Genetics Pt. 2

The deviation from mendelian genetics

Sometimes during the crosses the ratio obtained does not obey mendelian’s law. The first one is co-dominance. In human we have 4 type of main blood group which is A, B, AB and O. In co-dominance, more than one allele expresses the characteristics. In AB, A and B both express it’s characteristics. We can say that a multiple alleles control the same characteristic.

In snapdragon flower, the phenotypic ratio and genotypic ratio for F2 generation is the same which is 1:2:1. This is due to incomplete dominance where recombinant progeny is produced. A red flower and white flower cross would produce 1 red flower, 2 pink flower and 1 white flower. This is due to the intensity of the pigment cannot express itself in heterozygous state and produce a combination of the pigment red and white which is pink.

Then we’ve come to another case which is maternal effect. In this case we take the example of shell shape in gastropod. There are two main type of shape which is dexteral and sinistral (clockwise and anti-clockwise). For the maternal effect the progeny inherited the same trait as the maternal. Here’s an example:

D: Dexteral                                                                DD(M) x dd(F)

d: Sinistral                                                                 F1: All Dd (All sinistral)

Even though that D is dominant over d but due to maternal effect the maternal characteristic express itself in the progeny. Let’s take a look at another example:

DD(M,sinistral) x dd(F, dexteral)

All Dd (dexteral)

In genetics part 3 we would discuss on epistasis, extranuclear inheritance and linked genes.