question_answer 2)

Differentiate between the following: (a) Dominance and Recessive (b) Homozygous and Heterozygous (c) Monohybrid and Dihybrid  

Answer:

s Ans.       (a) Differences between dominant and recessive genes are follows:                

	Dominance	Recessive
1.	Dominant allele is able to express itself even in the presence of recessive allele.	Recessive allele is unable to express its effect in the presence of dominant allele.
2.	It does not require another similar allele to produce its effect on the phenotype, e.g., Tt is tall.	It produces its phenotypic effect only in the presence of a similar allele, e.g., tt is dwarf.
3.	Dominant allele or enzyme for expressing its effects, e.g., red colour of flower in pea.	The recessive allele forms an incomplete or defective polypeptide or enzyme so that the expression consists of absence of the effect of dominant allele, e.g., with flower colour in pea.

  (b) difference between homozygous and heterozygous are as follows:  

	Homozygous	Heterozygous
1.	It is pure for a trait and breeds true i.e., gives rise to similar homozygous individuals	Heterozygous individual is seldom pure and produces offspring with different genotypes on selfing, e.g., TT, Tt and tt on selfing of Tt individuals.
2.	Both the alleles of a trait are similar, e.g., TT, tt.	It carries dissimilar alleles, e.g., Tt.
3.	Homozygous individual can carry either dominant or recessive alleles but not both.	Heterozygous individual has both dominant and recessive alleles.
4.	It produces one type of gametes.	It produces two type of gametes.
5.	It does not show extra vigour.	The individual can show extra vigour called hybrid vigour or heterosis.

  (c) Differences between monohybrid and dihybrid crosses are  

	Monohybrid	Dihybrid
1.	It is a cross between two pure organisms in order to study the inheritance of a single pair of alleles.	It is a cross between two pure organisms of a species in order to study the inheritance of two pairs of alleles belonging to two different characters.
2.	It produces a phenotypic monohybrid ratio of 3 : 1 in F2 generation.	It produces a phenotypic dihybrid ratio of 9 : 3 : 3 : 1 in F2 generation.
3.	It produces genotypic ratio of 1 : 2 : 1 in F2.	It produces genotypic ratio of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 in F2.