T. H. Morgan and Linkage

• Morgan is known af father of Experimental genetics. Morgan conducted several  cross on Drosophila melanogaster   Which were similar as  cross of Mendel.

• Morgan  did not found ratio in F2 generation like Mendel of Dihybrid cross. 

• Morgan studied in Drosophila that some of the genetic were sex linked .

• One day Thomas Hunt Morgan observed a male fruit fly and he noticed this fruit was not similar to a normal fruit fly.

• Normally fruit flies or Drosophila melanogaster have red eyes. But this fly had white eyes. Morgan was surprised about this particular trait and tried to explore  how traits were inherited and distributed among the population of fruit flies. 

• Morgan selected  a simple breeding analysis to find the real fact  about the trait of  white eyes.

• Morgan was  the first person to demonstrated link the inheritance of a specific trait with a particular chromosome and realized the  unusual Pattern of Inheritance

Morgan Monohybrid cross 

• Morgan conducted a  cross between the white-eyed male fly and  red-eyed females. He got similar results  in the F1 generation as found by Mendel.

• All progeny were with red eyes and  white-eye recessive trait was still present as a Morgan again made a cross on the basis of the pattern of  the Mendel to calculate the F2 generation.

• He found a 3:1 ratio of red eyes. This result is very similar as first shown by Mendel . But in the F2 generation , white-eyed traits were present only in male and there was no female with white eyes.

• Morgan was first to make a relation with trait with sex. But according to the Mendelian experiments ,the 3:1 ratio of red eyes to white eyes would be shared equally among males  and females . 

• Morgan made a cross between a heterogeneous red-eyed female with a white-eyed male.

• He found the progeny in result of cross - one  white eyed female, one red eyed female, one white eyed male and one  red eyed male with ratio of 1:1:1:1  

• This made Morgan think about the linkage between the traits and sex chromosomes. 

Morgan dihybrid cross 

• He performed several dihybrid crosses in Drosophila melanogaster but he did not find the result as a dihybrid cross of Mendel.

• Finally he concluded that In Drosophila genes of some characters are not segregated and were responsible for the deviation in ratio of  this dihybrid cross.

Dihybrid cross - A

• He observed the result of his first dihybrid cross and stated that the genes of yellow and white wings are tightly linked and transferred as such to the next generation.

• It means progeny receive the same type of chromosomes from  their parents and  have less  chances of acquiring new characters or recombinant.

Dihybrid cross - B

• In the second dihybrid cross Morgan observed that genes of white and miniature wings are loosely arranged on  Chromosomes and segregated at higher rate.

• It means progeny do not receive the same type of chromosomes from  their parents and  have more   chances of acquiring new characters or recombinants.

Morgan postulation 

• The genes that are tightly linked on chromosomes due to the phenomenon of Linkage.

• These gene are not separated during gamete formation and inherited exactly the same to the offspring and there is less frequency of formation of recombinants.

• The genes that are not  tightly linked on chromosomes .There are  higher chances  for separation of these  genes  during gamete formation and inherited  into the offspring as per the parent.  These genes produce  less frequency of formation of recombinants.

• Linkage signifies the association between the gene on chromosome. This linkage is too high or too low.

• If genes are closely associated then the formation of recombinants takes place in less frequency. If genes are not closely associated it will lead to a higher frequency of recombinant.

• When gene of characters are located  on same chromosome than these gene contribute much more combination of parental gene.

• When gene of characters are located  on different chromosome than these gene contribute  less combination of parental gene.

• Alfred Sturtevant , disciple of Morgan used frequency of recombination of genetic pairs on same chromosome to measure the distance between gene and formulate" genetic map". Recent, Genetic map of  Alfred Sturtevant is used in Human genome project.

Why did Morgan select Drosophila for experiment

• Morgan selected fruit fly or Drosophila for the experiment because:

• These fly can grow in synthetic medium in laboratory.

• They can complete their life cycle about two week.

• They can produce more number of progeny in single mating .

• Male and female fruit fly are easily distinguished.

• Variations in Drosophila can observed with low power microscope.