Linkage – concept, types and theories

Linkage – Concept, Types and theories�

1. Linkage – Concept, Types and theories Vaishali S.Patil Professor , Department of Botany Shri Shivaji College of Arts, Commerce & Science Akola

2. Linkage- Genetic  linkage describes the way in which two genes that are located close to each other on a chromosome are often inherited together , during the meiosis phase of sexual reproduction. 1.Each   chromosome   contains more than one gene. 2.The genes for different characters may be either situated in the same chromosome or in different chromosomes. 3.When the genes are situated in different chromosomes, the characters they control appear in the next generation together or apart, depending on the chance alone. They assort independently according to Mendel’s law of independent assortment. 4.But if the genes are situated in the same chromosome and are fairly close to each other, they tend to be inherited together. This type of coexistence of two or more genes in the same chromosome is known as linkage.

3. The occurrence of different genes on the same chromosome. They show non- random assortment at meiosis. Characteristic of Linked Genes Genes on different chromosomes assort independently giving a 1: 1: 1: 1 test cross ratio. Linked genes do not assort independently but tend to stay together in the same combination as they were in the parents. Types of Linkage- Morgan and his co-workers by their investigation on the  Drosophila  found two types of linkage – 1.Complete linkage and 2.Incomplete linkage. Complete Linkage- The complete linkage is the phenomenon in which parental combinations of characters appear 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. These genes do not conform to Mendel’s principle of independent assortment they fail to produce the expected 9:3:3:1 ratio in a breeding situation involving the inheritance of two pairs of contrasting characters ( Dihybrid cross).

4. Complete Linkage: If linkage is complete, there should be parental combinations only and no recombination . Morgan (1919) reported a complete linkage in Drosophila. When ordinary male wild fly with grey body and normal wings was crossed with female having black body and vestigial wings, in F 1 , hybrids were all grey bodied and normal winged (with dominant characters). But when F 1 , male is back crossed with recessive female parent only two types of individuals in F 2  generation were produced instead of expected four. These two types were grey bodied and normal winged and black bodied and vestigial winged in equal number, thus indicating the complete linkage. But how do genes located on same chromosome assort independently? It may be due to exceptions in Mendel’s second law of independent assortment or due to some mechanism for genes on the same chromosome to separate and recombine during meiosis . Both the cases have been observed.

5. Incomplete linkage: Incomplete linkage produces new combinations of the genes in the progeny due to the formation of chiasma or crossing over in between the linked genes present on homologous chromosomes. When in sweet peas a cross is made between blue flower and long pollen (BBLL) with red flower and round pollen ( bbll ) in F 1  expected blue flower and long pollen ( BbLl ) heterozygous condition is got. (Fig. 5.47). However, test cross between blue and long ( BbLl ) and double recessive ( bbll ) gave blue long (43.7%), red round (43.7%), blue round (6.3%) and red long (6.3%). The parent combinations are 87.4% are due to linkage in genes on two homologous chromosomes, while in case of new combinations (12.6%) the genes get separated due to breaking of chromosomes at the time of crossing over in prophase-I of meiosis. New combinations in the progeny appeared due to incomplete linkage (Fig. 5.47).

6. Significance of Linkage: 1. Linkage does not permit the breeders to bring the desirable characters in one variety. 2. Linked characters are maintained for generations because linkage prevents the incidence of recombination. Thus linkage has one of the great significance for the living organisms in that it reduces the possibility of variability in gametes unless crossing over occurs.

7. Theories-1. W. Sutton and T. Boveri 1902) 2.Sutton (1903) and Bateson and Punnett (1906)3. T. H Morgan (1911) 1.W. Sutton and T. Boveri 1902 Theory- On the Boveri –Sutton chromosome theory is a fundamental theory of genetics which identifies chromosomes as the carriers of genetic material. It correctly explains the mechanism underlying Mendels laws of inheritance by identifying chromosomes with the paired factors required by Mendels laws. It also States that chromosomes are composed of linear structures of genes located at specific sites called loci along them.  It States simply that chromosomes, which can be seen in all dividing cells and pass from one generation to another, is the basis for all genetic inheritance. Over time random mutations involve changes in the DNA sequence of the gene. Genes are located on chromosomes.

8. The chromosome theory of inheritance is credited to papers by Walter Sutton in 1902 and 1903, as well as to independent work by Theodor Boveri during roughly the same period. Boveri was studying sea urchins , in which he found that all chromosomes must be present for normal embryonic development to take place. Suttons work with grasshoppers showed that chromosomes occur in matched pairs of maternal and paternal chromosomes which separate during meiosis and "may pose a physical basis of the law of heredity Mendel". 

9. 2.Coupling and Repulsion hypothesis of linkage: by Bateson and Punnett Bateson and Punnett in 1906 described a cross in sweat pea ( Latharus odoratus ) , where they find gene pairs do not assort independently as expected. While they were working on sweet pea, they noticed two variety of plant-plant with Blue flowers (BB) with long pollen grains (LL) and red flower (bb) with round pollen grain ( ll ). When the sweat pea variety having blue flower and long pollen grain (BBLL) were crossed with those of another variety having red flower and round pollen ( bbll ). In F1 generation, all the offspring having blue flower and long pollen ( BbLl ) were observed as expected. Because, blue color is dominant over red and long pollen is dominant over round. When these F1 offspring ( BbLl ) were test crossed with red flower and round pollen variety ( bbll ), they obtained ( BbLl ), ( Bbll ), ( bbLl ), ( bbll ) in the ration 7:1:1:7 instead of 1:1:1:1. This indicates that the dominant characters (Blue color and long pollen) tends to remain together. Similarly, the recessive characters (red color and round pollen) tends to remain together. This deviation or phenomenon is explained by  Coupling and Repulsion hypothesis  by Bateson and Punnett . .

10. Bateson and punnett conclude that- the pairs of genes from homozygous parents try to remains together in a cross due to which parental characters is more frequent. This is due to  coupling.   Two dominant gene for Blue flower and long pollen (BL) are linked in one plant while in other plant two recessive gene for red flower and round pollen ( bl ) are linked. Such linkage is called coupling. In another experiment, when a cross was made between sweet pea variety having blue flower and round pollen ( BBll ) with another variety having red flower and long pollen ( bbLL ) In F1 generation, all offspring with blue flower and long pollen were obtained ( BbLl ). When these F1 offspring ( BbLl ) were test crossed with red flower and round pollen variety ( bbll ), they obtained ( BbLl ), ( Bbll ), ( bbLl ), ( bbll ) in the ratio 1:7:7:1 instead of 1:1:1:1. It was observed in both experimental cases that plants with parental characters are obtained in excess number than the plants with recombinant characters . Also when dominant alleles are from different parents (heterozygous ) the frequency of recombination is more because of repulsion of gene (repulsion). In First experiment, BL are linked and such linkage is known as Coupling and in second experiment  bL  are linked and such linkage is known as Repulsion. The coupling and repulsion hypothesis is now discarded .

11. Chromosomal theory of linkage: by T. H Morgan Bateson and Punnett   failed to explain the exact reasons of coupling and repulsion Later,  T.H. Morgan  who found coupling and repulsion hypothesis incomplete, while performing experiments with  Drosophila,  in 1910. He proposed that the two genes are found in coupling phase because they are present on same chromosome and similarly on repulsion phase because they are preset on two different homologous chromosomes . There genes are then called  linked genes  and the phenomenon of inheritance of such linked genes is called  linkage  by Morgan. The term coupling and repulsion were replaced by the terms,  cis   and  trans  by ( Haldane , 1942). Morgan  stated the linked genes have the tendency to remain together in original combination because they are located on same chromosome. And the strength of linkage depends upon the distance between the linked genes in the chromosome. The concept of linkage by Morgan establish the foundation of Cytogenetics and develop the theory of linear arrangement of genes in the chromosomes and helps to construct genetic map of the chromosome.

12. According to Chromosomal theory of linkage: Chromosome contains genes. Genes lie in a linear order in a chromosome Distance between them is variable. Each gene has a definite locus in a chromosome. The genes which are close to each other, shows the phenomenon of linkage. The linked genes cannot be separated during gametogenesis (inheritance process),  they inherited together. Tendency of genes to remain linked is due to their presence on same chromosome. The distance between the linked genes determines the strength of linkage. The closer the distance stronger is the linkage strength. The linkage is not due to any relation between two genes but is simply because they happens to be located in the same chromosome.