Genetic Disorders

---

Impact of Genetic Disorders

Genetic disorders are far more common than is widely appreciated by the general public. They are not rare anomalies; they are a fundamental part of human medicine.

• Lifetime Prevalence: The estimated lifetime prevalence of genetic diseases is 670 per 1000 individuals. This means that over the course of a lifetime, more than half the population will experience a disease that has a genetic component.
• Early Gestation: It is estimated that 50% of spontaneous abortions (miscarriages) during the early months of pregnancy occur because the embryo has a demonstrable chromosomal abnormality incompatible with life.
• Newborns and Youth: About 1% of all newborn infants possess a gross (large-scale, easily visible) chromosomal abnormality. Furthermore, approximately 5% of individuals under age 25 develop a serious disease with a significant genetic component.

Genetics versus Genomics

While these terms sound similar, they represent different scales of study:

• Genetics: The study of single genes or a few specific genes and their phenotypic effects (the physical traits they produce). Example: Studying the single mutated gene that causes Cystic Fibrosis.
• Genomics: The comprehensive study of all the genes in the entire genome and how they interact with each other. Example: DNA microarray analysis of tumors is an excellent example of genomics in current clinical use. It looks at thousands of genes at once to understand a cancer's behavior.

The Spectrum of Human Diseases

Every human disease falls somewhere on a spectrum based on what causes it:

• Environmentally determined: Caused purely by outside factors (e.g., getting a sunburn or breaking a bone in a car accident).
• Genetically determined: Caused purely by DNA (e.g., Sickle cell anemia).
• Environmentally AND Genetically determined: A mixture of both. Someone might have a genetic predisposition to lung cancer, but smoking (environment) triggers it.

---

Some Definitions

Before looking at the specific diseases, we must clearly define the terminology used in medical genetics.

• Genetic Disorders: A heterogeneous (diverse) group of disorders caused by abnormalities in genes or whole chromosomes.
• Hereditary Disorders: These are derived from one’s parents and are transmitted through the germ line (sperm and egg cells) across generations. Therefore, these conditions are familial (they run in families).
• Congenital: Simply means "born with." It is crucial to note that not all congenital diseases are genetic (e.g., a baby born with syphilis acquired it from the mother during birth, which is environmental), and not all genetic diseases are congenital (e.g., Huntington's disease is genetic but symptoms don't appear until age 40).
• Mutations: A permanent change in the DNA sequence.

Germ Cells versus Somatic Cells

Where a mutation happens dictates whether it can be passed on to children:

• Germ Cell Mutations: Mutations that affect sperm or egg cells. These are transmitted to the progeny (offspring) and give rise to inherited diseases.
• Somatic Cell Mutations: Mutations that arise in the regular cells of the body (like skin, liver, or lung cells) after birth. Understandably, these do not cause hereditary diseases because they are not in the sperm or egg. However, they are immensely important in the genesis of cancers and some congenital malformations.

Classification of Mutations

Mutations are classified by the "size" of the DNA mistake:

• Genome mutations: The largest errors. The loss or gain of whole chromosomes. This gives rise to monosomy (missing a chromosome) or trisomy (having an extra one, like Trisomy 21 causing Down Syndrome).
• Chromosome mutations: The rearrangement of genetic material. A whole chunk of a chromosome might break off and attach somewhere else. These give rise to visible structural changes in the chromosome under a microscope. Most of these are highly destructive and incompatible with survival.
• Gene mutations: The smallest, but most common, errors. These may result in partial or complete deletion of a specific gene, or more often, affect just a single base (a single "letter" in the DNA code).

General Classification of Genetic Disorders

Genetic disorders are grouped into three massive categories:

1. Disorders related to mutant genes of large effect (Mendelian disorders).
2. Diseases with multifactorial inheritance.
3. Chromosomal disorders.

---

Mendelian Disorders (Mutant Genes of Large Effect)

Named after Gregor Mendel (the father of genetics), these disorders are the result of expressed mutations in single genes that have a very large, obvious effect on the body. An estimated 80% to 85% of these mutations are familial (inherited from parents).

Most of these diseases are recessive, meaning a person needs two bad copies of the gene to show symptoms. Because of this, many people carry these mutations without having any serious phenotypic effect themselves.

The Laws Governing Mendelian Inheritance

To understand how these traits are passed down, we rely on Mendel's fundamental laws, which were later proven by the discovery of meiosis (the cell division process that creates sperm and eggs).

• Law of Segregation (The "First Law"): States that when any individual produces gametes (sperm/eggs), the two copies of a gene separate, so that each gamete receives only one copy. A gamete will receive one allele or the other. In meiosis, the paternal and maternal chromosomes get physically separated, segregating the characters into two different gametes.
• Law of Independent Assortment (The "Second Law"): Also known as the "Inheritance Law," it states that alleles of different genes assort independently of one another during gamete formation. Mendel concluded that different traits are inherited independently of each other. Example: The gene for hair color is passed down completely independently from the gene for blood type. There is no relation between them.

---

Transmission Patterns of Single-Gene Disorders

Mendelian disorders follow three main transmission patterns: Autosomal Dominant, Autosomal Recessive, and X-Linked.

A. Autosomal Dominant Disorders

These disorders occur when you only need one mutant copy of a gene to show the disease. The abnormal gene is located on one of a pair of autosomes (the non-sex chromosomes, pairs 1-22).

• They are manifested in the heterozygous state.
• At least one parent of an index case (the patient) is usually affected.
• Both males and females are affected equally, and both can transmit the condition.
• New Mutations: Some patients do not have affected parents. Such patients owe their disorder to brand new (de novo) mutations in either the egg or sperm from which they were derived.
• Delayed Onset: In many autosomal dominant conditions, the age at onset is delayed. Symptoms and signs do not appear until adulthood (a prime example is Huntington disease, which often strikes in a person's 40s).

Modifying Factors in Autosomal Dominant Diseases:

The clinical features can be altered by two major phenomena:

1. Reduced Penetrance: Think of this as an "on/off" switch that fails. Some individuals inherit the mutant gene but are phenotypically completely normal. The gene is there, but it fails to penetrate and cause the disease.
2. Variable Expressivity: Think of this as a "volume dial." The trait is observed in all individuals carrying the mutant gene, but it is expressed very differently among individuals. One person might have a severe form, while their sibling has a very mild form.

Examples of Autosomal Dominant Disorders:

B. Autosomal Recessive Disorders

These disorders result only when both alleles at a given gene locus are mutants (homozygous state). If you have one good copy, it produces enough protein to keep you healthy.

• The trait does not usually affect the parents (they are just healthy carriers). However, siblings may show the disease.
• The condition appears in one-quarter (25%) of the brothers and sisters of an affected individual.
• Parents of the affected individual are often consanguineous (blood relatives, like first cousins). This increases the chance that both parents carry the exact same rare mutant recessive gene.
• The expression of the defect tends to be much more uniform than in autosomal dominant disorders (less variable expressivity).
• Complete penetrance is common (if you have two bad copies, you *will* get the disease).
• Onset is frequently very early in life (often seen in infants or toddlers).

Examples of Autosomal Recessive Disorders:

C. X-Linked Disorders

These are mutations on the sex chromosomes. Females are XX, males are XY.

• Almost all of these disorders are X-linked recessive.
• The Y Chromosome: Several genes are encoded in the male-specific region of the Y chromosome; all these are related to spermatogenesis. Males with Y-chromosome mutations are usually infertile.

Transmission Rules:

• An affected male does not transmit the disorder to his sons (because he gives his sons his Y chromosome, not his X).
• However, an affected male transmits the mutant X to all his daughters, making them all carriers.
• Sons of heterozygous carrier women have a 1:2 (50%) chance of inheriting the mutant gene and getting the disease.
• Female Protection: The heterozygous female usually does not express the full phenotypic change because she has a paired, normal allele on her other X chromosome to compensate. Males have no backup, which is why X-linked disorders predominantly affect males.

Examples of X-Linked Disorders:

---

Biochemical and Molecular Basis of Single-Gene Disorders

To truly understand Mendelian disorders, we must look at what the mutated gene actually failed to build. Genes are instructions for proteins. The defects fall into four main categories:

---

Disorders with Multifactorial Inheritance

These disorders do not follow simple Mendelian rules. Instead, they result from the combined actions of environmental influences AND two or more mutant genes having additive effects. No single gene is fully responsible.

• Interestingly, a massive number of normal phenotypic characteristics are governed by multifactorial inheritance, such as hair color, eye color, skin color, height, and intelligence. They exist on a spectrum because many genes are adding up together.
• The risk of expressing a multifactorial disorder is conditioned strictly by the number of mutant genes inherited. The more "bad" genes you inherit, the closer you get to the threshold of disease.
• The rate of recurrence of the disorder for all first-degree relatives of an affected individual is 2% to 7%. This means if a couple has a child with a multifactorial heart defect, the chance their next child has it is about 2-7% (much lower than the 25% or 50% seen in Mendelian disorders).