Question

Explain how a single gene effects two or more independent pathways (Ex: White eye phenotype for a friut fly). Propose five hypothesis (details needed).?

Answer 1

The phenomenon when a single gene affects or influences two or more independent pathways is called Pleiotropy.

In such cases , mutation in a single gene causes a change in several traits simultaneously as the gene coding for a product is used by a number of different cells or targets that have the same signalling function.

Gene pleitropy occurs when a gene product interacts with a multiple other proteins or catalyses multiple reactions. It might have a cascade like signalling function that affects various tergets.

Pleitropy is considered the reason for the occurance of disorders like phenylketonuria, albinism. sickle cell anemia and certain forms of autism and schizophrenia .

Phenylketonuria is caused by a defect in a single gene on chromosome 12 . Multiple systems , such as the nervous and the integumentary systems are affected .

One model for ocurance of pleitropy describes a single gene locus to the expression of a single trait. The locus affects the expressed trait only through changing the expression of other loci. Over time , the licus would affect two traits by interacting with a second locus.

It is believed that pleiotropy increases the phenotypic variation of more than one trait since a single mutation on a gene would have twice the effect.

Some examples of how a single gene can affect two or more independent pathways are--

*Albinism -- It is caused by the mutation of TYR gene . The mutation alters the production of melanin, a pigment that absorbs light and provide colouration to the skin. Several melanin related traits are affected throughout the organisms body like absence of colour in the eye , hair etc. Some forms of albinism affect the eye movement , light sensitivity and strabismus.

* Autism and schizophrenia-- Deletion in the 22q11.2 region of chromosome 22 has been associated with schizophrenia and autism. Although these two disorders are linked to the same gene, but they manifest in two completely different ways.

Chidhood manifestation of the gene disorder results in autism but if the manifestation is later in life , it might result in schizophrenia or other psychotic disorders.

* Sickle cell anemia -- This is a disease which results in the production of deformed red blood cells. It is caused by point mutation in the HBB gene . This gene encodes information to make the beta-globin sub-unit of hemoglobin.

The sickle shaped red blood cells lead to several complications like organ damage, risk of blood clots, strokes, high blood pressure and loss of vision.

* Chickens exhibit various traits affected by pleiotropic genes. There is the frizzled feather trait in which their feathers curl outwards and upwards instead of lying flat against the body. It is caused due to deletion in the genomic region coding for alpha-keratin. Several other abnormalities like increased metabolism, high food consumption , accelerated heart rate and delayed sexual maturity have been linked to the deletion of alpha keratin.

So, we can conclude that since the body works as a unit, it is not possible to keep any genetic change from affecting other body functions directly or indirectly.