Linkage

The genes which reside on the same chromosome are said to be linked, and this phenomenon is called linkage.

Linkage groups: All those genes which are located on the same chromosomes constitute a linkage group. For example, there are four (4) linkage groups in Drosophila melanogaster, 7 in Pisum sativum and 23 in man.

Each organism has thousands of genes but the number of chromosomes is limited .For example in man, there are more than twenty thousand genes on only 23 pairs of chromosomes. Therefore many genes must be present on each chromosome.

Discovery of linkage (William Bateson and R.C Punnet)

In 1906 W. Bateson and R.C. Punnet were study two characters of sweet pea, flower colour and shape of pollen.

They had crossed a variety of sweet pea having purple flowers and long pollen, with another variety having red flowers and round pollen. The gene for purple (P) is dominant over red (p) and genes for long pollen (L) is dominant over round (l). The F1 offsprings showed purple flowers with long pollen having genotypes PpLl.

P                                  PPLL (purple, long)                 x          ppll (red, round)

Gametes                     PL                                                                    pl

F1                                                        PpLl (purple, long)

Self Cross                               PpLl                             x                                  PpLl

Gametes                        PL        Pl         pL        pl                     PL        Pl         pL        pl

The F2 phenotypes deviated strikingly from the expected 9:3:3:1 ratio

F2 shows large number of parental combinations (purple long and red round) than recombinants (purple round and red long).

As a possible explanation of this, they proposed that the F1 had actually produced more parental gametes( PL & pl) then would be produced by Mendelian independent assortment because these were the gametic type in the original pure lines, the researchers thought that physical

coupling

(The tendency of the two dominant genes to remain together during gamete formation on F1 generation, when they come from the same parent, is called as coupling phase.) between dominant allele P and L and between the recessive alleles p and l might have prevented their independent assortment in the F1.

Repulsion Phase

The tendency of the two dominant genes to separate out during gamete formation, when they come from two different parents, is called as repulsive phase. Bateson and Punnet had crossed a plant having purple flowers and round pollen with a plant having a red flowers and long pollen. The F1 off springs showed purple flowers with long pollen having genotype PpLl. The F2 off springs showed purple long, purple round, red long and red round plants. the percentage of purple round and red long was more than expected ratios. It  is obtained because F1 forms gametes with PL or pl with lesser percentage than those with Pl and pL genes.

Parents            PPll (purple, round)                 X          ppLL (red, long)

Gametes                     Pl                                             pL

F1                                                        PpLl  (purple long)

Thomas Hunt Morgan

He found a similar deviation from Mendel’s second law while studying two autosomal genes in Drosophilla. One of these genes affects eye colour (pr, purple, and pr⁺, red) and the other affects wing length (vg, vestigial, and vg⁺, normal). The wild type alleles of both genes are dominant. Morgan crossed prprvgvg flies with pr⁺pr⁺vg⁺vg⁺ and then test crossed doubly heterozygous F1 females.

Parents            prprvgvg (purple, vestigial)                 x          pr⁺pr⁺vg⁺vg⁺    (Red, normal)

prvg                                                     pr⁺vg⁺

F1                                            pr⁺pr vg⁺vg

Test cross                            pr⁺prvg⁺vg      x          prprvgvg

Results of test cross

Obviously, these numbers deviate drastically from the Mendelian predictions of a 1:1:1:1 ratio, and they indicate the coupling of genes.

Morgan concluded that the genes for wing shape and eye colour are located on the same chromosome. The new combinations of the two characters were formed by crossing over.

Morgan suggested that the two genes in his analyses are located on the same pair of homologous chromosomes. Thus, in the first analysis, when pr and vg are introduced from one parent,they are physically located on the same chromosome, whereas pr and vg are on the homologous chromosome from the other parent

Fig: Inheritance of linked genes. Simple inheritance of two pairs of alleles located on the same chromosome pair.

New combinations of alleles arise from crossovers

Fig 2 Crossing-over in meiosis. The exchange of parts by crossing-over may produce gametic chromosomes whose allelic combinations differ from the parental combinations.

The linkage hypothesis explains why allele combinations from the parental generations remain together— because they are physically attached by the segment of chromosome between them. But how do we explain the appearance of the minority class of nonparental combinations? Morgan suggested that when homologous chromosomes pair in meiosis, the chromosomes occasionally break and exchange parts in a process called crossingover. Fig 2 illustrates this physical exchange of chromosome segments. The two new combinations are called crossover products.