Linkage in Sweet Pea

                                                         Linkage in Sweet Pea

Bateson and Punnet (1906) studied linkage in sweet pea (Lathyrus odoratus). In sweet pea, blue flower colour is dominant (BB) over red flower (bb). Long pollen grain is dominant (LL) over round pollen grain(ll).

They crossed blue flowered, long pollen sweet pea with red flowered, round pollen plant. In these plants, the genes for flower colour and pollen shape are located in the same chromosome. That is, they are linked. The linked genotypes of the parents are written as follows:

Blue flowered long pollen grained plant - BL/BL

Red flowered round pollen grained plant-bl/bl

The F1 plants were blue flowered with long pollen. The genotype is written as BL/bl.

The F1 plant (BbLI) was test crossed with double recessive plant (bl/bl). In the F2 generation, the plants appeared in the ratio 7:1:1:7 instead of the Mendelian test cross ratio 1:1:1:1.

Blue long-7

Blue round-1

Red long-1

Red round-7

It is a deviation of Mendel's dihybrid test cross. In the F2 the dominant alleles as well as the recessive alleles inherited together. Hence the parent like plants are more i.e 14 out of 16. The parental combinations are due to linkage. The new combinations are less just 2 out of 16. The new combi nations are due to crossing over. In the same sweet pea plant, Bateson and Punnet conducted another experiment. They crossed blue flowered round pollen grained pea plant with red flowered long pollen grained plant. All the F1 plants were blue long. When they are test crossed with red round, in the F2, the pea plants appeared in the ratio of 1 Blue long: 7 Blue round: 7 Red long: 1Red round.

In this experiment also, the parental combinations are more. ie, 14 out of 16. This means that the parental genes are linked together.