Autosomal recessive disorder, Genetic recombination, Linked DNA Markers

Question I'm stuck on:
In this pedigree below II 1 is affected with an autosomal recessive disorder. The disease status for II 2, II 3 and II 4 is unknown. A and B represent alleles at a locus which is tightly linked to the disease locus with recombination fraction of 0. On the basis of the linked markers II 2 can be told that:

(Check out the attachment for the pedigree.)

A) he is affected.
B) he is a carrier.
C) he is homozygous unaffected.
D) the marker results are not informative
E) All the above

Correct answer:
B) he is a carrier.


I watched the following two videos.:

1.


2.



I know what an autosomal recessive disorder is.

I think that the term "genetic recombination" refers to the fact that it is sometimes the case that the corresponding alleles of two sister chromatids are swaped and that that could lead to non-parental phenotypes.

I also think that the recombination fraction is the fraction that is greater than or equal to 0 and less than or equal to 1 which is the ratio of non-parental offspring to total offspring.

I suspect that the statement "A and B represent alleles at a locus which is tightly linked to the disease locus" means that alleles A and B are very close to the disease allele, and I suspect that that implies that the disease allele must also be located wherever the alleles A and B are located.

Having said that, I'm not exactly sure what linked markers are, and I'm not sure how to get that the correct answer is "B) he is a carrier.".

Could someone please help me figure out how to do this problem?

Any help would be GREATLY appreciated!