Over the long history of science, ideas about heredity and variability have changed. Back in the days of Hippocrates and Aristotle, people tried to carry out selection, trying to develop 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 the dominant and recessive traits on the example of peas. Today, scientists all over the world use his works to obtain new varieties of plants and animal species, most often the third Mendel law is applied - dihybrid crosses.
Crossbreeding Features
The principle of crossing two organisms, which differ in two pairs of properties, is called dihybrid. For crossbreeding, the scientist used homozygous plants, different in color and shape - they were yellow and green, wrinkled and smooth.
According to the third law of Mendel, organisms differ with 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.
Crossbreeding principle
During the experiments, the scientist revealed that a yellowish color and a smooth surface are dominant signs, and green color and wrinkling are recessive. When crossing peas with yellowish and smooth seeds with plants that have green wrinkled fruits, the hybrid generation F1 is obtained, which has a yellow color and a smooth surface. After self-pollination of F1, F2 were obtained, moreover:
- Of the sixteen plants, nine had smooth yellow hue seeds.
- Three plants were yellow and wrinkled.
- Three are green and smooth.
- One plant was green and wrinkled.
During this process, the law of independent inheritance was deduced.
Experiment result
Before the discovery of the third law, Mendel established that when monohybrid breeding of parental organisms that differ in one pair of traits, two types can be obtained in the second generation in the ratio of 3 and 1. When crossing, when a pair with two pairs of different properties is used, the second generation produces four types , and three of them are the same, and one is the other. If we continue crossing the phenotypes, then the next crossing will result in eight specimens 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 according to the genotype in F2 during monohybrid crossing occurred according to the 1: 2: 1 principle, in other words, there were three different genotypes, and when digibrid crossing, nine genotypes, and when trihybrid crossing, offspring with 27 different types of genotypes is formed.
After conducting the study, the scientist formulated the law of independent gene inheritance.
The wording of the law
Long experiments allowed the scientist to make a grand discovery. A study of the heredity of peas made it possible to create the following formulation of Mendel’s third law: when two pairs of heterozygous individuals are crossed that differ in two or more pairs of alternative properties, genes and other characters are inherited independently from each other in a ratio of 3 to 1 and combined all possible variations.
The basics of cytology
Mendel’s third law is applicable when genes are located in different pairs of homologous chromosomes. Suppose A is a gene of yellowish coloring of seeds, and is a green color, B is a smooth fruit, and B is wrinkled. When crossing the first generation of AAVB and aavv, plants with the genotypes AaBb and AaBb are obtained. This type of hybrid got the F1 mark.
When gametes are formed from each pair of allele genes, only one gets into it, and it can happen that together with A, gamete B or B gets in, and gene a can connect to B or B. The result is only four types of gametes in equal amounts: AB, Av, Av, Av. When analyzing the results of the cross, 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 the case of monohybrid crosses.
Features of solving problems
When solving problems, you should not only know how to formulate the third law of Mendel, but also remember:
- Correctly identify all the gametes that form the parental instances. This is possible only with an understanding of the purity of gametes: 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 types of genes.
Understanding how problems are solved is easier with an example. This will help to quickly master the principle of crossing according to the third law.
Task
Suppose a cat has a black tint over white and a short coat over a long one. What is the likelihood of giving birth to short-haired black kittens in individuals of digeterozygous according to the indicated signs?
The condition of the task will look like this:
A - black wool;
a - white wool;
in - long hair;
B - short hair.
As a result, we get: w - AaBb, m - AaBb.
It remains only to solve the problem in a simple way, dividing all the properties into four groups. The result is the following: AB + AB = AABB, etc.
During the decision, it is taken into account that the gene A or a of one cat always connects with the gene A or a of the other, and gene B or c only with gene B or another animal.
It remains only to evaluate the result and you can find out how many and what kind of kittens will be obtained with dihybrid crosses.