Pleiotropy : An Overview MCQ - Practice Questions with Answers

• There are instances where a single gene can exhibit multiple phenotypic expressions. Such a gene is called a pleiotropic gene. 
• The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways which contribute towards different phenotypes.

Instance 1 of pleiotropy:

• When we discussed Mendel’s experiments with purple-flowered and white-flowered plants, we didn’t mention any other phenotypes associated with the two flower colors.
• However, Mendel noticed that the flower colors were always correlated with two other features: the color of the seed coat (covering of the seed) and the color of the axils (junctions where the leaves met the main stem). 
• In plants with white flowers, the seed coats and axils were colorless, while in plants with purple flowers, the seed coats were brown-gray and the axils were reddish. 
• Thus, rather than affecting just one characteristic, the flower color gene actually affected three.

Instance 2 of pleiotropy:

• Another example can be the gene that controls starch-synthesis as well as shape of seeds in pea plants.
• It has two alleles (B and b). 
• Starch is synthesised effectively by BB homozygotes and therefore, large starch grains are produced. 
• In contrast, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains. 
• After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled.
• Heterozygotes produce round seeds, and so B seems to be the dominant allele. 
• But, the starch grains produced ar of intermediate size in Bb seeds. 

Instance 3 of pleiotropy:

• Phenylketonuria is a disease which occurs in humans. 
• The disease is caused by mutation in the gene (present on chromosome 12) that codes for the enzyme phenylalanine hydroxylase (single gene mutation). 
• It results in accumulation of phenylalanine and is not converted to tyrosine.
• This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.