Hereditary diseases are medical conditions passed down from one generation to the next. These conditions arise from changes in an individual’s genetic material, which can then be transmitted from parents to their children. This article explains the basic mechanisms through which these conditions are inherited.
The Genetic Blueprint: DNA, Genes, and Chromosomes
Our bodies operate based on instructions contained within deoxyribonucleic acid, or DNA. DNA provides the code to build and maintain an organism. These instructions are organized into functional units called genes, which are specific segments of DNA. Genes carry the information for particular traits or functions, such as eye color or how cells produce certain proteins.
Genes are arranged on larger structures known as chromosomes. Each human cell contains 23 pairs of chromosomes, totaling 46. One set of 23 chromosomes comes from the mother, and the other from the father, ensuring each person inherits two copies of most genes. Different versions of a gene are known as alleles; an individual inherits two alleles for any given gene, one from each parent. These alleles can be dominant, meaning only one copy is needed for a trait to be expressed, or recessive, requiring two copies for expression.
Common Patterns of Hereditary Disease Transmission
Hereditary diseases often follow predictable patterns based on how genes are inherited. One common pattern is autosomal dominant inheritance, where only one copy of an altered gene on a non-sex chromosome (autosome) is sufficient for a person to develop the condition. An affected individual has a 50% chance of passing the condition to each child, regardless of the child’s sex. Examples include Huntington’s disease and Marfan syndrome.
Autosomal recessive inheritance requires two copies of the altered gene, one from each parent, for the disease to manifest. Individuals with only one copy of the altered gene are considered carriers; they do not show symptoms but can pass the gene to their children. If both parents are carriers, each child has a 25% chance of inheriting two altered copies and developing the condition, a 50% chance of being a carrier, and a 25% chance of inheriting two normal genes. Cystic fibrosis and sickle cell anemia are examples of autosomal recessive disorders.
X-linked inheritance involves genes located on the X chromosome, one of the two sex chromosomes. Since males have one X and one Y chromosome (XY) and females have two X chromosomes (XX), X-linked conditions affect males more frequently and severely. For X-linked recessive conditions, a male with an altered gene on his single X chromosome will develop the condition because he lacks a second X chromosome to compensate. Females are unaffected carriers if they have one altered X chromosome and one normal X chromosome, but can pass the altered gene to their sons. Hemophilia and red-green color blindness are examples of X-linked recessive disorders.
Genetic Changes and Disease Development
Hereditary diseases arise from genetic mutations, which are changes in the DNA sequence. These changes can range from alterations in a single “chemical letter” of DNA to larger rearrangements. When a gene’s DNA sequence is altered, the instructions it carries for making proteins can be corrupted or become incomplete. Proteins are molecules that perform most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.
A mutation can lead to the production of a faulty protein, a protein that is missing entirely, or one that does not function correctly. For example, a point mutation, a change in a single DNA base, can alter the protein produced, as seen in sickle cell anemia. Deletions (loss of DNA segments) or insertions (addition of DNA segments) can cause frameshift mutations, severely disrupting the protein’s code. These errors in the genetic blueprint cause hereditary diseases when they occur in reproductive cells.
More Complex Inheritance Scenarios
Beyond common Mendelian patterns, some hereditary conditions involve more intricate inheritance mechanisms. Polygenic inheritance describes diseases influenced by multiple genes, interacting with environmental factors. Conditions like heart disease, type 2 diabetes, and certain cancers do not follow simple inheritance patterns because many genes contribute to their development, alongside lifestyle and environmental influences. These traits show a continuous range of variation rather than distinct categories.
Mitochondrial inheritance is a pattern where diseases are inherited exclusively from the mother. Mitochondria, organelles outside the cell nucleus that generate energy, contain their own DNA (mtDNA). Because only egg cells contribute mitochondria to the developing embryo, any genetic alterations in mtDNA are passed solely from the mother to all her children. Affected males will not pass the condition to their offspring.
Chromosomal abnormalities involve larger-scale changes in the number or structure of chromosomes. Unlike single-gene disorders, these conditions result from an entire chromosome being extra, missing, or having significant structural rearrangements. Down syndrome, for instance, is caused by an extra copy of chromosome 21. These abnormalities can arise during the formation of egg or sperm cells or early fetal development and can lead to various developmental challenges.