Crossing over

Crossing over can be defined as the recombination of linked gene brought as a result of interchange of corresponding parts between the chromatids of a homologous pair of chromosome so, as to produce new combinations.

As per Morgan experiment complete linkage is very rare. i.e. atleast some amount of recombination take place during meiotic prophase. Janseen(1909) observed the chromosomes of Salamander and found some X shaped structure which he proposed as breakage of chromosomal segment. But later on Morgan classified the genetical significanceOf the structure and named it as chiasma. Morgan further suggested that the interchange of allele takes place as a result of breakage and reunion of homologous chromosome which he termed as crossing over. So, crossing over can be named as an out comes of incomplete linkage.

Example- The wild Drosophila having grey body and long wings was crossed with black body and vestigial wing. In F1 all of the individual become grey long. In a test cross of F1 female hybrid with double recessive male 4 types of phenotypes were obtained in F2 i.e. in addition to original parental combination two new variety were obtain and they are grey vestigial and black long.

It happens due to the fact that the F1 female hybrid produces 4 different type of gamete i.e. 2 normal non cross over gamete and the 2 crossover gamete.

Mechanisms of crossing over

Observation under microscope as given idea regarding the behavior of chromosome during meiosis. In real sence crossing over is lengthy process which occurs at the diplotin stage of mitotic prophase 1. The different stages are-

(1)Synapsis:-

During  zygotene stage of meiotic prophase1. The maternal and parental chromosome of a homologous pair comes close together and they lie side by side. It happens due to mutual attraction between the allele genes. This type of pairing is called synapsis and the paired chromosomes are now known as Bivalents. According to C.D Darlington the chromosome with a single pairs chromatids represents and unsaturated and unbalanced state. So in order to become saturated they must pair.

(2)Duplication

In the diplotene stage each of the homologous chromosome of a bivalent splits longitudinally into 2 sister chromatids. So the bivalent now contains 4 chromatids and it is known as tetrad or tetravalent.

(3)Crossing over

During diplotin stage when the paired chromosome start separating, the non sister chromatids remain in contact at one or more point and thus, establish one or more exchange of segment. The point of contact is known as chiasmata. At the point of chiasma the non sister chromatids at first break and then re joint with exchange of segment. Thus the new chromatid which are formed contain the segment derive from the two non sister chromatids.

Here Stern and Hotta (1969) have described that this breakage of chromatids at the region of chiasma is brought about by a nuclear enzyme called Endonuclease and again rejoint
By the enzyme ligase. According to recent finding  a little amount of DNA synthesis take place during this time for repairing the broken segments.

(4)Terminalization

After the competition of crossing over the non sister chromatids start their separation due to the force of repulsion. This separation usually begins from the point of centromere towards the end of chromosome so that the chiasma starts moving just like the zipper of a chain. This movement of chiasma is called Terminalization. Thus, the new chromatids acquire a changed genetic constitution having a part of original chromatid and a part of other chromatids. After that the chromatid condensed and get shorten.

Theories of crossing over-

(i)Contact first theory (by serebrovsky):-

(Picture)

According to this theory the two chromatids of homologous chromosome undergoing crossing over at first touch each other then cross each other. So at the point of contact breakage occurs. These broken segment again reunite to form new combination.

(ii)Brcakage first theory (By Muller):- 

   (Picture)

According to this theory the chromatids undergoing crossing over first of all break into 2 without any crossing over and after than the broken segments of non sister chromatid reunite to form new combination.

(iii)Strain Theory (By Darling ton):-

According to this theory the breakage in chromosome i.e. in chromatids is due to strain caused by pairing and Latter on the brcakage parts and again reunite.