Does each gene cause a single trait or does each gene cause multiple traits?

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Please find below the solution/link to the asked query:
Each gene can cause both i.e. single trait as well as multiple traits.

There are many traits that are controlled by the single gene. Some of which are as follows:
  • Presence of Freckles: Freckles are produced as a result of variation of MC1R gene while other genes are involved in influencing the trait. 
  • Red/green color blindedness is controlled by a single gene.
  • PTC tasting or Phenylthiocarbamide receptors is controlled by a single gene, the tasters usually carry one copy of the tasting version of the gene.
  • Similarly there are traits such as Huntington diseasePhenylketonuria , Hemophilia A  that involves only one gene.
Many human traits are controlled by more than one gene. These traits are called polygenic traits (or characteristics). The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.
 
An example of a human polygenic trait is adult height.

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