Linkage

LINKAGE

The genes situated in the same chromosomes and are fairly close each other they tend to inherit together this type of coexistence of two or more genes in the same chromosomes is called linkage.The hereditary units or genes that determine the characters of an individual are carried in the chromosomes determination of various usually has many genes for the determination of various different characters.The genes for different characters may be either situated in the same chromosomes or in the different chromosome.When the genes are situated in different chromosomes ,the characters they control appear in the next generation either together or apart,depending on the chance alone.

HISTORICAL

The hypothesis that linked genes to remain in their original combinations because of their locations in the same chromosomes was advanced by T.H.Morgan,in 1911.But the prior to Morgan, W.Sutton and T.Bover(1902),Sutton(1903)and Bateson and Punnet(1906)had given some hints about the phenomenon of linkage.It is strange that Mendel could not detect the phenomenon of linkage in pea plant since extensive linkage studies of Lamprecht(1961)have demonstrated that seven genes used by Mendel belonged to only four linkage groups.The views of various geneticists about the phenomenon of linkage can be represented as follows:

1. Sutton-Boveri Chromosomes Theory of Heredity

The formal statement of the chromosomes theory of heredity is usually credited to both Walter Sutton .He is an American graduate student who studied meiosis in grasshopper and confirmed Montgomery's conclusion that associations were brought only between paternal.He also showed that presence of the complete set of the chromosome was important for survival.In 1903 these investigators recognized independently that the behavior of Mendel’s genes during the production of gametes in peas precisely paralleled the behavior of the chromosomes at meiosis:genes are in the pair of chromosomes.The members of a gene pair segregate equally into gametes.It was therefore concluded that genes or Mendelian factor were located on chromosomes.This generalization is now known as a chromosomes theory of inheritance.This theory provides a physical basis of heredity.

2. Sutton’s View on Linkage

Sutton (1903) predicted that the chromosomes are the bearers of the units of heredity or genes and since the number of chromosomes,therefore each chromosome must contain a number of genes.He further stated that since the chromosomes move as units during the meiosis to the gametes,all the genes which are situated in the same chromosomes will be linked together.As a result, each species of animals and plants would have a specific number of groups of linked genes which would correspond with the number of chromosomes found in that species.Unfortunately, Sutton could not provide his predictions experimentally.

3. Bateson and Punnet’s Coupling and Repulsion Hypothesis

Bateson and Punnet formulated the ‘hypothesis of coupling and repulsion explain the unexpected F₂ results of a dihybrid cross between a homozygous sweet pea(Lathyrus odoratus)having dominant alleles for blue or purple flowers (RR) and long pollen grains ( Ro-Ro)with another homozygous double recessive plant(rr,roro)with red flowers and round pollen grains.Further,when they crossed blue or purple round (RR ro ro)with red long (rr Ro Ro)the F₁ hybrids were found to be heterozygous blue or purple long (Rr Ro ro).The F₁ hybrid when test crossed with recessive (rr ro ro)present,the tests cross ratio was 1 blue or purple long:7 red long:7 blue or purple round :1 red round as has been taken.Batten and Punnett could not explain the exact reasons of coupling and repulsion and it was T.H.Morgan who while performing,experiments with Drosophila in 1910,found that coupling or repulsion was not complete.

4. Morgan’s Views on Linkage

Morgans stated that the pairs of genes of homozygous parent tend to enter in the same gametes and to remain together,whereas same genes from heterozygous parent tend to enter in the different gametes and remain apart from each other. He further stated that the tendency of linked genes remaining together in original combination is due to their location on the same chromosome.According to him the degree or strength of linkage depends on upon the distance between the linked genes in the chromosomes.Morgan’s concept about the linkage developed the theory of linear arrangement of genes in the chromosomes which helped the cytogenetics in the construction of genetics or linkage maps of the chromosomes.

5. Chromosome Theory of Linkage

Morgan’s along with Castle formulated the chromosomes theory of linkage which is as follows:

• The genes that show the phenomenon of linkage are situated in the same chromosomes and these linked genes usually remain bounded by the chromosomal material so that they cannot be separated during the process of inheritance.
• The distance between the linked genes determines the strength of linkage.The closely located genes show strong linkage then the widely located genes which show the weak linkage.
• The genes are arranged in the linear fashion in the chromosomes.

KINDS OF LINKAGE

T.H.Morgan and his co-workers by their investigation on the Drosophila and other organisms have found two types of linkage , viz.,complete linkage and incomplete linkage.

1.Complete Linkage

The complete linkage is the phenomenon in which parental combination of characters appears together for two or more generations in a continuous and regular fashion.In this type of linkage, genes are closely associated and tend to transmit together.

Example.The genes for bent wings (bt)and sharpen bristles (svn)of the fourth chromosomes mutant of Drosophila melanogaster exhibit complete linkage.

Complete linkage in male Drosophila.In most of the organisms crossing over takes place both in males and females.But in male Drosophila and female silkworm,Bombyx mori crossing over takes place either very rarely or not at all.In 1919,T.H.Morgan mated gray bodies and vestigial winged (b⁺vg/b⁺vg)fruit flies with flies having black bodies and normal wings(bvg⁺/bvg⁺).Here the use of the testcross is very important.This is a contrast to the situation in an F₁ selling where there are two sets of meiotic divisions to consider one for the F₁ male parental gametes and one for the F₁ female.

2.Incomplete linkage.

The linked genes do not always stay together because homologous non-sister chromatids may exchange segments of varying length with one another during meiotic prophase.This sort of exchange of chromosomal segments in between homologous chromosomes is known as crossing over.The linked genes which are widely located in chromosomes and have chances of separation by crossing over are called incompletely linked genes and the phenomenon of their inheritance is called incomplete linkage.

Example.The incomplete linkage has been reported in female Drosophila and various other organisms such as tomato,maize,pea,rice,poultry and man,etc.The examples of incomplete linkage have been considered as Drosophila and maize.

1.Incomplete linkage in female Drosophila.When F₁ females of Morgan’s classical cross in Drosophila between gray,vestigial and black,normal or long were test-crossed to double recessive males,all four types of progeny were obtained.

2.Incomplete linkage in maize.In Zea mays, a case of incomplete linkage between the alleles for color and shape of the seed has been observed by Hutchison.When a maize plant with seeds having colored and full endosperm (CS/CS)is crossed with another plant having recessive alleles for colorless,shrunken seeds(cs/cs),the F₁ heterozygotes are found with the phenotype of colored fill and genotype of CS/cs.

LINKAGE GROUPS

All the linked genes of chromosomes form a linkage group. Because all the genes of chromosomes have their identical genes on the homologous chromosome,so linkage groups of the homologous chromosome are considered as one.However,in organisms the female or male sex having dissimilar sex chromosomes (e.g.,human beings,Drosophila,fowl,etc),one more linkage occur then the haploid numbers.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.