Question

Two linked genes a and b show 30% recombination. The individual of a dihybrid cross ++/++ ab/ab shall produce gametes as
(a) ++ 80; ab 20
(b) ++40; ab 40; +a10; +b10
(c) ++35; ab35; +a15; +b15
(d) ++50; ab

Answer 1

Hint: In recombination crossing over takes place between two chromosomes of different organisms which leads to the exchange of genetic material and offspring which obtain a combination of both traits. It breaks the old linkage of genes and forms a new linkage.

Complete answer:
Two gene linkages show 30% recombination which means that the remaining 70% are nonrecombinant type and 70 % crossing over will not take place. So in nonrecombinant, the progeny will resemble both of the parents by 35%. In case of crossing over progeny will show the character of both parent genes that’s why from 30% of recombinant genes +a is 15% and +b is 15%.

Additional Information: Bateson and Punnett propose the coupling and repulsion hypothesis for crossing over, which was replaced by Morgan’s chiasma type hypothesis. Chiasma is related to chiasmata, according to Morgan chiasmata leads to crossing over of chromosomes. Crossing over is of four types,
-Single crossing over takes place between non-sister chromatids of homologous pairs.
-Double-crossing over takes place between chromatids which occurs at two-place, it also leads to reciprocal crossing over and complementary crossing over.
-Multiple crossing over is less common and takes place at 4 to 5 points of the chromosome.
-Somatic crossing over is the exchange of chromosomes which present in somatic cells.
So, the correct answer is ‘++35; ab35; +a15; +b15’.

Note: Large distances between genes, high temperature, X- rays, and chemicals affect the linkage whereas sex having a heteromorphic chromosome shows strong linkage. The strength of linkage also increases with an increase in age. Proper linkage was first reported in male drosophila.