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Molecular Genetics

Gene switch Gene switch Genes are sections of the genetic code (contained in DNA) which perform a specific function and contain the instructions to produce proteins for a specific purpose. This function can be switched on or off (activated or repressed) by proteins that bind to DNA. Some gene therapies take advantage of this by switching certain genes on or off in order to treat a genetic disorder. The complete core concepts of molecular genetics like DNA replication, gene expression and gene regulation are extensively presented here.

Learning Objectives

After completing the topic, the student will be able to:

  • Explore when, where and how DNA replication takes place and what happens if there is a mistake during DNA replication.
  • Understand, draw and explore the process of DNA replication, including all the enzymes and molecules that are necessary.
  • Summarize the molecular mechanism involved in transcription and translation phenomena.
  • Identify how DNA mutations influence protein structure and summarize how mutations may cause cancer.
  • Co–relate and understand the genetic concepts of bacteria and viruses.
  • Distinguish the different gene transfer mechanisms in bacteria (conjugation, transformation and transduction).
  • Investigate how operons act as on-off switches for transcription and allow for production of genes only when needed.
  • Apply the concepts of genetics of viruses and bacteria to produce novel products using recombinant DNA technology.
  • Apply the concepts of transcription and translation for the synthesis of desired fragments of DNA both by in vivo and in vitro methods.
Secret behind blue eyes Secret behind blue eyes Whether because of its exotic hue or its novelty, blue eyes have long been a subject of fascination for historians, poets, artists, and scientists.
Molecular Genetics

The traits such as brown, blue or gray eyes; black, brown or blond hair are transmitted from parents to offspring through discrete heritable units called "genes" which can be sorted and passed along, generation after generation, in undiluted form. Chromosomes inside the cell nucleus carry the genetic traits through genes and every new embryo formed out of merging of the cells (usually one male and one female) carries half the genes of each parent (genes of 23 chromosomes from mother and 23 from father for total of 46). The sharing of genes from the parents leads to inheritance and the varying dominance of different genes, causes some traits to appear unevenly instead of averaging out.

For example, the secret behind blue eyes is genetic mutation. Eye color is determined by the amount and type of pigments in the eye's iris and the variations in eye color such as blue, brown, green and others are attributed to varying ratios of eumelanin produced by melanocytes in the iris. The melanin content of the iris pigment epithelium, the melanin content within the iris stroma, and the cellular density of the iris stroma contributes to the color of eye. In all eyes, the iris pigment epithelium contains the black pigment, eumelanin. The color variations among different irises are typically attributed to the melanin content within the iris stroma. The density of cells within the stroma affects how much light is absorbed by the underlying pigment epithelium. OCA2 gene polymorphism, close to proximal 5'- (pronounced "five prime") regulatory region, explains most human eye-color variation.

Cross between blue and brown – colored eyes. Cross between blue and brown–colored eyes.

In humans, brown eyes are dominant over blue eyes. If a couple who are both heterozygous for brown eyes reproduce with one another, will the child have brown eyes or blue eyes? If we consider, B = brown eyes and b = blue eyes; the cross will be as follows.

P1: Bb × Bb

Genotypic ratio (GR): ¼ BB: 2/4 Bb: ¼ bb
Phenotypic ratio (PR): ¾ or 75% brown eyes: ¼ or 25% blue eyes
The child could have brown eyes or blue eyes. The probability that the child could be brown-eyed is 75% while the probability that it could be blue-eyed is only 25%.

Genetics of blue eyes

Blue eyes contain low amounts of melanin within the iris stroma; longer wavelengths of light tend to be absorbed by the underlying iris pigment epithelium, and shorter wavelengths are reflected and undergo Rayleigh scattering. The type of melanin present is eumelanin. The inheritance pattern followed by blue eyes is considered similar to that of a recessive trait. Eye color inheritance is considered as a polygenic trait, meaning that it is controlled by the interactions of multiple genes.

Recent research by Hans Eiberg from Department of Cellular and Molecular Medicine at the University of Copenhagen revealed that people with blue eyes have a single common ancestor and that genetic mutation lead to blue eyes while initially everyone had only brown eyes.

The study was published in Human Genetics which suggests that a mutation in the 86th intron of the HERC2 gene reduced expression of OCA2 gene with subsequent reduction in melanin production. The authors concluded that the mutation may have arisen in a single individual in the Near East or around the Black Sea region 6,000-10,000 years ago during the Neolithic revolution.

Eiberg stated, "A genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch, ' which literally 'turned off' the ability to produce brown eyes". The genetic switch is located in the gene adjacent to OCA2 and rather than completely turning off the gene, the switch limits its action, which reduces the production of melanin in the iris. In effect, the turned-down switch diluted brown eyes to blue eyes.

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