The phenomenon of inheritance of genes together and to retain their parental combination even in the offspring is termed as linkage.
Mendel’s principle of independent assortment shows exception. It came to notice when Bateson and Punnet noted that two pairs of alleles in pea plant did not assort independently.
They crossed pea plants having blue flower (B) and long pollen (L) with pea plants having red flowers (b) and round pollen (l). A test cross between heterozygous blue long (BbLl) and double recessive red round (bbll) shows that the frequency of parental combination is 7 times greater than the non parental combinations. The expected result should have been different i.e., the total parental frequency should have been equal to the recombinant or non parental frequency.
The greatest frequency of parental combinations and the lesser frequency of the recombinants indicate that the genes did not assort independently. The two parental genes tend to move together and appear to be linked together. Bateson and Punnet describe it as genetic coupling and repulsion. Morgan while working on Drosophila melanogaster described the above as linkage.
Chromosome theory of linkage
Morgan and Castle formulated the chromosome theory of linkage which has the following characteristics:
- Genes are arranged in a linear fashion in the chromosomes.
- Genes that show linkage occur on the same chromosome.
- The linked genes remain in their original combination during the course of inheritance.
- The distance between the linked genes determines the degree or strength of the linkage.
The characters for which ratios were obtained by Mendel for segregation and independent assortment were present on different chromosomes. But, in pea, as in other organisms there would always be more genes than chromosomes. Therefore, each chromosome should carry many genes. All the genes located on the same chromosomes are termed as linked genes and the phenomenon as linkage. Each such chromosome forms one linkage group. The number of linkage groups would thus be equal to haploid number of chromosomes in an organism.
Under these circumstances, there could be two possibilities amongst genes present on the same chromosome:
- The linked genes fail to separate independently and are, therefore, always exceptions to Mendels’ principle of independent assortment.
- Linked genes too have a mechanism of their separation and recombination during mitosis and behave as if these are assorting independently or present on different chromosomes.
Coupling and Repulsion:
Linkage was first demonstrated by Bateson and Punnet (1906) in sweet pea (Lathyrus Odoratus). This demonstration did not give Mendelian ratios.
Coupling phase or cis-phase
The cross consisted of two individuals of sweet pea. In this plant blue flowers (B) are dominant over red flowers (b) and long pollen (L) are dominant over round pollen (l).
A plant homozygous for blue flowers and long pollen (BBLL) was crossed with another individual homozygous for red flowers and round pollen (bbll). F1 progeny shows blue flowers and long pollen, the genotype being (BbLl). A test cross between F1 hybrid (BbLl) and its double recessive parent (bbll) results in a ratio, 7:1:1:7, instead of 1:1:1:1 . This indicates the tendency of dominant alleles to remain together. A similar tendency for recessive alleles was also observed. This was termed gametic coupling, coupling phase or cis phase.
Repulsion phase or trans-phase
Similarly when two such dominant or recessive alleles come from different parents, they tend to remain separate. This is termed as repulsion phase or trans phase. In repulsion phase, one of the parents had blue flowers and round pollen (BBll). The other parent had red flowers and long pollen (bbLL). F1 hybrid shows blue flower and long pollen (BbLl). A test cross between F1 hybrid (BbLl) and its double recessive parent (bbll) results in a ratio, 1:7:7:1, instead of normal 1:1:1:1. This shows that dormant alleles coming from different parents remain separate and do not form many recombinations. The same is true for recessive alleles.
Bateson termed his results as coupling and repulsion hypothesis. Later it was found that both these are parts of the same phenomenon of linkage. The frequencies obtained in the test cross in Lathyrus showing linked genes can be compared with those obtained in a test cross where independent assortment takes place. It shows that a test cross involving two independently assorting genes (dihybrid test cross) produces 4 phenotypes with 25 percent each. Out of these, two parental frequencies ( 50%) are equal to the total recombinant frequencies (50%).
In Lathyrus (sweet pea), however, frequency of parental combination is more (87.4%) than the frequency of recombinants (12.6%). This indicates that these two parental genes do not separate independently but tend to move together. Such genes are, therefore, linked. A pair of genes may thus be termed linked if their recombination frequency in a test cross is lower than the expected value of 50%. In other words, parental combination should have frequency of more than 50%.
Types of linkage:
A cross between wild type Drosophila with grey body and vestigial wings (BBvv) and Drosophila with black body and long wings (bbVV) produces F1 offspring, all of which have grey body and long wings (BbVv).
Back cross of these F1 male hybrids with a double recessive female having black body and vestigial wings (bbvv), produce offspring of two types in equal proportion. These offspring resemble the two grandparents.
The result indicates that grey body character is inherited together with the vestigial wings. It means that the genes are linked together. Similarly black body character is associated with the long wings.
In the above example, the offsprings exhibit only the parental combination. Since new non parental combinations are not formed, this is a complete linkage.
In majority of cases homologous chromosomes undergo breakage and reunion during gametogenesis. During reunion the broken pieces of chromatids exchange (i.e., crossing over) producing some non parental (i.e., new combination). Therefore the linkage is incomplete.
Crossing of one variety of maize having coloured and full seeds with another variety of colourless and shrunken seed coat produces F1 hybrid with coloured and full seed coated.
But in a test cross when the F1 female hybrid are cross pollinated with the pollen from a plant having colourless and shrunken seeds (double recessive) four types of seeds are produced.
1. Coloured full (CScs)
2. Colourless shrunken (cscs)
3. Coloured shrunken (Cscs)
4. Colourless full (CScs)