Human Genetics and Genetic Disorders

Sickle-cell disease is a hereditary blood disorder, characterised by red blood cells that assume an abnormal, rigid, sickle shape. Sickling decreases the cells' flexibility and results in a risk of various life-threatening complications. This sickling occurs because of a mutation in the haemoglobin gene.

A frameshift mutation is a genetic mutation caused by indels (insertions or deletions) of a number of nucleotides in a DNA sequence that is not divisible by three. Due to the triplet nature of gene expression by codons, the insertion or deletion can change the reading frame .

Mitochondrial neurogastrointestinal encephalopathy syndrome is a rare autosomal recessive mitochondrial disease. Unlike typical mitochondrial diseases caused by mitochondrial DNA (mtDNA) mutations, MNGIE is caused by mutations in the TYMP gene, which encodes the enzyme thymidine phosphorylase.

Only one mutated copy of the gene will be necessary for a person to be affected by an autosomal dominant disorder. Each affected person usually has one affected parent. The chance a child will inherit the mutated gene is 50%. Autosomal dominant conditions  have reduced penetrance, which means although only one mutated copy is needed, not all individuals who inherit that mutation go on to develop the disease.

The complete set of observable traits of the structure and behaviour of an organism is called its phenotype. These traits arise from the interaction of its genotype with the environment.

Topoisomerases regulates the overwinding or underwinding of DNA. The winding problem of DNA arises due to the intertwined nature of its double-helical structure.

Cancer genome sequencing is the whole genome sequencing of a single, homogeneous or heterogeneous group of cancer cells. It is a biochemical laboratory method for the characterisation and identification of the DNA or RNA sequences of cancer cell(s).

Gene targeting can be permanent or conditional. Conditions can be a specific time during development / life of the organism or limitation to a specific tissue.

Turner syndrome 45,X, encompasses several conditions in human females, of which monosomy X (absence of an entire sex chromosome, the Barr body) is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent or has other abnormalities.

Tritanopia is a very rare colour vision disturbance in which there are only two cone pigments present and a total absence of blue retinal receptors. Blues appear greenish, yellows and oranges appear pinkish, and purple colours appear deep red.

SLOS Chromosome SLOS is a metabolic disorder caused by a mutation in the DHCR7 (7-dehydrocholesterol reductase) gene on chromosome 11. People who have SLOS are unable to make enough cholesterol to support normal growth and development.

The typical eukaryotic nuclear intron has consensus sequences defining important regions.  Near the 3' end there is a branch site.

In squamous cell carcinoma, a protein from the epidermal growth factor receptor (EGFR) gene is often overexpressed in conjunction with polysomy of chromosome 7, so chromosome 7 can be used to predict the presence of EGFR in squamous cell carcinoma.

Each Okazaki fragment is preceded by an RNA primer, which is displaced by the procession of the next Okazaki fragment during synthesis.

Here a portion of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder.

Reverse transcriptase is added, along with deoxynucleotide triphosphates (A, T, G, C). This synthesises one complementary strand of DNA hybridised to the original mRNA strand.

A housekeeping gene is typically a constitutive gene that is transcribed at a relatively constant level. The housekeeping gene's products are typically needed for maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions.

RET mutation is responsible for increased risk of medullary thyroid cancer, as well as multiple endocrine neoplasia 2, in which pheochromocytoma and parathyroid adenoma also occur.

In Barrter's syndrome there is a mutation, which inhibits the activity of the NaK2Cl cotransporter. It is the molecule normally inhibited by frusemide. The clinical effects of Barrter''s syndrome are therefore the same as excess frusemide and include loss of sodium, potassium and water. The bodies response to this is to raise aldosterone levels in an attempt to conserve sodium.

Gamma C deficiency is responsible for about 40% of cases of SCID and, together with JAK 3 deficiency, results in a T-B+NK- phenotype. It is also utilised by the receptors for IL-2, -4, -7, -9 and -21.

In some cases, recombinant DNA can have deleterious effects even if it is not expressed. One mechanism by which this happens is insertional inactivation, in which the rDNA becomes inserted into a host cell's gene.

Cri-du-chat is caused by a deletion (the length of which may vary) on the short arm of chromosome 5. Multiple genes are missing, as a result of this deletion and each may contribute to the symptoms of the disorder. One of the deleted genes is TERT (telomerase reverse transcriptase).

A back mutation or reversion is a point mutation that restores the original sequence, and hence, the original phenotype.

The desired DNA fragment is inserted into a vector, usually a plasmid, for transfer to the receiving chromosome. P lasmids are circular DNA molecules found in the cytoplasm of bacteria that bond with the desired DNA fragment with the help of the joining enzyme, DNA ligase, to create the resulting recombinant DNA.

In this method the fragments are amplified upon primers attached to a solid surface and form DNA colonies, or DNA clusters. This method is used in the Illumina Genome Analyser sequencers.