BOTANY Higher Secondary Second Year - Textbooks Online

3.3. Linkage and crossing over
The tendency of genes or characters to be inherited together because
of their location on the same chromosome is called linkage. Many
hybridization experiments were conducted both on plants and animals
based on Mendel’s work. The results of certain dihybrid crosses did not
confirm the law of independent assortment. It states that the inheritance of
genes of each pair in a dihybrid during gamete formation is independent
of the other.
Blue flower Red flower
Parent long pollen round pollen
BBLL bbll
Gametes BL bl
F 1 generation BbLl (Blue long)
Dihybrid test cross BbLl bbll
Gametes BL B1 bL bl
BbLl Bbll bbLl bbll
Phenotype Blue Blue Red Red
long round long round
Observed percentage
frequency
44 6 6 44
Observed ratio 7 : 1 : 1 : 7
Expected ratio 1 : 1 : 1 : 1
Mechanism of linkage - coupling in Lathyrus odoratus
In 1906, William Bateson and Reginald Punnett conducted experiments
in sweet pea, Lathyrus odoratus to confirm Mendel’s dihybrid testcross.
They observed an exception to the independent assortment of two genes
in this plant. Here, blue flower (B) is dominant over the red flower (b)
and long pollen (L) dominant over round pollen (l). They crossed true
breeding plants having blue flower with long pollen (BBLL) and red flower with
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bl