g., a Robertsonian translocation between chromosomes 14 and 21 can cause familial Down syndrome)

Effects and Variability of Chromosomal Abnormalities

The phenotypic effects of chromosomal abnormalities depend on which chromosomes and gene regions are involved.

• Down syndrome (trisomy 21): associated with intellectual disability (typically mild to moderate), characteristic facial features, hypotonia (low muscle tone), and an increased risk of congenital heart defects (occurring in about 40–50% of affected individuals).
• Turner syndrome (45,X): characterized by short stature, ovarian dysfunction leading to infertility, webbed neck, and sometimes cardiac or kidney abnormalities.

Mosaicism is an important source of variability. It occurs when nondisjunction happens during an early mitotic division after fertilization, producing two or more genetically distinct cell populations in the same individual. Someone with mosaic Down syndrome, for example, has some cells with the normal 46 chromosomes and others with 47. This typically results in milder symptoms compared to full trisomy 21, and the severity depends on the proportion and location of affected cells.

Methods of Genetic Screening

Prenatal and Preimplantation Genetic Screening

Several screening and diagnostic tools are available during pregnancy, each with different timing, invasiveness, and accuracy.

• Non-invasive prenatal testing (NIPT): Analyzes cell-free fetal DNA circulating in the mother's blood. It screens for common trisomies (21, 18, and 13) and can be performed as early as 10 weeks. NIPT is a screening test, not a diagnostic one, so positive results are typically confirmed with amniocentesis or CVS.
• Chorionic villus sampling (CVS): A small sample of placental tissue (chorionic villi) is removed and analyzed for chromosomal abnormalities and specific genetic disorders. Performed between 10 and 13 weeks of pregnancy. Because it samples placental cells, it carries a small risk of miscarriage (roughly 0.5–1%).
• Amniocentesis: A needle is used to withdraw a small amount of amniotic fluid containing fetal cells. These cells are cultured and analyzed for chromosomal and genetic abnormalities. Performed between 15 and 20 weeks of pregnancy, also with a small miscarriage risk.
• Preimplantation genetic diagnosis (PGD): Used during in vitro fertilization (IVF). One or a few cells are removed from an embryo at the blastocyst stage and tested for specific genetic disorders or chromosomal abnormalities before the embryo is transferred to the uterus.

Newborn and Carrier Screening

Newborn screening is performed shortly after birth (typically via a heel-prick blood test) to identify treatable genetic conditions before symptoms appear. Every state in the U.S. mandates screening, though the specific panel of conditions varies.

• Phenylketonuria (PKU): An inherited metabolic disorder where the body cannot break down the amino acid phenylalanine. Without treatment (a strict low-phenylalanine diet started in infancy), toxic buildup causes severe intellectual disability. With early detection and dietary management, children develop normally.
• Congenital hypothyroidism: The thyroid gland does not produce sufficient thyroid hormone, which is critical for brain development. If untreated, it leads to developmental delays and growth failure. Early hormone replacement therapy prevents these outcomes.

Carrier screening identifies individuals who carry one copy of a recessive allele without showing symptoms. This is particularly useful for family planning.

• Carriers are typically asymptomatic but can pass the allele to offspring. If both parents are carriers of the same autosomal recessive disorder, each child has a 25% chance of being affected.
• Carrier screening is often recommended for individuals with a family history of a specific disorder or those from populations with higher carrier frequencies (e.g., Tay-Sachs disease in Ashkenazi Jewish populations, sickle cell trait in populations of African descent).

Ethical Considerations of Genetic Screening

Privacy, Discrimination, and Informed Consent

Genetic screening generates highly personal information, raising significant concerns about who has access to it and how it can be used.

• The Genetic Information Nondiscrimination Act (GINA), passed in 2008, prohibits discrimination based on genetic information in health insurance and employment in the United States. However, GINA does not cover life insurance, disability insurance, or long-term care insurance, which is a notable gap.
• Informed consent is a requirement for genetic screening. Patients must understand the purpose of the test, what conditions it screens for, the possibility of uncertain or unexpected results, and the potential psychological impact before agreeing to testing.
• Genetic counseling plays a central role in helping individuals and families interpret results, understand inheritance risks, and make informed decisions about medical care or family planning.

Equity, Accessibility, and Appropriate Use

Access to genetic screening is not equal across populations. Cost, insurance coverage, geographic location, and education all influence who benefits from these technologies. These disparities can widen existing health inequalities when some populations receive early diagnoses and interventions while others do not.

Preimplantation genetic diagnosis raises particularly complex ethical questions:

• PGD was developed to screen for serious genetic diseases, but its potential use for selecting non-medical traits (sex, physical characteristics) raises concerns about so-called "designer babies."
• The boundary between preventing disease and enhancing traits is not always clear, and different societies draw that line in different places.

The broader use of genetic information for non-medical purposes (such as predicting athletic ability or behavioral traits) raises additional concerns about genetic determinism, the idea that genes alone define a person's potential. Genetic information is probabilistic, not deterministic, and misunderstanding this distinction can lead to discrimination or harmful social pressure. The psychological and social impact of screening results on individuals and families also deserves careful consideration, particularly when results are ambiguous or reveal unexpected information about family relationships.