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

When the trait is controlled by single gene then it will follow the Mendel’s law. But when the trait or morphological expression is controlled by multiple genes then it shows different degrees of expression between the dominant and recessive expression, depending on the genetic makeup like number of dominant and recessive alleles of respective gene.

In above mentioned case, the morphological expression is length of the corolla. The trait is not following the Mendel’s law and in F1 generation, the expression of trait is in between the dominant and recessive, that is intermediate length of the corolla.

The F2 and F3 generation not following the Mendel’s laws that is various degrees of expression observed, instead of dominant or recessive trait.

In case of multiple alleles, number of dominant alleles in the offspring will determine the degree of expression of trait. Number of dominant alleles of gene add the effect of expression is called as additive cumulative effect.

For example, if the corolla length is controlled by three genes (A, B, and C). the offspring with all dominant gene (AABBCC) shows longest length of the corolla. It contains all dominant alleles of three genes. While offspring with all recessive gene (aabbcc) shows minimum corolla length. It contains all recessive alleles of three genes.

Offspring with 5 dominant alleles like AABBCc or AaBBCC or AABbCC etc. has length of corolla smaller than dominant offspring but longer than offspring with 4 dominant offspring (aaBBCC, AaBBCc) like this the number of dominant alleles present in the offspring determines the degree of expression of that trait.

Offspring with all dominant alleles shows longest corolla. The length of the corolla varies in degree as follows in the offspring

Six dominant alleles (longest corolla) is greater than five dominant allele greater than four dominant alleles greater than three dominant alleles greater than one dominant allele greater than all recessive alleles

So multiple alleles controlling length of the corolla does not follow the Mendel’s law.