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 characteristics cannot be observed in the progeny if linkage is present?

There are three genes $a, b, c$. The percentage of crossing over between $a$ and $b$ is $20\%$,between $b$ and $c$ is $28\%$,and between $a$ and $c$ is $8\%$. What is the sequence of genes on the chromosome?

Thomas Hunt Morgan and his colleagues performed experiments on which organism to verify the chromosomal theory of inheritance and later demonstrated linkage?

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:

$A$ pure breeding pea plant with the dominant phenotype of character $A$ and recessive phenotype of character $B$ was crossed with another pure breeding plant with the recessive phenotype of $A$ and dominant phenotype of character $B$. An offspring of this cross was crossed with a homozygous recessive for both the characters $A$ and $B$. The following results were obtained:
$22$ were phenotypically dominant for $A$ and recessive for $B$.
$4$ were phenotypically dominant for both $A$ and $B$.
$4$ were phenotypically recessive for both $A$ and $B$.
$24$ were phenotypically recessive for $A$ and dominant for $B$.
These results show that: