After reading this article you will learn about the principle of independent assortment of genes.
Independent assortment is defined as random distribution of alleles to the gametes that occurs when genes are located on different chromosomes. The distribution of one pair of alleles is independent of that of other genes located in non-homologous chromosomes. This principle, is as a matter of fact, extension of law of segregation to 2 or more gene situations provided the genes are located on different chromosomes.
However, similar results are possible when 2 genes are located on same chromosome provided they are so far apart that there is always crossing over between the two genes.
The diagrammatic representation of this principle is shown in Fig. 5.2:
Thus, the typical phenotypic ratio of 9: 3: 3: 1 is a consequence of independent segregation (assortment) of 2 genes located on 2 different chromosomes.
As a matter of fact independent segregation is occurring while formation of gametes from F1 as shown below:
Thus, T segregates from t and Y from y during meiosis. However, T has freedom to go with Y and y and similarly t has freedom to go with Y and y resulting into 4 kinds of gametes, TY, Ty, tY, ty in equal frequencies. When these 4 kinds of gametes combine with each other in all possible combinations, a typical 9:3:3:1 ratio is obtained in the F2 generation which we get after selfing (F1 x F1) of F1.
A 3: 1 F2 ratio is the result of single gene (monogenic) segregation and 9: 3: 3: 1 ratio obtained with 2 genes is a di-genic segregation. The 9: 3: 3: 1 ratio can also be easily obtained by applying the law of probability called the ‘product rule’ which states that when 2 or more events are occurring independently, the chance of events occurring together is the product of probabilities of individual events.
On this basis under law of segregation, it follows that:
For I gene pair, probability of getting tall plants in F2 = 3/4
The probability of getting dwarf plants in F2 = 1/4
For II gene pair, probability of getting yellow pods in F2 = 3/4
The probability of getting green pods in F2 = 1/4
Now on the basis of product rule of probability applicable to independent events as above (segregation of tall vs dwarf is independent to that of yellow vs green), the probabilities of occurrence of various possible events together shall be as follows:
Similarly, various genotypic ratios can also be derived, for example:
Tri-hybrid Ratio:
A cross involving parental lines differing for 3 pairs of genes is known as tri-hybrid. Principle of independent assortment could be easily extended to 3 gene situation either by checkerboard method as given in Fig. 5.2
For 2 gene situation or by-product rule of probability as illustrated below:
It should be clearly understood that TtYy Rr will form 8 kinds of gametes. T will segregate from t, Y from y and R from r. After segregation of individual genes during meiosis, and their combinations into gametes, following 8 gametes will be formed:
TYR, TYr, TyR, Tyr, tYR, tYr, tyR, tyr
Now onwards if we treat each gene separately, the F2 phenotypic ratio for each character will be as follows:
Now all possible combinations of phenotypes can be easily derived as follows in F2.
Thus, the typical tri-genic phenotypic ratio is 27: 9: 9: 3: 9: 3: 3: 1
The same ratio can be obtained by forked line method as given in Fig. 5.3:
Similarly, genotypic ratio can be easily derived by any of the 3 methods, viz., checkerboard method, probability product rule method and forked line method.
This is illustrated in Fig. 5.4:
By now it should be clear that the relationship between the number of genes involved in a cross, gametes, F2 phenotypes, F2 genotypes and all possible combinations in F2 assuming complete dominance under assortment of genes shall be as given in Table 5.1.
