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I was studying monohybrid crosses in genetics where the character considered is stem height. Whenever I came across punnett squares, I used to calculate the probability of the genotype of the offsprings which can be formed as a result of a cross between 2 heterozygous plants. In the F1 generation, these are obtained by crossing homozygous dominant(Tall) and homozygous recessive plant(Short). Now when these plants of the F1 generation are crossed, each plant produces 2 types of gametes where each gamete is having only 1 allele of the gene.

Now my question is that when we cross these plants, is it necessary to get 'one- fourth' dwarf plants and 'three-fourth' tall plants? Doesn't the punnett square only gives the probability of the possible genotypes of the offsprings formed, while the real cross could result in say 50-50 percent tall or dwarf plants? Is it necessary that the actual cross follows the probability of predicted by the punnett square ?

If no then why is this statement given in my book that 'One fourth of the plants obtained in the cross were dwarf while the rest were dwarf'? Couldn't this proportion be anything other than this proportion ? If so then how could this proportion be obtained isn't that a chance factor?

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while the real cross could result in say 50-50 percent tall or dwarf plants?

Sure it's possible, but if you have dozens of offspring, the odds of you being very far away from the 1:3 ratio are very small. If you look hard enough, you can find Mendel's actual results, and a chi-square test is the usual way to show that his results do not deviate significantly from the expected ratios.

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  • $\begingroup$ That is these results( punnett square) are the most probable results $\endgroup$
    – p0803
    Commented Jul 8, 2019 at 18:02

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