G. Mendel in the genetic experiments used a hybridological method. He crossed the peas plants differing in one or several signs. Then the scientist analyzed the nature of manifestation of signs at posterity.
Instruction
1. Clean lines are the grades plants having some constant sign, for example, yellow or green color of seeds. Monohybrid crossing – crossing of two clean lines of the plants differing only in one sign. At dihybrid crossing, individuals at whom consider differences on two signs undertake.
2. Let, for example, there is a clean line of peas with yellow smooth seeds, and the line with green and wrinkled. Signs are defined by couples of genes, at the same time one couple of genes codes color of seeds, another – their form. Yellow coloring and a smooth form – prepotent genes, green coloring and rugosity of seeds – recessive.
3. In the first generation all seeds of peas will be yellow and smooth, under the law of uniformity of hybrids of the first generation. Here the phenomenon of complete dominance is observed: only prepotent genes are shown, and recessive are suppressed.
4. For the further solution of a task on dihybrid crossing it is necessary to fill Pennet's lattice. Plants of the first generation of F1, merging among themselves, will give four types of gametes: AB, Ab, aB and ab. Draw a framework of the rectangular table dimension four on four. From above over columns designate gametes. Similarly paint gametes to the left of lines. It reminds a game in sea battle.
5. All possible combinations of these four types of gametes will give in the second generation of 9 various genotypes: AABB, AaBB, AABb, AaBb, aaBB, AAbb, aaBb, Aabb, aabb. But only four phenotypes will be observed: yellow – smooth, yellow – wrinkled, green – smooth, green – wrinkled. Ratio of observed phenotypes 9:3:3:1.
6. If separately to consider proportions between yellow and green peas, they will make 3:1, as well as in a case with monohybrid crossing. The same treats also smoothness or rugosity of seeds.
7. So, the rule of splitting is carried out for mono - and dihybrid crossing is identical. Therefore it is possible to draw a conclusion that genes and signs coded by them at dihybrid crossing are inherited independently of each other. The law of independent inheritance of signs is fair only when genes are located in different nonhomologous chromosomes.