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CBSE Class 12-science Answered

describe mendels dihybrid cross experiment
Asked by ap996969 | 18 Feb, 2019, 11:17: PM
answered-by-expert Expert Answer
  • Consider a cross of pea plants having round and yellow seeds and plants with wrinkled and green seeds. In this cross, the two pairs of characters involved are seed shape and seed colour.
  • The letter capital R is used to denote round shaped seeds (RRYY) and capital Y to denote yellow coloured seeds (rryy).
  • The two parent plants are then crossed, RRYY × rryy.
  • When the gametes of P1 generation are formed, each pair of alleles segregates independently of the other. Hence, during gamete formation RRYY plants produce gametes RY. Similarly, the other parent rryy produces gametes ry.
  • Mendel observed that phenotypically, the F1 plants were with round yellow seeds. The genotype of these F1 hybrids was RrYy. The genotype shows R which is dominant over r with respect to seed shape and results in round seeds. The genotype also contains Y which is dominant over y with respect to seed colour and results in yellow coloured seeds. All F1 plants are called dihybrids as they have two sets of contrasting characters and have the genotype RrYy.
  • The F1 plants produced four types of gametes: ry, RY, rY and Ry during gamete formation.
  • When F1 plants are self-pollinated, they produce a new generation of plants known as F2 or second filial generation. The four types of male gametes and four types of female gametes produce in all 16 different mating combinations. The 16 different types of F2 individuals can be easily represented with the help of a Punnett square.
  • F2 generation obtained by self-pollination of F1 plants exhibits 16 plants with different genotypes. Out of a total of 16 plants, 9 plants are with round and yellow seedsRRYY, 3 are with wrinkled and yellow seedsrrYY, 3 are with round and green seedsRRyy and 1 is with wrinkled and green seed rryy. Thus, the phenotypic ratio of F2 plants is 9 : 3 : 3 : 1.
  • Genotypically, there are 9 different types of plant combinations, RRYY, RrYy, Rryy, rryy, RRYy, rrYy , Rryy, Rryy and rrYY. The genotypic ratio is 1 : 4 : 1 : 1 : 1: 2 : 2 : 2 : 2
  • The 9:3:3:1 ratio of each phenotype of the seeds in F2 generation is called dihybrid ratio. 
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