An individual homozygous for genes cd is crossed with wild type and ${\mathrm{F}}_1$ crossed back with the double recessive. The appearance of the offspring is as follows
+ + →        903
cd →         897
+d →         98
c+ →         102

The distance between the genes c and d is ----------------.

The following family tree traces the occurrence of a rare genetic disease. The filled symbols signify the individuals with the disease, whereas the open symbols signify healthy individuals.

Based on this information, the disease is most likely to be:

Assertion (A): The proportion of parental gene combinations was much lighter than non-parental type

Reason (R): Gene combination is due to linkage of the two genes

Give one significant difference between the following:

Linkage and crossing over