Did You Know?
Genes have a 'memory.' The lives of your grandparents - the air they breathed, the food they ate, even the things they saw - can directly affect you, decades later, despite your never experiencing these things yourself.
~ BBC, Ghost in Your Genes
~ BBC, Ghost in Your Genes
"They are in you and me; they created us, body and mind; and their preservation is the ultimate rationale for our existence. They have come a long way, those replicators. Now they go by the name of genes, and we are their survival machines."
- Richard Dawkins
- Richard Dawkins
DNA or deoxyribonucleic acid is the macromolecule that contains all the information required for the development and functioning of an organism. This is achieved through sets of DNA sequences called genes, which are responsible for a particular phenotype, trait or function. These units of heredity are arranged together, along with their regulatory sequences, to form chromosomes.
In addition to the DNA present in nucleus, the mitochondria of each cell also possess their own genetic component called mitochondrial DNA. These mitochondrial genes are responsible for certain vital components of the cellular processes. Genetic diseases are the result of mutations, deletions, repetitions and other anomalies in one or more genes as well as chromosomal aberrations. Given below is a list of several genetic diseases and disorders, categorized on the basis of their inheritance and genetic complexity.
Single Gene Disorders
As the name suggests, this set of genetic disorders refers to those arising due to mutations in a single gene. Based on the location and the inheritance pattern of the gene, these diseases are further classified into the following types.
Genetic diseases that are manifested in the presence of a single copy of a mutated/abnormal autosomal gene are called autosomal dominant diseases. In other words, abnormal gene inherited from one parent can cause a disease, even if a normal copy of the gene is inherited from the other parent.
- Alagille syndrome
- Alexander disease
- Apert syndrome
- Optic atrophy type 1
- Gardner syndrome
- Hereditary spherocytosis
- Legius syndrome (LS)
- Marfan syndrome
- Marshall syndrome
- Huntington's disease
- Stone man syndrome
- Neonatal Onset multisystem inflammatory disease
- Liddle's syndrome
- Stickler syndrome
- Noonan syndrome
- Pallister-Hall syndrome
- Peutz-Jeghers syndrome
- Rubinstein-Taybi syndrome
- Russell-Silver syndrome
- Tuberous sclerosis
- von Hippel-Lindau syndrome
- Worth syndrome
- Neurofibromatosis type I
- Best's disease (Best's vitelliform macular dystrophy)
This set of diseases are manifested only when two copies of the mutated gene are inherited. If one gene is defective whereas the other inherited gene is normal, the individual is simply a carrier of the gene, and does not suffer from the disease.
- Alpers' disease
- Oculocutaneous albinism
- Canavan syndrome
- Dubin-Johnson syndrome
- Hemoglobin C disease
- Refsum disease
- Sickle cell anemia
- Alström syndrome
- Xeroderma pigmentosum
- Shwachman-Diamond syndrome
- Hurler syndrome
- Bloom syndrome
- Congenital adrenal hyperplasia
- Phenylketonuria (PKU)
- De Barsy syndrome
- Cystic fibrosis
- Primary carnitine deficiency
- Familial dysautonomia
- Gaucher's disease
- Laron syndrome
- Scheie syndrome
- Usher syndrome
- Dubowitz syndrome
- Pseudoxanthoma elasticum
- Louis-Bar syndrome
- Tay-Sachs disease
- Werner syndrome (adult progeria)
The disorders that arise due to defects or mutations in genes located on the X chromosome are termed X-linked disorders. An X-linked dominant disorder arises when a single copy of the gene leads to expression of the abnormal protein. A characteristic of X-linked disorders is that an affected father does not pass on the defect to sons, since sons inherit X-chromosome from their mothers. These are more commonly observed in females.
- Aicardi syndrome
- Coffin Lowry syndrome
- X-linked hypophosphatemia (familial vitamin d-resistant rickets)
- Rett syndrome
- Giuffrè-Tsukahara syndrome
- Danon disease (glycogen storage disease type IIb)
In this set of disorders, a single copy of defective gene located on the X-chromosome results in the manifestation of symptoms. They are more common in males, and families with such a condition, often have an affected male in each generation. An affected female will always pass on the condition to her sons.
- Color blindness
- Ocular albinism
- Barth syndrome (BTHS)
- Menkes disease
- X-linked ichthyosis (XLI)
- Aarskog-Scott syndrome (AAS)
The diseases resulting due to defects or anomalies in genes present on the Y chromosome are termed Y-linked disorders. The Y chromosome has very genes and Y-linked genetic diseases are rarely observed. These occur only in males and the son of an affected father will always inherit the disease.
The two known examples of Y-linked disorders are:
- Y chromosome infertility
- Swyer syndrome
This subset include diseases that are associated with several genes but result due to mutation in any of them. Their inheritance depends on the location of the particular gene affected.
- Corneal dystrophy
- DiGeorge syndrome
- Waardenburg syndrome
- Ehlers-Danlos syndrome (EDS)
- Congenital hypertrichosis
- Alpha-1 antitrypsin deficiency
- Polycystic kidney disease
- Charcot-Marie-Tooth disease (CMT)
- Duane syndrome
- Proteus syndrome
- Fragile X syndrome
- Spinocerebellar ataxia
- Osteogenesis imperfecta
- Treacher Collins syndrome
- Myotonic hypertrophy
- Hutchinson-Gilford progeria syndrome
"Famous Single Gene Disorders"
- Miles Davis: Sickle cell anemia
- Fred Rogers: Color blindness
- Vincent van Gogh: Acute intermittent porphyria
- Bob Flanagan: Cystic fibrosis
- Woodrow Wilson "Woody" Guthrie: Huntington's disease
- Hemophilia: Members of the European royal family
The disorders that arise due to the absence of a particular chromosome, presence of an additional chromosome or due to structural anomalies (deletions, duplication or translocation) in a chromosome are termed chromosomal disorders. Such anomalies occur randomly during the formation of an egg or sperm in the gonads, and most of them are not inheritable. However, certain structurally defective chromosomes may be inherited leading to passing of the disease from one generation to the next.
The human karyotype depicting the complete set of 46 chromosomes has been given below.
- Patau syndrome (trisomy 13)
- Down's syndrome (trisomy 21)
- Edwards syndrome (trisomy 18)
- Williams syndrome (WS) (deletion 7q.11.23)
- Smith-Magenis syndrome (17p syndrome)
- Jacobsen syndrome (11q deletion disorder)
- Wolf-Hirschhorn syndrome (deletion 4p.16.3)
- XYY syndrome (Extra Y chromosome)
- Turner syndrome (monosomy X)
- Klinefelter syndrome (XXY trisomy)
- Triple X syndrome (XXX Syndrome)
- Cri du chat syndrome (5p deletion syndrome)
- Potocki-Shaffer syndrome (11p deletion syndrome)
- Pallister-Killian syndrome (tetrasomy 12p mosaicism)
"Famous Chromosomal Disorders"
- Christopher Joseph "Chris" Burke: Down's syndrome
Polygenic and Multifactorial Disorders
Genetic diseases that arise due to defects in more than one gene as well as non-genetic factors are termed polygenic or multifactorial genetic diseases. The non-genetic factors include environment, lifestyle, diet, habits, etc. These diseases often run in families but do not follow a definite pattern of inheritance.
The susceptibility to these diseases with respect to the different factors has been illustrated below.
- Diabetes mellitus
- Alzheimer's disease
- Cerebral palsy
- Angelman syndrome
- Bardet-Biedl syndrome
- Prader-Willi syndrome
- Bipolar disorder
- Cleft lip and palate
- Hirschsprung's disease
- Macular degeneration
- Neural tube defects (NFD)
- Skeletal dysplasias
- Androgen insensitivity syndrome
- Cardiovascular conditions
- Iron-deficiency anemia
- Addison's disease
- Parkinson's disease
- Asperger's syndrome
- Muscular dystrophy
- Lou Gehrig's disease
- Spina bifida
- Epidermolysis bullosa
- Fibromuscular dysplasia
- Guillain-Barré syndrome
- Pierre Robin syndrome
- Congenital/acquired hip dysplasia
"Famous Multifactorial Disorders"
- John F. Kennedy: Addison's disease
- Thomas Edison: Asperger's syndrome
- Adolf Hitler: Parkinson's disease
- Lou Gehrig: Amyotrophic lateral sclerosis (Lou Gehrig's disease)
Mitochondrial DNA-associated Disorders
Diseases that are characterized by alterations in mitochondrial function due to defects in nuclear or mitochondrial DNA (mtDNA) are collectively called mitochondrial diseases. The inheritance of these disease is often complex and depends on several factors. However, the set of diseases that arise due to mutations in mtDNA are often inherited maternally.
The examples of these diseases have listed below.
- Pearson syndrome
- Kearns-Sayre syndrome (KSS)
- Leber hereditary optic neuropathy (LHON)
- Myoclonic epilepsy and ragged-red fibers (MERRF)
- Maternally-inherited diabetes and deafness (MIDD)
- Neuropathy, ataxia, and retinitis pigmentosa (NARP)
- Chronic progressive external ophthalmoplegia (CPEO)
- Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS)
Unfortunately, the list of genetic diseases is an unending one, and new additions occur frequently, thereby presenting a new challenge to researchers and the medical community. However, the following words by Richard Dawkins seem to provide a great inspiration in understanding these diseases, and finding a way to counterbalance their errors.
"Let us understand what our own selfish genes are up to, because we may then at least have a chance to upset their designs, something that no other species has ever aspired to do."