Question

The inheritance rule was initially discovered by Mendel and it depends on clear or qualitative traits. The theory was based on a single gene that was responsible for one character. However, expression of many traits are not governed by a single gene, rather can involve multiple genes as well as additive effects. This kind of inheritance pattern is known as quantitative and is demonstrated by East (1916) as well as others. He performed experiments using 2 homozygous, pure-breeding lines of Nicotiana longiflora that differed in corolla length. After crossing these lines, F1 offspring had an intermediate length for corolla and had same variation level as in parents. But found a very different result for F2 and F3 than expected. Discuss the reasons for this result and if it agrees/disagrees with Mendel’s law. Using obtained knowledge so far, what kind of cumulative effects would you add to this variation?

Answer 1

Traits which are controlled by more than one gene are called as polygenic characters. Since each gene contributes to the formation of final character, such characters are also called as cumulative characters or quantitative characters. When a cross is made between two parents for a polygenic character, all F1 individuals show an intermediate character. But when F1 and F2 are self crossed, we see many variations in the the development of that character. It is because the different genes controlling this character are located on different chromosome and hence assort independently during gametogenesus. As such many new combinations are formed which represents the trait in different degrees. The in heritance pattern agrees with Mendelism except unit characters.