Question

in what way is linkage in opposition to Mendel law of independent assortment

Answer 1

Genetic Linkage: Genetic linkage is the tendency of
DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction.
Now, what would happen if two genes didn't follow independent assortment?
always have stayed together, and so might the green and wrinkled alleles.

To see how this could work, imagine that the color and shape genes are physically stuck together and cannot be separated, as represented by the boxes around the alleles in the diagram below. For instance, this could happen if the two genes were located very, very close together on a chromosome.

Rather than giving a color allele and, separately, giving a shape allele to each gamete, the F1 dihybrid plant would simply give one “combo unit” to each gamete: a YR allele pair or a yr allele pair.

We can use a Punnett square to predict the results of self-fertilization in this case. If the seed color and seed shape genes were in fact always inherited as a unit, or completely linked, a dihybrid cross should produce just two types of offspring, yellow/round and green/wrinkled, in a 3:13:13:1 ratio. Mendel's actual results were quite different from this (the 9:3:3:19:3:3:19:3:3:1 ratio we saw earlier), telling him that the genes assorted independently.