Gregor Mendel, the father of genetics made various interesting observations during his study of inheritance regarding the colour of different plant components. Significantly, Mendel also observed that the plants with coloured seed coats had coloured leaf axils ( a part that connects leaves to stems) and coloured flowers. Mendel also noticed that pea plants had colourless seed coats with no pigmentation on their axils and white-coloured flowers. The colour of the seed coat was always concerned with specific axil and flower colour.
Today we come to the conclusion that Mendel’s observations were based on the result of pleiotropy which is the phenomenon wherein an individual gene plays its role in multiple phenotypic traits. Here, the gene of seed coat colour was not the only one responsible for its colour but also for the axil pigmentation and flower.
A human genetic disorder known as the Marfan syndrome is caused due to the mutation in a single gene yet it affects various aspects of growth and development that include vision, height, and heart functioning. All these are examples of pleiotropy or a gene that affects multiple characteristics.
When we talk about Mendel’s experiments with white-coloured flower and purple coloured plants, we do not think about phenotypes concerned with the colours of two flowers. However, Mendel observed that the colours were always related to two different features: the seed coat colour and the colour of axils.
A plant bearing white-coloured flowers consist of colourless axils and seed coats, whereas plants with purple flowers have brown-grey coloured seed coats with reddish axils. Hence, instead of affecting only one characteristic, the colour gene affects three.
Genes of this kind that control multiple and unrelated features are called pleiotropic where the term pleio refers to many and tropic indicates effects. In this manner, the discrete phenotypes could be identified back to a defect in one gene with different jobs.
Moreover, the alleles of pleiotropic genes are transferred in a similar way as alleles of genes affect the single traits. Although a phenotype has various elements, these elements are expressed as a package and the recessive and dominant versions of the package would be visible in the offspring of two heterozygotes in a 3:1 ratio.
Human Genetic Disorders
The genes which are affected by human genetic disorders are mostly pleiotropic. For example, a person with a hereditary disorder known as Marfan syndrome could have a set of unrelated symptoms that involve the following:
- Abnormal tall height
- Dislocation of the eye lens
- Lean fingers and toes
Heart problems that include the aorta, large blood vessels that carry blood away from the heart, ruptures.
The above symptoms do not seem relevant but as it turns out, it could be traced back to the mutation of an individual gene. This gene encodes a protein into chains and makes fibrils that provide flexibility and strength to the body’s connective tissues. Mutations that are responsible for Marfan syndrome minimize the amount of functional protein formed by the body and results in lesser fibrils.
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