Question

Give a genetic explanation for the following cross. When a tall pea plant with round seeds was crossed with a dwarf pea plant with wrinkled seeds then all the individual of F1-populations were tall with round seeds. However selfing among $F_1$-population led to a 9 : 3 : 3 :1 phenotypic ratio. [5]

Answer 1

When a tall pea plant with round seeds (TTRR) was crossed with a dwarf pea plant with wrinkled seeds (ttrr), in which letter T represents dominant tall height and t represents for recessive dwarf height, R for dominant round shaped seeds and r for recessive wrinkled seed shape.
The genotype of the parents can then be written as TTRR and ttrr. The gametes are TR and tr. Upon fertilisation they produce the F1 hybrid TtRr (All tall plants with round seeds).
Based upon such observations Mendel proposed the law of dominance and law of segregation which state that offspring that are hybrid for a trait will have only the dominant trait in the phenotype and both the trait segregate separately at the time of gamate formation [2.5]

$\begin{array}{cccc}\text{Parents}& \text{TTRR}& \times & \text{ttrr}\\ & \text{Tall Round}& & \text{Dwarf wrinkled}\\ & \text{TT}\downarrow \text{RR}& & \text{tt}\downarrow \text{tr}\\ \text{Gametes}& \text{(TR)}& & \text{(tr)}\\ F_1\text{generation}& \text{TtRr}& & \text{(Tall Round)}\end{array}$

However selfing among $F_1$-population led to a 9 : 3 : 3 :1 phenotypic ratio.

$F_2$ generation

$\begin{array}{ccccc}& \text{TR}& \text{Tr}& \text{tR}& \text{tr}\\ \text{TR}& \text{TTRR}& \text{TTRr}& \text{TtRR}& \text{TtRr}\\ \text{Tr}& \text{TTRr}& \text{TTrr}& \text{TtRr}& \text{Ttrr}\\ \text{tR}& \text{TtRR}& \text{TtRr}& \text{ttRR}& \text{ttRr}\\ \text{tr}& \text{TtRr}& \text{Ttrr}& \text{ttRr}& \text{ttrr}\end{array}$

Phenotypic ratio : 9 : 3 : 3 : 1 (Dwarf Wrinkled):
(Tall Round) : (Tall Wrinkled) : (Dwarf Round)

Based upon such observations on dihybrid crosses (crosses between plants differing in two traits), Mendel proposed a second set of generalisation called "Law of Independent Assortment" which states that, that the alleles of two (or more) different genes get sorted into gametes independently of one another. [2.5]