Primary source of allelic variation is
Independent assortment
Recombination
Mutation
Polyploidy
Alleles are
The first "geneticist'' was
$BB =$ for black colour alleles
$bb =$ for brown colour alleles
Offspring of a cross between a black mouse and brown mouse allowed to interbreed than find out the percentage of black coat in them
Dominant allele are expressed in
Assertion : Cross of $F_1$ individual with recessive homozygous parent is test cross.
Reason : No recessive individual are obtained in the monohybrid test cross.