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Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYPrinciples of Inheritance and Variation4 Marks
Question
Explain the inheritance of one gene by Punnett Square ?
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Answer

→ Punnett Squre:
→ The production of gametes by the parents, the formation of the zygotes, the F, and F₂ plants can be understood from a diagram called Punnett Square (By Reginald C. Punnett).
→ It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
→ The Punnett Square shows,
TT → Tall (Male) (♂)
tt → Dwarf (Female) (♀)
Produced Gametes (T, t)
Tt→ F Progeny (self-pollinated)
→ The F₁ plant of the genotype Tt when self- pollinated, produces gametes of the genotype T and t in equal proportion.
→ When fertilisation takes place, the pollen grains of genotype T have a 50 per cent chance to pollinate eggs of the genotype T, as well as of genotype t.
→ Also pollen grains of genotype t have a 50 per cent chance of pollinating eggs of genotype T, as well as of genotype t. As a result of random fertilisation, the resultant zygotes can be of the genotypes TT, Tt or tt.
→ From the Punnett square it is easily seen that 1/4th of the random fertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt.
→ Though the F₁ have a genotype of Tt, but the phenotypic character seen is 'tail'.
→ At $F_2$, 3/4 of the plants are tall, where some of them are TT while others are Tt.
→ Externally it is not possible to distinguish between the plants with the genotypes TT and Tt. Hence, within the genotypic pair Tt only
one character 'T' tall is expressed.
→ Hence the character T or 'tall' is said to dominate over the other allele or 'dwarf character.
→ It is thus due to this dominance of one character over the other that all the $F_1$ are tall (though the genotype is Tt) and in the $F_2$ 3/4 of the plants are tall (though genotypically 1/2 are Tt and only 1/4th are TT).
Genotypic ratio3 : 1 (Tall: Dwarf)
Phenotypic ratio1:2:1 (TT: Tt: tt)
→ The 1/4 1/2 1/4 ratio of TT Tt: tt is mathematically condensable to the form of the binomial expression (ax + by)², that has the gametes bearing genes T or t in equal frequency of 1/2. The expression is expanded as given below:
→ (1/2T + 1/2t)²= (1/2T + 1/2t) X (1/2T + 1/2t)
= 1/4 TT + 1/2 Tt + 1/4 tt
→ Mendel self-pollinated the F₂ plants and found that dwarf F₂ plants continued to generate dwarf plants in F3 and F4 generations. He concluded that the genotype of the dwarfs was homozygous - tt. Image

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