If both of the man's marker alleles were 13 repeats in length, we would not have been able to do the analysis because both the normal and mutant allele of the disease locus would be linked to the same size marker. You can also see that we need to be able to distinguish between the husband s and wife s marker alleles. As you know, homologous recombination occurs during meiosis. Every time a person makes an egg or sperm, there is on average 1 recombination per chromosome. The probability of a recombination event occurring between the marker and the disease-causing gene is very small if the gene and the marker are very close together.
Geneticists describe the degree of linkage in terms of centiMorgans.
By the way I introduced the example, it looks very easy to do. To do so we had to go further back in the family to trace the inheritance of the both the disease locus and the linked polymorphism. So we can conclude that copy of chromosome 1 that our patient inherited from his mother carries the disease allele and the marker allele with 13 repeats. The copy of chromosome 1 he inherited from his father carries a normal allele and a marker allele with 7 repeats. The combination of alleles found on each chromosome is referred to as a haplotype .