Polymorphisms and Genetc Variability

Phenotypic Polymorphisms

When individuals differ in a discreet character, the different forms are called morphs A population is said to display phenotypic polymorphism for a character if two or more distinct morphs are each represented in high enough frequencies to be readily noticeable. (Obviously, the defination of “readily noticeable” is somewhat subjective, but a population is not considered polymorphic if it consists primarily of a single morph and other morphs are extremely rare.)

In contrast, height variation in human population does not show phenotypic polymorphism because it does not consist of distinct and separate morphs – heights vary along a continuum. Nonetheless, polymorphisms play a role in such characters at the genetic level. The heritable component of height is the result of such genetic polymorphisms for alleles at the several loci that influence height.

Genetic variability in humans

Your text discusses the basis for average heterozygosity and explains its distinction from nucleotide variability. Average heterozygosity tends to be greater than nucleotide variability. This is because a gene can consist of thousands of base pairs of DNA. A difference at only one these bases is sufficient to make two alleles of that gene different and contribute to average heterozygosity. For example, the average heterozygosity of Drosophila is 14% (meaning that it is heterozygous at about 14% of its loci), whereas its nucleotide variability is 1%. Compared to the nucleotide variability of humans (0.1%), we can say that Drosophila individuals are 10 times more different to each other than humans.

 

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