The distance between two linked genes on a chromosome is measured in cross-over units,which is:

The linkage map of $X$-chromosome of fruit fly has $66$ units with yellow body gene $(y)$ at one end and bobbed hair $(b)$ gene at the other end. The recombination frequency between these two genes ($y$ and $b$) should be

Two genes $R$ and $Y$ are located very close on the chromosomal linkage map of maize plant. When $RRYY$ and $rryy$ genotypes are hybridized,then $F_{2}$-segregation will show

Briefly mention the contribution of $T.H.$ Morgan in genetics.

Define/Explain the following terms:
$1.$ Linked genes
$2.$ Locus

In $Lathyrus$ $odoratus$,a hybrid blue-flowered and long-pollen plant is test-crossed with a homozygous recessive red-flowered and round-pollen plant. How many parental types are obtained when genes are present in the $cis$ stage in parents (in $\%$)?