32. (b) (i) How would you find out whether a given tall garden pea plant is homozygous or heterozygous ? Substantiate your answer with the help of Punnet square.
(ii) Given below are \(F_2\) phenotypic ratios of two independently carried monohybrid crosses.
(1) 1 : 2 : 1
(2) 3 : 1
Mention what does each ratio suggest.
32. (b) (i) How would you find out whether a given tall garden pea plant is homozygous or heterozygous ? Substantiate your answer with the help of Punnet square.
(ii) Given below are \(F_2\) phenotypic ratios of two independently carried monohybrid crosses.
(1) 1 : 2 : 1
(2) 3 : 1
Mention what does each ratio suggest.
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Solution and Explanation
A tall pea plant can be TT (homozygous) or Tt (heterozygous), and both look tall.
To find which it is, do a test cross. Cross the tall plant with a homozygous recessive dwarf plant (tt).
Then study the offspring.
Step 2: Case 1: the tall plant is homozygous (TT):
TT makes only T gametes and the dwarf parent makes only t gametes.
| Gametes | t | t |
|---|---|---|
| T | Tt | Tt |
| T | Tt | Tt |
All offspring are Tt, so all are tall. No dwarf plant appears.
Step 3: Case 2: the tall plant is heterozygous (Tt):
Tt makes T and t gametes in equal numbers.
| Gametes | t | t |
|---|---|---|
| T | Tt | Tt |
| t | tt | tt |
Offspring: 2 Tt (tall) and 2 tt (dwarf), a 1 : 1 ratio of tall to dwarf.
Step 4: Conclusion of the test cross:
If all offspring are tall, the plant is homozygous (TT).
If tall and dwarf appear in a 1 : 1 ratio, the plant is heterozygous (Tt).
Step 5: Part (ii): Meaning of the ratios:
(1) 1 : 2 : 1 in the \(F_2\) means the phenotypic ratio equals the genotypic ratio. This points to incomplete dominance (or co-dominance), where the heterozygote looks different.
(2) 3 : 1 in the \(F_2\) means complete dominance, as in Mendel pea crosses, where the heterozygote looks like the dominant parent.
Final Answer:
Test cross with a dwarf (tt) plant: all tall means TT, and 1 tall : 1 dwarf means Tt. The ratio 1 : 2 : 1 shows incomplete dominance or co-dominance, and 3 : 1 shows complete dominance.
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