Non Mendelian Inheritance
It denotes the non-Mendelian inheritance pattern shown by a trait. Mendel derived the basis of inheritance patterns based on the monohybrid and dihybrid crosses. These crosses were based on the following assumptions:
(i) A single gene locus regulates or determines one particular trait.
(ii) A gene could have 2 alternate forms known as alleles, where one allele is provided by a parent each.
(iii) There is a dominant- recessive relation between the alleles. But there are exceptions to these assumptions due to which non-Mendelian inheritance patterns could be observed.
Incomplete Dominance
According to Mendal, one allele form would dominate over the other, such that only one parental phenotype could be observed in the progeny. But there are cases where a third phenotype is expressed in the hybrid progeny that is not similar to any one of the parental phenotypes but is intermediate of the two.
For instance, consider the cross between 2 pure-breeding lines red and white. Pink colour an intermediate phenotype is expressed in F1 progeny. This type of inheritance can also be observed in sweet peas and carnations.
When F2 progeny are interbred then both the parental phenotypes along with the hybrid phenotype is recovered from the cross. Red, pink and white flowers are expressed in the ratio 1:2:1. In such inheritance, different genotypes express or yield different phenotypes: Rr = pink, rr = white, and RR = red.
The molecular reason behind this inheritance can be understood, one copy of the red allele is not enough to express the required amount of pigment that could result in a dominant phenotype as in the case of a heterozygote.
Co-dominance
If both alleles of a gene are expressed fully together then it shows co-dominance. A heterozygote will express the phenotype of both alleles. An example of this is the ABO blood group type alleles.
Multiple Alleles
In normal instances, a gene locus in a diploid organism can have 2 alleles, one derived from each parent. But some genes have multiple alleles that are more than just 2 alternate forms, such that a trait can be determined by multiple alleles as in the ABO blood group. For instance, there are 3 alleles in the case of the ABO blood group: IA, IB and i.
ABO group, there are three alleles: A, B, and O. Usually these alleles are designated IA, IB, and i. Concerning the O allele, both alleles A and B are dominant. Allele A and B are not dominant to one another. As a result, there are only 6 possible genotypes, these express 4 blood groups: AB, B, O and A. A and B blood groups may have heterozygous or homozygous genotype constitutions. When both alleles A and B are present (IAIB), both are expressed.
These alleles code for enzyme sugar transferase that in RBCs adds a sugar moiety to a glycoprotein on their membrane. The alleles A and B code for enzymes that catalyze the addition of a different sugar while in the case of the O allele no sugar is added due to a dysfunctional protein. Genotype ii expresses the O blood group phenotype.
The reason for this odd pattern is that the A and B alleles both encode a sugar transferase enzyme that adds sugar to a glycoprotein in the red blood cell membrane. Each allele encodes an enzyme that adds a different sugar. The O allele, on the other hand, encodes a defective protein and adds nothing.
Multiple Interactive Gene Loci
Different genes that occur on different loci can affect the expression of the same trait as in the feather colour trait in budgies. It is the mutual interactions or interplay between these genes that result in the phenotype. These genes can be part of the same metabolic pathway, where they catalyze different steps.
For instance, in birds, the colour of feathers is determined by 2 factors: the iridescence produced by the tiny ridges and the primary pigment deposited. In the case of budgies, yellow-pigmented may or may not be deposited on the feathers based on the genotype.
The presence of blue ridges is influenced by another gene loci. The interaction of these 2 genes results in different phenotypes such as green, yellow, white and blue feathers. For instance, pigment and iridescence are present in green birds. If two birds that are heterozygous at both loci are crossed then they will produce the 9:3:3:1 ratio typical of a dihybrid cross.
Many genes are involved in determining the coat colour of mice like agouti and albino genes. Normal alleles at both these loci will produce a grey coat mouse. White mice result when the albino gene is homozygous recessive, while black coat colour results if the agouti gene is homozygous recessive.
In across between mice heterozygous for genes at both loci will not yield a typical dihybrid ratio, instead, a ratio of 9 agouti mice, 3 black mice and 4 albinos result. This result can be interpreted by the fact that the recessive homozygous condition of the albino gene inhibits the deposition of pigments in the coat of the mice.
