LINKAGE
A chromosome bears many genes.the genes present in a chromosome have tendency to transmit together in the same sequence . the genes that transmits together in the same sequence are called linked genes and the phenomenon is called as linkage.
Linkage is of two type :
COMPLETE LINKAGE
If the recombinants of a test cross are not formed or only parental combinants of characters are seen then the linkage is called as complete linkage.T.H morgan observed complete linkage in male drosophila. morgan crossed a grey body long winged drosophila with black body vestigeal wing drosophila.in f1 generation,all grey body long winged drosophila were formed.when test cross is made between f1 male and black body vestigeal wing drosophila then he got only parental combinations of characters.
genetic diagram showing complete linkage in male drosophila :
INCOMPLETE LINKAGE
In the incomplete linkage,new recombinants in a test cross are formed. the recombinants ratio will be less than 50%.formation of new recombinants is due to exchange of genes in the process of crossing over.
Incomplete linkage in female drosophila
when a grey body long wing drosophila is crossed with black body vestigeal wing drosophila then in the f1 generation all grey body long wing drosophila are produced.the grey body long wing characters are thus dominant characters.in a test cross in which the f1 female drosophila is crossed with double recessive male drosophila then recombinants are also formed by 17%.the parental combinants are formed by 83%. this shows incomplete linkage. it can be explained by following genetic diagram :
Incomplete linkage in maize
when a coloured full seed maize is crossed with colourless shrunken seed maize,then only coloured full seed maize are obtained in f1 generation.when a test cross is made between f1 hybrid and double recessive parent then coloured full seed maize and colourless shrunken seed maize were obtained by 48% and 48% i.e parental characters by 96%.new recombinants were formed by 4% i.e colourless full seed maize and coloured shrunken seed maize. this shows incomplete linkage.
Genetical diagram:
A chromosome bears many genes.the genes present in a chromosome have tendency to transmit together in the same sequence . the genes that transmits together in the same sequence are called linked genes and the phenomenon is called as linkage.
Linkage is of two type :
COMPLETE LINKAGE
If the recombinants of a test cross are not formed or only parental combinants of characters are seen then the linkage is called as complete linkage.T.H morgan observed complete linkage in male drosophila. morgan crossed a grey body long winged drosophila with black body vestigeal wing drosophila.in f1 generation,all grey body long winged drosophila were formed.when test cross is made between f1 male and black body vestigeal wing drosophila then he got only parental combinations of characters.
genetic diagram showing complete linkage in male drosophila :
INCOMPLETE LINKAGE
In the incomplete linkage,new recombinants in a test cross are formed. the recombinants ratio will be less than 50%.formation of new recombinants is due to exchange of genes in the process of crossing over.
Incomplete linkage in female drosophila
when a grey body long wing drosophila is crossed with black body vestigeal wing drosophila then in the f1 generation all grey body long wing drosophila are produced.the grey body long wing characters are thus dominant characters.in a test cross in which the f1 female drosophila is crossed with double recessive male drosophila then recombinants are also formed by 17%.the parental combinants are formed by 83%. this shows incomplete linkage. it can be explained by following genetic diagram :
Incomplete linkage in maize
when a coloured full seed maize is crossed with colourless shrunken seed maize,then only coloured full seed maize are obtained in f1 generation.when a test cross is made between f1 hybrid and double recessive parent then coloured full seed maize and colourless shrunken seed maize were obtained by 48% and 48% i.e parental characters by 96%.new recombinants were formed by 4% i.e colourless full seed maize and coloured shrunken seed maize. this shows incomplete linkage.
Genetical diagram:
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