Question

Two linked genes a and b show 20% recombination. The individuals of a dihybrid cross between $++/--\times ab/ab$ shall show gametes.
(a) $++80:ab:20$
(b) $++50:ab:50$
(c) $++40:ab40:+a10+b:10$
(d) $++30:ab30:+a20:+b:20$

Answer 1

Hint: The frequency of the dihybrid cross is related to the formation of the recombinant gametes which is found to be around 50%. The combined recombinant frequency of the parent is double the frequency of an individual.
Complete Step by Step Answer:
When the two genes are linked they show a minimal rate of recombination as they are closer to each other and will not possibly undergo crossing over. In the case of a dihybrid cross, the recombinants formed are 20%, so the combined frequency of the parental combinations $++$ and $ab$ will be 80% and the frequency of the individual is half of the parental will be $++40:ab40:+a10+b:10$.
- The genetic linkage is the ability of the chromosomes to have the DNA sequences arranged closer to each other.
- During the process of meiosis in sexual reproduction, these sequences will be together on the same chromosomes even after the crossing over of the chromatids.
- Those gametes that do not separate and remain together even after crossing over are called linked while those that form different gametes from the parental combinations are called recombinant gametes.
- This shows Mendel’s second law of inheritance, called the Law of independent assortment.
- This results in the formation of 50% recombination frequency in the recombinant gametes as they will consist of 2 out 4 gametes.
- The gametes that are linked or similarly the games that are linked will show a recombination frequency of less than 50%.
- As it is found that the combined frequency of parental combinations is 80% so the individual frequency will be its half and it will be 40%.
So, the correct answer is ‘$++40:ab40:+a10+b:10$’.

Note: The observation of the recombination and linkage frequency was done by the British geneticists William Bateson. Reginald Punnett, and Edith Rebecca Saunders in 1905. They performed their experiment based on Mendel’s law of inheritance in the pea plant. The detailed study on the linkage was done by Thomas Hunt Morgan and in his honor, the unit of genetic linkage is known as Centimorgan (cM) .