After the discovery of the mode of inheritance for a single pair of contrasting characters, Mendel proceeded to investigate the simultaneous inheritance of two different pairs of contrasting characters and the hybrids are called dihybrids. Mendel made experiments with garden Pea keeping view the simultaneous inheritance of two different pairs of contrasting characters out of each pair was dominant in F1 generation and it was always the same contrast which was dominant in single trait inheritance. However he always obtained a 9:3:3:1 ration in F2 generation of which 9 were of dominant parental type, 3:3 were of two different recombinant types which did not exist previously and one was showing both successive contrasts.
From the results of his breeding experiments Mendel realized correctly that the assortment of two different pairs of alleles is completely at random (i.e. depends on chance) This prinaple of inheritance was called the “law of independent Assortment”. In its simplest form it can be stated that the alleles for two different pairs of contrasting characters are transmitted independent of one another during their inheritance to the next generation.
Mendel crossed a variety of pea having yellow round seeds (both dominant characters) with one having green wrinkled seeds (both recessive characters). The seeds obtained from these two pure forms were all yellow and round i.e. they showed (both dominant characters). When the hybrid plants (F1) produced from these seeds were self fertilized they gave four types of seeds (1) rounded and yellow (2) wrinkled and yellow (3) rounded and green (4) wrinkled and green in the ratio of 9:3:3:1, i.e. in F2 generation there were nine yellow round, green wrinkled. All the yellows taken together were to the greens as 3:1; all the rounds taken together were to the wrinkled as 3:1 but some of the yellows were now wrinkled and some of the greens were now round.
The results obtained from the above experiments may be explained as follows:
The dominant yellow colour of the seed may be represented by the gene Y and the recessive green colour by the gene Y. The dominant roundness of the seed may be represented by the gene R and the recessive wrinkleness by the gene r. The gametes of the yellow round parent will carry the gene YR and those of the green wrinkled Yr. The zygote formed by the union of two such gametes, will have YR Yr, and will give rise yellow round seed, showing both dominant characters. Now when the hybrid arising from this zygote forms its gametes the members of each pair of contrasting characters behave independently of those of the other pair that is a gamete bearing a gene for the roundness of the seed may also bear a gene for yellow or green cotyledons. Similarly a game bearing wrinkleness of seed may also bear a gene for green or yellow cotyledons. Thus there will be four kinds of male gametes and four kinds of female gametes and the four kinds in each case will be YR, Yr, yR, yr. On fertilization any one of the four female gametes will unite with any one of the four male gametes and hence there will be sixteen possible combinations.
.jpg)