State Mendel’s law of independent assortment . Explain a Dihybrid cross & its result obtained by him?

Mendel’s Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

Dihybrid Cross :

Mendel performed another set of experiments where he crossed two (True breeding pea lines) Parent pea plants which differed in two pairs of contrasting traits. The hybrids thus produced are called Dihybrids and the cross termed as Dihybrid cross. where the two parents differ in two pairs of contrasting traits, for eg; seed colour and seed shape.

Parent Pea plants with genotype YYRR and yyrr with phenotype Yellow, Round seeds and Green, wrinkled seeds respectively were crossed with each other. The gametes contain alleles ‘YR’ and ‘yr’ as female eggs and male pollens respectively. The F1 generation have genotypes ‘YyRr’ of phenotype Yellow , round seeds. The two characters in consideration the seed color and the seed shape. The F1 generation, both seed color and seed shape exhibited the monohybrid ratio of 3 : 1, independent of each other, for their expression in two contrasting traits.

 

 

The F2 generation produced have phenotypes Yellow, Round : Yellow, wrinkled : Green , Round : Green , wrinkled in the ratio 9 : 3 : 3 : 1 while the genotypes are-

Yellow , Round –         YYRR, YYRr, YyRR, YyRr

Yellow wrinkled –        YYrr, Yyrr

Green Round –           yyRR, yyRr

Green Wrinkled –        yyrr.

As can be seen in the Punnet square. Thus 9 : 3 : 3 : 1 is the dihybrid ratio.

The Dihybrid Ratio is simply calculated by –

(3 Yellow : 1 Green )  ( 3 Round : 1 Wrinkled ) = 9 Yellow Round :3 Yellow Wrinkled : 3 Green Round : 1 Green Wrinkled.

During gamete formation, 50% gametes receive allele for Yellow seed color and 50% gametes receive allele for Green seed color.

Similarly, the alleles for seed shape also segregate and 50% gametes receive the allele for Round seed shape and 50% receive allele for wrinkled seed shape.

So there are 4 types of gametes produced during selfing of F1 generation.

These are, 50% with ‘Y’ allele and 50% with ‘y’ allele.

50% of gametes with ‘Y’ allele have ‘R’ allele and 50% have ‘r’ allele.                                       

Similarly, 50% of gametes with ‘y’ allele have ‘R’ allele and 50% have ‘r’ allele.Thus, the four possible gametes are produced are RY, Ry, rY and ry 25% each of total gametes produced.