What is the name of the pea species used by Bateson and Punnett in their experiments?

In a chromosome map of maize,two genes $R$ and $Y$ are located very close to each other. When $RRYY$ and $rryy$ genotypes are crossed,the $F_2$ generation analysis shows:

What is the reason for the absence of hybrids?

In a test cross involving $F_1$ dihybrid flies,more parental-type offspring were produced than the recombinant-type offspring. This indicates

Which of the following is true about linkage?
$I.$ It is a phenomenon in which more recombinants are produced in $F_{2}$-generation.
$II.$ More parental combinations are produced in $F_{2}$-generation.
$III.$ Genotypes which are present in $F_{1}$ hybrid reappear in high frequency in $F_{2}$-generation.
$IV.$ It is a phenomenon in which two chromosomes are linked.

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