Linkage, Recombination and chromosome mapping in eukaryotes
Linkage, Recombination and chromosome mapping in eukaryotes
Linkage
Morgan’s concept of Linkage
- Morgan (1910) while working on Drossophila stated that coupling and repulsion are two aspects of the same phenomenon, which is described as linkage. He defined linkage the tendency of the genes, as present in the same chromosome, to remain in their original combination and to enter together in the same gamete. Morgan and Castle formulated the chromosomal theory of linkage. It has following characteristics:
- Genes that show linkage are situated in the same chromosome.
- Genes are arranged in linear fashion in the chromosome i.e., linkage of genes is linear.
- The distance between the linked genes is inversely proportional to the strength of linkage. The genes which are closely located show strong linkage, where as those which are widely separated have more chances to get separated by crossing over.
- Linked genes remain in their original combination during course of inheritance.
The chromosomal theory of linkage is widely supported from the cytological studies. It has helped in the construction of linkage maps of chromosomes (the distance between the genes is determined by the percentage of cross overs).
Arrangement of Linked Genes
- In an individual heterozygous for two pairs of linked genes, the linkage can be either of the two types.
- The dominant alleles of both the gene pair when located on one chromosome and their recessive alleles in the other chromosome of the pair, this arrangement is known as cis-arrangement (Coupling linkage) and the heterozygous with such arrangement (AB/ab) are known as cis-heterozygote.
- The dominant allele of one pair and the recessive gene of other pair are located on one chromosome of the pair and the recessive allele of the first pair and dominant allele of the second pair are located in the second chromosome pair (Ab/aB). This arrangement of a dominant and recessive gene in the same chromosome of the chromosome pair is known as trans-arrangement (repulsion linkage) and heterozygote with such arrangement is known as trans-heterozygote
Linkages are two types
Complete linkage
- 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 the genes are closely associated and tend to transmit together.
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 chromosome is known as crossing over.
- The linked genes which are widely located in chromosome and have chances of separation by crossing over are called incompletely linked genes and the phenomenon of their inheritance is called incomplete linkage.
