CBSE Class 10 Answered

explain dihybrid cross?

Asked by Dolly | 04 Feb, 2015, 07:21: PM

Expert Answer

A cross that involves plants with two pairs of contrasting characters is called a dihybrid cross.

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 and capital Y to denote yellow coloured seeds. The genotype of parent plant is RRYY. The other parent has wrinkled seeds which are green in colour. The letter small r denotes wrinkled seeds and small y denotes green coloured seeds. The genotype of the parent is 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. The gametes RR or YY are not produced by them. Similarly, the other parent rryy produces gametes ry. The gametes rr or yy are not produced by them.

In each of the gametes, a pair of alleles is represented by only one of its members. In P₁ generation, the parent RRYY produces 2 gametes, RY and RY and parent rryy produces 2 gametes, ry and 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. On the other hand, parental plants P₁ produced only two types of gametes RY and ry. The gametes produced by F₁ plants that are similar to those produced by P₁plants are called parental combinations. The two other types of gametes rY and Ry produced by F₁ plants during gamete formation are called recombinations.

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 seeds RRYY, 3 are with wrinkled and yellow seeds rrYY, 3 are with round and green seeds RRyy 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 genotype RRYY is a pure dominant with homozygous round and homozygous yellow seeds. The genotype rryy is a pure recessive with homozygous wrinkled and homozygous green seeds.
The 9:3:3:1 ratio of each phenotype of the seeds in F₂ generation is called dihybrid ratio.