Throughout the long history of science, ideas about heredity and variability have changed. Back in the time of Hippocrates and Aristotle, people tried to carry out breeding, trying to bring out new types of animals, plant varieties.
When carrying out such work, a person learned to rely on the biological laws of inheritance, but only intuitively. And only Mendel managed to derive the laws of inheritance of various traits, identifying dominant and recessive traits using the example of peas. Today, scientists around the world use his work to obtain new varieties of plants and animal species, most often the third law of Mendel is used - dihybrid crossing.
Crossing features
Dihybrid is the principle of crossing two organisms that differ in two pairs of properties. For dihybrid crossing, the scientist used homozygous plants, different in color and shape - they were yellow and green,wrinkled and smooth.
According to Mendel's third law, organisms differ from each other in various ways. Having established how traits are inherited in one pair, Mendel began studying the inheritance of two or more pairs of genes responsible for certain properties.
Crossing principle
During the experiments, the scientist found that the yellowish color and smooth surface are dominant features, while the green color and wrinkling are recessive. When peas with yellowish and smooth seeds are crossed with plants that have green wrinkled fruits, the F1 hybrid generation is obtained, which is yellow and has a smooth surface. After self-pollination of F1, F2 were obtained, moreover:
- Out of sixteen plants, nine had smooth yellow seeds.
- The three plants were yellow and wrinkled.
- Three - green and smooth.
- One plant was green and wrinkled.
During this process, the law of independent inheritance was derived.
Experimental result
Before the discovery of the third law, Mendel established that with monohybrid crossing of parent organisms that differ in one pair of traits, two types can be obtained in the second generation in a ratio of 3 and 1. When crossing, when a pair with two pairs of different properties is used, in the second generation produces four species, and three of them are the same, and one is different. If you continue to cross phenotypes, then the next cross will be eightinstances of varieties with a ratio of 3 and 1, and so on.
Genotypes
Deriving the third law, Mendel discovered four phenotypes in peas, hiding nine different genes. All of them received certain designations.
The splitting by genotype in F2 with monohybrid crossing occurred according to the principle 1:2:1, in other words, there were three different genotypes, and with dihybrid crossing - nine genotypes, and with trihybrid crossing, offspring with 27 different types of genotypes are formed.
After the study, the scientist formulated the law of independent inheritance of genes.
Law wording
Long experiments allowed the scientist to make a grandiose discovery. The study of the heredity of peas made it possible to create the following formulation of Mendel's third law: when crossing a pair of individuals of a heterozygous type that differ from each other in two or more pairs of alternative properties, genes and other traits are inherited independently of each other in a ratio of 3 to 1 and are combined in all possible variations.
Fundamentals of Cytology
Mendel's third law applies when genes are located on different pairs of homologous chromosomes. Suppose A is a gene for yellowish seed color, a is a green color, B is a smooth fruit, c is wrinkled. When crossing the first generation of AABB and aavv, plants with the genotype AaBv and AaBv are obtained. This type of hybrid has received the mark F1.
When gametes are formed from each pair of genes, an allele falls into itonly one, in this case it can happen that together with A the gamete B or c gets, and the gene a can connect with B or c. As a result, only four types of gametes are obtained in equal quantities: AB, Av, av, aB. When analyzing the results of crossing, it can be seen that four groups were obtained. So, when crossing, each pair of properties during decay will not depend on the other pair, as in monohybrid crossing.
Features of problem solving
When solving problems, you should not only know how to formulate Mendel's third law, but also remember:
- Correctly identify all gametes that form parent instances. This is possible only if the purity of gametes is understood: how the type of parents contains two pairs of allele genes, one for each trait.
- Heterozygotes constantly form an even number of gamete varieties equal to 2n, where n are hetero-pairs of allelic gene types.
Understanding how problems are solved is easier with an example. This will help you quickly master the principle of crossing according to the third law.
Task
Let's say that a cat has a black shade that dominates white, and short hair over long. What is the probability of the birth of short-haired black kittens in individuals who are diheterozygous for the indicated traits?
The task condition will look like this:
A - black wool;
a - white wool;
v - long hair;
B - short coat.
As a result we get: w - AaBv, m - AaBv.
It remains only to solve the problem in a simple way, separating all the propertiesinto four groups. The result is the following: AB + AB \u003d AABB, etc.
During the decision, it is taken into account that gene A or a of one cat is always connected with gene A or a of another, and gene B or B only with gene B or in another animal.
It remains only to evaluate the result and you can find out how many and what kind of kittens will result from dihybrid crossing.
