Question

Give a genetic explanation for the following cross. When a tall pea plant with round seeds was crossed with a drawf pea plant with wrinkled seeds then all the individual of f₁ - populations were tall with round seeds. However selfing among F₁-population led to a 9 :3 :3: 1 phenotypic ratio.

Answer 1

Genetic explanation of the cross : If a pea plants with two pairs of contrasted characteristics like tall/dwarf and round/wrinkled seeds are crossed the F₁ progeny would have all tall plants with round seeds according to the periwinkle of dominance. This implies that tallness is dominant over dwarfness and round seed is dominant over wrinkled seeds. In F₂ progeny, there would be some tall plants with round seeds and dwarf plants with wrinkled seeds. However, there would be some plants with recombination of characters such as tall plants with wrinkled seeds and dwarf plants with round seeds. This depicts that tall-dwarf and round/lwrinkled traits are inherited independent of each other following law of independent assortment. which states that when two traits are taken in a cross the segregation of one trait is independent to the segregation of other character. 

The following cross Punnett square explains this :

F₂ generation:

Phenotypic ratio :

9/16 Tall Round : 3/16 Tall Wrinkled : 3/16 Dwarf Round : 1/16 Dwarf wrinkled ( 9:3:3:1).