🪢 Linkage

• Sutton and Boveri proposed the Chromosome Theory of Inheritance.
• According to chromosome theory of inheritance, it is well established that many genes are located in each chromosome in a linear fashion. It may therefore be expected that all genes located in same chromosome would move to same pole during cell division. As a consequence, such genes will fail to show independent segregation and would tend to be inherited together.
• This tendency of genes to remain together in their original combination during inheritance is called linkage.
• Mendel’s Law of Independent Assortment is applicable only when the genes are located in different chromosomes while linkage refers to the genes located in the same chromosome.
• Linkage in plant: Discovered by Bateson & Punnet (1906) worked on Sweet Pea (Lathyrus odoratus).
• Linkage in animal: Discovered by T.H. Morgan worked on Drosophila.
• Here both the genes were located on the same chromosome, therefore they inherited together. Such genes are called linked genes and the group of such genes are known as linkage group. Such genes have tendency to remain together during the process of inheritance.
• The strength of linkage depends upon the distance between the linked genes. Such linked genes can be separated by the process of crossing over or recombination. The linked genes of relatively close loci transmit together from generation to generation but of relatively distant loci are frequently separated by crossing over.
• The crossing over varies from 0-50% and will never cross 50% and the non-crossover varies from 50-100%.
• Bateson & Punnet formulated the hypothesis ‘Coupling and repulsion hypothesis’ to explain the lack of independent assortment in the result obtained from the dihybrid cross between homozygous sweet pea (Lathyrus odoratus) having a dominant allele for blue or purple flowers and long pollen with another homozygous double recessive plant with red flowers & round pollens. But they could not explain the exact reasons.
• Later, T. H. Morgan put forth the Theory of Linkage and concluded that coupling and repulsion were two phases of single phenomenon, linkage.
• Father of Drosophila (fruit fly) genetics: T.H. Morgan.
• Gynandromorph: Gyn (Female) + andro (Male) + morph (form)
• Means some part of body is female & other parts are male e.g. Drosophila

Types of Linkage

👉🏻 Linkage is generally classified on the basis of three criteria viz.,

• Crossing over
• Genes involved
• Chromosomes involved

(i) Based on crossing over

• Linkage may be classified into (a) complete and (b) incomplete / partial depending up on absence or presence of recombinant phenotypes in test cross progeny.
• Complete linkage: It is known in case of males of Drosophila and females of silkworms, where there is complete absence of recombinant types due to absence of crossing over.
• Incomplete / partial linkage: If some frequency of crossing over also occurs between the linked genes, it is known as incomplete / partial linkage. Recombinant types are also observed besides parental combinations in the test cross progeny. Incomplete linkage has been observed in maize, pea, Drosophila female and several other organisms.

(ii) Based on genes involved

• Depending on whether all dominant or some dominant and some recessive alleles are linked together, linkage can be categorized into (a) Coupling phase and (b) Repulsion phase
• Coupling phase: All dominant alleles are present on the same chromosome or all recessive alleles are present on same chromosome.
  • TR/TR 🚣🏼 tr/tr → Coupling phase
• Repulsion phase: Dominant alleles of some genes are linked with recessive alleles of other genes on same chromosome.
  • Tr/Tr 🚣🏼 tR/tR → Repulsion phase

(iii) Based on chromosomes involved

• Based on the location of genes on the chromosomes, linkage can be categorized into (a) autosomal linkage and (b) X-chromosomal linkage / allosomal linkage / sex linkage
• Autosomal linkage: It refers to linkage of those genes which are located in autosomes (other than sex chromosomes).
• X-chromosomal linkage / allosomal linkage / sex linkage: It refers to linkage of genes which are located in sex chromosomes i.e. either ‘X’ or ‘Y’ (generally ‘X’)

Characteristic features of Linkage

• Linkage involves two or more genes which are located in same chromosome in a linear fashion.
• Linkage reduces variability.
• Linkage may involve either dominant or recessive alleles (coupling phase) or some dominant and some recessive alleles (repulsion phase).
• It may involve either all desirable traits or all undesirable traits or some desirable and some undesirable traits.
• It is observed for oligo-genic traits as well as polygenic traits.
• Linkage usually involves those genes which are located close to each other.
• The strength of linkage depends on the distance between the linked genes. Lesser the distance, higher the strength and vice versa.
• Presence of linkage leads to higher frequency of parental types than recombinants in test cross. When two genes are linked the segregation ratio of dihybrid test cross progeny deviates significantly from 1 : 1 : 1 : 1 ratio. (Produce more parental type)
• Linkage can be determined from test cross progeny data.

Linkage and pleiotropy

• A close association between two or more characters may result either due to linkage or pleiotropy or both.
• Pleiotropy refers to the control of two or more characters by a single gene.
• A tight linkage between two loci can be often confused with pleiotropy.
• The only way to distinguish between linkage and pleiotropy is to find out a crossover product between linked characters.
• Intermating in segregating populations may break a tight linkage, but a huge population has to be raised to find out the crossover product. If a cross over product is not found in spite of repeated intermating, there seems to be the case of pleiotropy rather than linkage.

Linkage groups

• Linkage group refers to a group of genes which are present in one chromosome.
• In other words, all those genes which are located in one chromosome constitute one linkage group. The number of linkage groups is limited in each individual.
• The maximum number of linkage groups is equal to the haploid chromosome number of an organism.
• For example there are ten linkage groups in corn (2n = 20), seven in garden pea (2n = 14), seven in barley (2n = 14), four in Drosophila melanogaster (2n = 8) and 23 in man (2n = 46).

Detection of linkage

• Test cross is the most common method of detecting the linkage.
• In this method, the F1 heterozygous at two loci (AB/ab) is crossed to a double recessive parent (ab/ab) and the phenotypic ratio of test cross progeny is examined.
  • If the phenotypic ratio of test cross progeny shows 1:1:1:1 ratio of parental and recombinant genotypes, it indicates absence of linkage.
  • If the frequency of parental types and recombinant types deviate significantly from the normal dihybrid test cross ratio of 1:1:1:1, it reveals presence of linkage between two genes under study.
• Another way to detect the presence or absence of linkage is to self-pollinate the individual heterozygous at two loci. If there is complete dominance at each locus and no epistasis, the segregation ratio of the progeny will be 9:3:3:1. Presence of linkage either in coupling or repulsion phase will lead to significant deviation from 9:3:3:1 ratio. The deviation of observed values from the expected ratio is tested with the help of Chai square test.

Significance of Linkage in Plant Breeding

• Linkage limits the variability among the individuals.
• Linkage between two or more loci controlling different desirable characters is advantageous for a plant breeder. A linkage between genes controlling two different desirable characters will help in simultaneous improvement of both the characters.
• Linkage is undesirable when desirable and undesirable genes are linked together.
• The estimates of genetic variances for quantitative characters are greatly influenced by the presence of linkage.