Crossing over, Mechanism of crossing over, Types of crossing over- on the basis no. of chiasma formation

Crossing over-
Complete linkage is very rarely found in nature. In most of the organism, it has been seen that the offspring formed from the parents are also produced some offsprings having characters different from their parents.

Crossing over is a natural genetic recombination process which occurs during the pachytene stage of the prophase I of meiosis.

The process of separation of linked genes in this way was called crossing by MORGAN.

According to MORGAN- The exchange of segment between non - sister chromatids of homologous chromosomes is called crossing over.

Mechanism of crossing over-
Mechanism of crossing over completed in following steps:-

1) Synapsis-  The process of crossing over occurs before the formation of gametes during meiotic division.
During leptotene of prophase I of meiotic division, the homologous chromosome begins to become distinct and during its zygotene stage of prophase I, the synapsis or paring between the two homologous chromosome is initiated.

This pair of homologous chromosome are called bivalents.

2) Duplication of chromosomes-
The synapsis is followed by duplication of chromosomes.
During duplication each homologous chromosome of bivalent splits longitudinally and form two identical sister chromatids which remain attached to each other at centromere.

At this stage, each bivalent contains four chromatids so it is called tetrad.

3) Crossing over or chiasma formation- During pachytene of prophase-I non - sister chromatids of bivalents break at one or more corresponding points due to endonuclease enzyme and after exchange of chromosomal segments, they again unite together.

In this way, the breakage and reunion of chromatids is called crossing over.

During diplotene, a cross (X) like structure is formed at the point of crossing over in bivalent which is called chiasma or chiasmata.

The no. Of crossing over or chiasmata formation depends on the length of the chromosomes because the longer the chromosomes, the Greater the no. Of chiasmata.

4) Terminalisation - After chiasma formation the non - sister chromatids begins to separate slowly from centromere towards chiasma and chiasma itself begins to show movement towards the terminal ends of tetrad in zipper fashion. This movement of chiasma is called Terminalisation.

The homologous chromosomes get Completely separated from each other by this process.

Types of crossing over:-
On the basis of no. Of chiasma formation-
1) Single cross over
2) Double cross over
3) Multiple cross over

1) single cross over- In this, only one chiasma is found between two non - sister chromatids of bivalents as a result of which two non cross over type and two single cross over type chromatids are formed.

2) Double cross over type- In this, two chiasma are formed between two non - sister chromatids of bivalents as a result of which two double cross over and two non - cross over type of chromatids are formed.

3) Multiple cross over- In this, more than two chiasma are formed between two non - sister chromatids and thus crossing over takes place at more than two points on the same chromosome.

linkage- statement, definition, types of linkage, example of linkage, Morgan experiment ( drosophila cross) , HUTCHINSON maize cross

Linkage:- 

LINKAGE is the tendency of two or more genes to live together in the same chromosome during the genetic transmission.

Introduction:-

Large no. Of genes are found in each species of plants to produce various characters.

The no. Of genes is from thousands to lakhs while the no. Of chromosomes are constant and limited.

The no. Of chromosomes more than 100 is rarely found. Hence, obviously numerous genes should be present on single chromosome.

Since, Chromosomes reach to the gametes by meiosis i.e. chromosomes acts as a carrier of genes to next generation.

Definition:- such genes which are situated on one chromosome only and are always inherited together in coming generation are called linked genes and this phenomenon is called linkage.

Such characters whose genes are present on same chromosome the law of independent assortment is not applicable.

This fact is firstly reported by BATSON and PUNETT  when they failed to observe the independent assortment in two allelic pairs of sweet pea.

They reported that the results obtained in F-2 generation by performing two dihybrid crosses for colour of flower and length of pollen grain were totally different from the law of independent assortment.

The phenotypic ratio expected was 9:3:3:1 but here the phenotypic ratio of offsprings of dihybrid test cross of such type of character were 1:7:7:1.

BATSON and PUNNETT reported that the above dihybrid crosses are due to coupling and repulsion tendencies of both the characters of genes.

They explained that when two dominant genes come together from one parent only then they inherited into the coming generation together due to which the offspring bearing parental kind of characters are obtained in much larger no. And the offsprings bearing new kind of characters or recombinants are less in no. .

Coupling - This type of tendency of two dominant genes is called coupling.

Types of linkage:-

Linkage are of two types:-

1) Complete linkage- according to MORGAN- "Complete linkage is When recombinants are not formed in linked genes, that is those linked genes in which crossing over does not occur".

It is found among such genes which are situated very close to each other on the same chromosome due to which crossing over does not occur between them. This type of linkage occurs very rarely and it is studied for only some characters of male drosophila .

Example:- MORGAN (1911) crossed an ordinary wild type drosophila with grey body long wings (BBVV) with another drosophila (mutant type) with black body and vestigeal wings (bbvv).

All the hybrid of F-1 generation are with grey body and long wings (BbVv). I.e. phenotypically like the wild type of parents.

now a male of F-1 generation having genotype (BbVv) is back crossed with double recessive female (test cross) with black body and vestigeal wings having genotype (bbvv) then only parental combinations are formed in F-2 generation and no appearance of any new combinations.

The results of above crosses indicate that grey body character is inherited together with long wings. It implies that these genes are linked together. Similarly, black body character is inherited together with vestigeal wings.

Since only parental combination of 

Character will appear in F-2 generation and no new or non parental combination appears.

This shows complete linkage.

Incomplete linkage:-

When some recombinants are formed in linked genes i.e. some crossing over occurs between linked genes, it is called incomplete linkage.

This type of linkage is found  among such genes that are situated far apart on the same chromosome and expected to get separated from each other during crossing over.

Such type of genes are called incompletely linked genes and this condition of their inheritance is called incomplete linkage.

Incomplete linkage is very common and has been studied in almost all the organism.

Example:- 

HUTCHINSON crossed a variety of maize having homozygous coloured and full seeds (CCSS)  with a variety having heterozygous colourless and shrunken seeds (ccss) .

All the F-1 plants produced coloured and full seeds ( CcSs).

But in a test cross, when such F-1 females (CcSc) heterozygous are cross pollinated with the pollen from the plant having colourless and shrunken seeds (ccss) double recessive, four types of seeds are produced.

1) coloured and  full seeds

2) colourless and shrunken seeds

3) coloured and shrunken seeds

4) colourless and full seeds

It is clear that, the parental combination are more in number (96.4%) then the new combinations (3.6%). These genes are located in same chromosome and only 3.6% individual these genes are separated by crossing over. This is an example of incomplete linkage.

Differences between Respiration in plants and Respiration in animals

All  parts of plant (Like stem, root and leaves) performs Respiration individually while in animals Respiration perform through a single unit.

In plants Respiration occurs at a slow rate while in animals it occurs at much faster rate.

In plants, during Respiration little transport of gases occurs from one part of plant to other while in animals the gases are usually transported over long distances during Respiration.

Pumping action of blood / way of blood circulation/ contraction and relaxation of heart/beating of heart/Double circulation

Heart is really a double pump. The left side of heart (left atrium and left ventricle) acts as one pump which pumps blood into the whole body, except the lungs.

 The right side of heart (right atrium and right ventricle) acts as another pump which pumps blood only into the lungs Left atrium receives oxygenated (blood containing oxygen) blood from the lungs via pulmonary vein.

 When left atrium receives the blood it relaxes or expands. 

Then Left atrium contracts so it can transfer the blood to the left ventricle. 

When left ventricle receives the blood it relaxes or expands so it can collect the blood. 

 Then left ventricle contracts so that it can pump the blood out through aorta for the circulation in the body. 

 Right atrium receives the deoxygenated (blood containing carbon di oxide) blood from the body via vena cava. 

 When right atrium receives the blood it relaxes or expands . 

 Then the right atrium contracts so that it can transfer the blood to the right ventricle.

 When the right ventricle receives the blood it relaxes or expands so that it can receive the blood or dilates.

Then the right ventricle contract so that it can pump the blood out through pulmonary artery to lungs.

Law of Independent Assortment -

"When Crosses are made between plants with two or more than two Contrasting characters, the inheritance of these characters occurs independently like, the inheritance of one Character does not affect the inheritance of Other Characters".

Example and mechanism of independent Assortment:-

• When dihybrid Cross between  plants with to pairs of contrasting characters is made between homozygous round and Yellow seeded plant (Genotype RRYY) and homozygous wrinkled and green seeded plant (genotype rryy) both the plants, being homozygous for both the Characters, at first only same type of gametes are poroduced i.e. 'RY' and 'ry' which produce F1 hybrid (genotypety RrYy ) after fertilization . All plants of F1 generation Round Seeded with green coloured Cotelydon. It is so because the Round shape of Seed is dominant over wrinkled seed and Yellow colour of  cotyledon are dominant over green coloured Cotelydon.
• When the plants of F1 generation (Hybrid) are self pollinated, then F1 hybrid being heterozygous for the genes of both type of free characters, produce four types of gametes 'RY', 'rY', 'Ry' and 'ry'.
• As All the four types of gametes of F1 hybrid fuse with each other by all the possible Combinations, forming 4 types of phenotype plants and 9 types of genotypic plants In F2 generation.
• In F2 generation following 4 types of seed of plants are obtained:-

1. Round Seed with Yellow cotyledon
2. Round Seed with green cotyledon 
3. Wrinkled seed with Yellow Cotyledon
4. Wrinkled seed with green Cotyledon

• The phenotypic ratio in these plants ds 9:3:3:1  respectively. This ratio is called dihybrid ratio.
• It is clear that round character is not always found with yellow character and  similarly wrinkled character is not always found with green character as they are present in parents.
• The plants with new combinations are also obtained. I.e. round with green and wrinkled with yellow.
• This proves that the character of Shape of seed and character of colour of cotyledon are always assorted independently. In this way, this two characters give rise to new combinations in F2 generation.

Law of Segregation Or Law of Purity of gametes with example and mechanism of segregation

Law of Segregation Or Law of Purity of gametes :- Given by Mendel→ 

"When a pair of Constrasting characters remained together in F1 hybrid then both of them live together without Contaminating or blending or intermingling with each ather and get Segregated Completely and in pure form during the formation of gametes"

Example and mechanism of segregation-

• when homozygous red flowered plants are crossed with homozygous white flowered plant than parent plant always produce only one type of gammat as they are homozygous genotype that is red flowered plant and white flowered plant produce gametes 'R' and 'r' respectively.

• The gametes of both the plant, after cross pollination, produce F1 hybrid whose genotype is 'Rr' and phenotype is Red.

• Both the genes (R and r) of red and white colour respectively present together in F1 hybrid, but only red colour is expressed because the gene 'R' of red colour is dominant over the gene 'r' of white colour.

• Both the genes (R and r) live together for a long time in F1 hybrid but they do not affect each other. They neither mix together nor contaminate each other.

• During the gamete formation of F1 hybrid, two types of gametes will form in equal number. Half of the gametes will contain gene 'R' sand the other half will contain the gene 'r' .

• During self pollination this gametes  unite together in three possible combinations- 'RR', 'Rr' and 'rr' and two types of phenotypic plants will produce in F2 generation in which 75% are red flower plants and 25% are white flower plants .

• The appearance of white colour in F2 generation shows that gene 'r' of white colour lives together with the gene 'R' of red colour in F1 hybrid but 'r' gene neither mixes with it nor contaminate by it and gets separated from it during the gamete formation.

This law given by Mendel is called "Law of segregation" or "Law of purity of gametes".

Mendel's law of dominance with example

• Law of dominance:- Given by Mendel- "when  two pure homozygous plants taking only one pair of contrasting characters are crossed, only  one  character of a pair  expresses itself in the F1 generation and other is suppressed itself"

In this way, in F1 generation only one type of phenotype is expressed.

The character which is expressed in F1 generation is called Dominant and the character which is suppressed is called recessive.

The "law of dominance" is not universally applicable.

• Example:-

Law of dominance can be explained by monohybrid cross. If a cross is made between pure red flowered plant and pure white flowered pea plant, all plants obtained in F1 generation are Red flowered pea plants, though these plants  contain the genes (Rr)  of both the characters. In the experiment, red colour is dominant character while white colour is a recessive character.