- Home
- Standard 12
- Biology
With the help of Punnett square describe monohybrid experiment.
Solution
Punnett square was developed by a British geneticist Reginald $C$. Punnett.
It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
The possible gametes are written on two sides usually the top row and left columns.
All possible combinations are represented in boxes below in the squares, which generates a square output form.
The punnett square shows the parental tall $TT$ (male) and dwarf $tt$ (female) plants, the gametes produced by them and the $\mathrm{F}_{1}$ 'Tt' progeny.
The $F_{1}$ plants of genotype $T t$ are self-pollinated.
The symbols $q$ and $\sigma^{\top}$ are used to denote the female (eggs) and male (pollen) of the $F_{1}$ generation respectively.
The $\mathrm{F}_{1}$ plant of the genotype $Tt$ when self – pollinated, produces gametes of the genotype ' $\mathrm{T}^{\prime}$ and ' $\mathrm{t}$ ' in equal proportion.
When fertilisation take place, the pollen grains of genotype $'T'$ have a $50$ percent chance to pollinate eggs of the genotype ' $\mathrm{T}$ ' as well as of genotype $'t'.$
Also pollen grains of genotype ' $\mathrm{t}$ ' have a $50$ percent chance of pollinating eggs of genotype ' $\mathrm{T}^{\prime}$ as well as of genotype ' $\mathrm{t}$ '.
As a result of random fertilisation the resultant zygote can be of the genotypes $TT$, $Tt$ or $tt$.
From the punnett square it is easily seen that $1 / 4^{\text {th }}$ of the random fertilisation lead to $TT$, $1 / 2$ lead to $\mathrm{Tt}$ and $1 / 4^{\text {th }}$ to $\mathrm{tt}$.
Though the $\mathrm{F}_{1}$ have genotype of $Tt$ but the phenotypic character seen is 'tall'.
At $F_{2}, 3 / 4^{\text {th }}$ of the plants are tall, where some of them are $TT$ while others are $\mathrm{Tt}$.
Externally it is not possible to distinguish between the plants with the genotypes $'TT'$ and $Tt.$
Hence within the genophytic pair $'Tt'$ only one character $"T' $ tall is expressed.
Hence the character $"T'$ or 'tall' is said to dominate over the other allele $'t'$ or dwarf character.
