Morgan and his group observed that when genes are grouped on the same chromosome,some genes are very tightly linked and show:

The following experiment is the result of a dihybrid cross performed by Morgan. Identify the proportions of $P, Q, R,$ and $S$.

When two genes are situated very close to each other in a chromosome,

When normal and mutant alleles are present on opposite chromosomes of a homologous pair,the heterozygotes are called as:

Select the incorrect statement$(s)$ from the options given below with respect to dihybrid cross.
$I.$ Tightly linked genes on the same chromosome show higher recombinations.
$II.$ Genes far apart on the same chromosome show very few recombinations.
$III.$ Genes loosely linked on the same chromosome show similar recombinations.

Drosophila has four pairs of chromosomes. How many linkage groups does it have?