Question

Define the following: [5 MARKS]
(a) Genetics
(b) Punnett square
If a pea plant homozygous (pure) for tall is crossed with one homozygous (pure) for dwarf. What will be the appearance of (i) F1 and (ii) F2 progenies?
[The character of tallness (T) is dominant over dwarfness(t)]

Answer 1

a) 2 Marks
b) 3 Marks

(a) Genetics is the study of the transmission of body features from parents to offspring and the laws related to such transmission.
(b) Punnett square is a simple diagram in which the different types of gametes of one parent (female) are placed along one side of the square and those of the other parent, i.e.(male) are placed along the other side.
Homozygous (pure) tall plant is designated as - TT
Homozygous (pure) dwarf plant is designated as - tt
(i) A cross between homozygous tall parent and homozygous dwarf parent will be as follows:


The progenies of F1 generation will be all heterozygous tall (hybrids).

(ii) The F1 hybrids are self-pollinated and crossed to produce F2 progenies which are as follows:



Therefore, three plants will be tall and one plant will be dwarf in F2 generation. Out of the three talls, one will be homozygous and two will be heterozygous.
Results
(i) All plants in F1 generation are heterozygous tall (Tt). These are all hybrids.
(ii) In F2 generation, the phenotypic ratio of tall and dwarf is 3:1. Out of the four plants, one is pure tall, two are heterozygous tall and one is pure dwarf. Thus, the genotypic ratio is 1 (TT) :2 (Tt): 1 (tt).