Answer :

a) A dihybrid cross upto F2 generation between pea plants bearing violet coloured axial flowers and white coloured terminal flowers and their phenotypic ratio is as follows:


To find out the phenotypic ratio of a dihybrid cross, we can take an example for the same:


A cross between pea plants bearing violet coloured axial flowers (AAVV) and white coloured terminal flowers (aavv) is made where axial (A) and violet (V) traits are dominant and terminal trait (a) and white colour trait (v) is recessive allele.



F1:


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In the F1 progeny, all flowers show violet flowers which are axial.


Thus all the offspring in F1 are AaVv i.e. all are violet and axial.


Now gametes in F2 are: AV, Av, aV, av


Therefore:


F2:


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Therefore as it is evident from the Punnett Square: The phenotypic ratio for the progeny F2 is four possible phenotypes (9:3:3:1):- 9 axial violet, 3 terminal violet, 3 terminal white, 1 terminal white.


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(b) Mendel's law of inheritance: There are two laws of inheritance formulated by Mendel: the First Law or Law of Dominance and the Second Law or Law of Segregation


Law of Dominance states that the characters are controlled by factors which occur in pairs. One member of the pair is dominant while another is recessive or dominated by the other member of the pair. This law thus explains the expression of only one of the parental characters in a monohybrid cross in the F1 progeny while expression of both during the F2 progeny.


Law of Segregation: This law states that the alleles do not show any blending and both the characters are recovered as such in the F2 generation even though one isn't visible in the F1 progeny. It also states that the factors or alleles of a pair would segregate from each other such that each gamete receives only one of the two factors.


OR


a) In the Galapagos Islands, Darwin observed small black birds which were later called Darwin’s Finches. He could see how there were many varieties of finches which had developed or evolved in the same island. The original seed-eating features were there along with many other forms with altered beaks. These different types of beaks had arisen to help them become insectivorous. This process of evolution of different species in a given area starting from a starting point and radiating to other areas is called adaptive radiation which was seen in Darwin’s finches.


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b) Darwin's view on evolution differed from de-Vries in the following way:


• Vries concluded that mutation was responsible for evolution and not the minor heritable variations that Darwin suggested.


Mutations are random and directionless while Darwinian variations are small and directional.


• Evolution for Darwin's theory was gradual while for deVries theory of mutation, evolution caused speciation and hence was called saltation which is a single step large mutation.


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