Exchange of genetic material between chromatids of homologous chromosomes during meiosis is called

How many types of gametes will be produced by a $O$ Drosophila having following arrangement of
two genes ($y*$ and $w*$) on $X-$chromosome?

In Mendelism, linkage was not observed due to

Give scientific reasons : Linkage and crossing over of genes are alternatives of each other.
 

Select the correct statement from the ones given below with respect to dihybrid cross.