Sign epistasis occurs when one mutation has the opposite effect when in the presence of another mutation. This occurs when a mutation that is deleterious on its own can enhance the effect of a particular beneficial mutation. For example, a large and complex brain is a waste of energy without a range of sense organs, however sense organs can be more useful if the organisms brain is better able to process the information.

The first sex-linked gene ever discovered was the lacticolor X-linked recessive gene in the moth Abraxas grossulariata by Leonard Doncaster.

Epistasis has a large influence on the shape of evolutionary landscapes, which leads to profound consequences for evolution and evolvability of traits.

Hemophilia A is a blood clotting disorder caused by the mutation of factor VIII gene and leads to the deficiency of factor VIII. It was once thought to be the royal disease found in the descendants of Queen Victoria.

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.

DNA gyrase relieves strain while double-strand DNA is being unwound by helicase. This causes negative supercoiling of the DNA.

Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body. Even in a homozygous dominant or recessive female, the condition may not be expressed fully. Example: Baldness in humans.

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

A nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon or a nonsense codon in the transcribed mRNA, and possibly a truncated and often non functional protein product.

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance.

DNA is synthesised from 5' to 3', thus when copying the 3' to 5' strand, replication is continuous. Phosphodiester links form between the 3' to 5' and nucleotides can be added with the aid of the enzyme DNA polymerase for the continuous leading strand.

Insertion of one base will alter the reading frame and usually leads to a stop codon eventually.

Primase adds RNA primers onto the lagging strand, which allows synthesis of Okazaki fragments from 5’ to 3’. However, Primase creates RNA primers at a much lower rate than that at which DNA polymerase synthesizes DNA on the leading strand.

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

With reference to the genetic code, the conversion of ATC to GTC is isoleucine to valine,  both are nonpolar amino acids. Therefore, this substation is most likely to be conservative.

The spliceosome is assembled from snRNAs and protein complexes. The spliceosome removes introns from a transcribed pre-mRNA, a kind of primary transcript. snRNA (small nuclear RNA) is a component of the splicosome.

A Robertsonian translocation in balanced form results in no excess or deficit of genetic material and causes no health difficulties.

The mutation in the melanocortin-4 receptor protein (MC4R) which is a component of leptin-melanocortin pathway was detected to be related to obesity. The common monogenic obesity disorder was represented by MC4R deficiency. The obesity was accounted due to homozygous and heterozygous mutations in MC4R.

A monosomic disorder results when a chromosome pair fails to separate during either meiosis 1 or 2, a process known as nondisjunction. Nondisjunction can result in monosomy or trisomy. Turner syndrome occurs when part or all of the second sex chromosome is absent, resulting in a 45X karyotype.

Individuals who are able to provide the necessary food, shelter and avoid predation reproduce more successfully than others. Over generations, the characteristics of the population change as those individuals more successful in reproduction populate the species. The process in which the frequency of certain traits within a species change by uneven reproduction rates caused by natural selection is called evolution.

The lac repressor is a tetramer of identical subunits. Each subunit contains a helix-turn-helix (HTH) motif capable of binding to DNA. The operator site where repressor binds is a DNA sequence with inverted repeat symmetry.

Darwin's ideas were inspired by the observations that he had made on the Beagle voyage, and by the work of a political economist, the Reverend Thomas Malthus, who in An Essay on the Principle of Population, noted that population (if unchecked) increases exponentially, whereas the food supply grows only arithmetically; thus, inevitable limitations of resources would have demographic implications leading to a struggle for existence.

In positive repressible operons, the activator proteins are normally bound to the pertinent DNA segment. However, when an inhibitor is bound by the activator, it is prevented from binding the DNA. This stops activation and transcription of the system.

It is a genetic state in which the fingers are unusually short in heterozygotic condition. But, this condition is lethal during early years to homozygous recessive individuals due to major skeletal defects.

True or genomic polyploids are derived from the hybridisation of parents with dissimilar genome sets.

XX male syndrome occurs when there has been a recombination in the formation of the male gametes, causing the SRY-portion of the Y chromosome to move to the X chromosome. When such an X chromosome contributes to the child, the development will lead to a male because of the SRY gene.

Some people can curl up the sides of their tongue to form a tube shape. Tongue rolling is controlled by a single gene. However, people can learn to roll their tongue as they get older, suggesting that environmental factors also influence the trait.

Non-penetrance means that a person has inherited the genetic change, but that it does not manifest the associated phenotype. A skipped generation for a dominant trait is an example of non-penetrance.

Presymptomatic testing involves determining whether an individual has inherited a gene mutation that exhibits age-dependent penetrance.

Maternal uniparental disomy (UPD) for chromosome 15 results in Prader–Willi syndrome; paternal UPD would lead to Angelman syndrome.

Human skin colour ranges in variety from the darkest brown to the lightest pinkish-white hues. Human skin pigmentation is the result of natural selection. Skin pigmentation in human beings evolves primarily to regulate the amount of ultraviolet radiation penetrating the skin, controlling its biochemical effects.

SLOS Chromosome SLOS is a metabolic disorder caused by a mutation in the DHCR7 (7-dehydrocholesterol reductase) gene on chromosome 11. This gene codes for an enzyme that is involved in the production of cholesterol. People who have SLOS are unable to produce enough cholesterol to support normal growth and development.

This state results from an abnormal swap over of genetic material between chromosomes. This swap over occurs as a chance event in the father of an affected person during the formation of sperm cells. The translocation affects the gene in charge that is SRY for development of a foetus into a male.

This type of mutation takes place between two chromosomes and interchanges their chromosome segments. In other words, a portion of chromosome is transferred on to another chromosome.

They form a molecular record of the evolutionary history and statistics of the parental genome.

This gene regulates calcium in melanocytes and is important in the process of melanogenesis.

Thermal disruption at elevated temperature increases the rate of depurination (loss of purine bases from the DNA backbone) and single-strand breaks. For example, hydrolytic depurination is seen in the thermophilic bacteria, which grow in hot springs at 40-80 °C.

Promoter bashing is a technique used in molecular biology to identify how certain regions of a DNA strand, commonly promoters, affect the transcription of downstream genes.

47, XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An incident in chromosome separation during anaphase II (of meiosis II) called non disjunction can result in sperm cells with an extra copy of the Y-chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y-chromosome in each of the body's cells.

Natural selection acts on the phenotype or the observable characteristics of an organism, but the genetic (heritable) basis of any phenotype that gives a reproductive advantage may become more common in a population.

An inducible gene is a gene whose expression is either susceptible to environmental change or dependent on the position in the cell cycle.

Allolactose is an isomer of lactose and is the inducer of the lac operon. Lactose is galactose-(β1->4)-glucose, whereas allolactose is galactose-(β1->6)-glucose. Lactose is converted to allolactose by β-galactosidase in an alternative reaction to the hydrolytic one.

Helicases are proteins that are a type of a molecular motor. They use the chemical energy in nucleoside triphosphates, predominantly ATP, to break hydrogen bonds between bases and unwind the DNA double helix into single strands. These enzymes are essential for most processes where enzymes need to access the DNA bases.

Arginine is an α-amino acid. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG are the triplets of nucleotide bases or codons that code for arginine during protein synthesis.

Sickle-cell disease is a hereditary blood disorder, characterised by red blood cells that assume an abnormal, rigid and 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.

Centromere protein C 1 is a centromere autoantigen and a component of the inner kinetochore plate. The protein is required for maintaining proper kinetochore size and a timely transition to anaphase. A putative pseudogene exists on chromosome 12.

It is referred to the situation in which a gene does not have a vertical origin (that is direct inheritance from parent to offspring) but comes from an unrelated genome. It is recognised that this type of transfer occurs between bacteria, and that it also has taken place between the genomes of the cell organelles like mitochondria, chloroplasts and the nuclear genomes.

Q-banding is a fluorescent pattern obtained using quinacrine for staining. The pattern of bands is very similar to that seen in G-banding. They can be recognised by a yellow fluorescence of differing intensity. Most part of the stained DNA is heterochromatin. Quinacrin (atebrin) binds both regions rich in AT and in GC, but only the AT-quinacrin-complex fluoresces.

H2A is important for packaging DNA into chromatin. Since H2A packages DNA molecules into chromatin, the packaging process will effect the gene expression. H2A has been correlated with DNA modification and epigenetics. H2A plays a major role in determining the overall structure of chromatin.

Toc1 gene, also known as Timing of CAB Expression 1 gene, is an oscillating gene found in the plants that is known to control the expression of CAB. It has been shown to affect the period of circadian rhythms through its repression of transcription factors. This was found through mutations of toc1 in plants that had shortened period of CAB expression.