When two genes are situated very close to each other on a chromosome:

Incomplete linkage is $A$. Complete linkage is $B$. Choose the correct option for $A$ and $B$.

What is the reason for the absence of hybrids?

When a cluster of genes shows linkage behaviour,they:

Discuss why Drosophila has been used extensively for genetical studies.

$A$ cross is made between two homozygous individuals, one with the wild type $(+, +)$ and the other with the mutant type $(a, b)$. In the $F_2$ generation (or test cross progeny), out of $1000$ individuals, $700$ are parental types. Calculate the distance between genes $a$ and $b$.