If there are 7 chromosomes, there are 7 linkage groups. Dorsophila with four chromosomes has four linkage groups; Maize with 10 Chromosomes has 10 linkage groups etc.
Sometimes in exceptional cases as in the Gaudens and Valens varieties of Oenothera all chromosomes may form one single large linkage group. This however is a special situation brought about by translocation.
Classification of linkage:
There are two types, complete and incomplete. In complete linkage, there is virtually no recombination. The same characters appear in unison for several generations. This type of linkage however is rare in nature and occurs due to absence of cross over male individuals of Drosophila and certain other insects show complete linkage. In incomplete linkage which is quite prevalent in biological world, the linked genes get separated due to crossing over during meiosis.
The extent of separation of the linked genes depends upon the distance between the two genes. In addition to this, interchromosomal linkage or false linkage as Longley (1945) calls it is brought about due to cytological irregularities like translocation.
Measurement of linkage:
The easiest method to obtain linkage in terms of percentage value is to cross two individuals differing in two pairs of genes. The F is back crossed to the double recessive parent to obtain the parental and recombination. The percentage of parental combinations in the total progeny gives the strength or percentage linkage between the two genes responsible for the characters. A higher percentage of parental combinations indicate a strong linkage between the two genes and a higher percentage of recombination is indicative of weak linkage. The strength of linkage between any two pairs of genes is inversely proportional to the distance between them, lesser the distance stronger is the linkage and longer the distance, weaker is linkage.