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The transmission of information genetics through sexual reproduction

 The statistical laws of transmission of characters hereditary in diploids

Sexual reproduction is a biological phenomenon that ensures the
transmission of genetic information from one generation to another. The characters of the new individuals are different from those of their parents and their siblings, but they possess the same number of chromosomes. This observation can be explained by the succession of two phenomena:
-  Meiosis: it ensures the formation of gametes (haploid sex cells (n)).
-  Fertilization: it ensures the union of two parental gametes and the formation of an egg (cell diploid (2n)), which will give rise to a new individual.

-The genetic importance of meiosis and fertilization

- The genetic importance of meiosis

Meiosis is a cellular phenomenon that takes place, at the level of the genital organs, by the succession of two cell divisions (reductional and equational). From a cell diploid mother (2n) we obtain four haploid cells (n) which will turn into gametes.

- The stages of meiosis

- Reduction division: it is preceded by a phase of DNA replication (interphase)  during which the homologous chromosomes (tetrads) separate, and divide into two daughter cells with n chromosomes (haploid).

- Equational division: it takes place without DNA replication, during this division, the chromatids of each chromosome separate, and divide into four daughter cells haploids, which will be the source of gametes. Meiosis, essential for the formation of gametes, takes place at puberty in animals, and at maturity in plants. This phenomenon occurs in two cell divisions

cell division
image 1:cell division
The roles of meiosis

a- Reduction in the number of chromosomes and the amount of DNA (Reduction of genetic information)

Reduction of genetic information
image 2:  
Reduction of genetic information

Meiosis takes place by the succession of reductional division and equational division.
- Reduction division involves the reduction of the number of chromosomes by the separation of homologous chromosomes, without cracking of their centromeres, and without reduction in the amount of DNA during anaphase 1.
- Equational division results in the reduction of the amount of DNA from 2Q to Q by the separation of chromatids (daughter chromosomes), after cracking of the centromeres during anaphase 2.
b- Diversity of genetic information in gametes (chromosomal shuffling) Hereditary characteristics are controlled by genes (fragments of DNA) having precise loci and stable on homologous chromosomes determined in diploid (2n) individuals of the same
species. 
During the formation of gametes, the diversity of genetic information is ensured by a chromosomal shuffling, the demonstration of the latter can be demonstrated by the study of fewer than two characters, that is to say, two genes represented by two pairs of alleles.
These genes are either carried by the same pair of homologous chromosomes, they are two genes linked, or carried by two pairs of different homologous chromosomes, these are two genes independent. There are two types of chromosomal shuffling:
b-1 / Intra-chromosomal shuffling
In prophase 1 of reduction division, during the pairing of homologous chromosomes
(tetrads), there may be an exchange of the fragments of their chromatids through the formation of a chiasma (crossover), there is an exchange of paternal and maternal alleles. It is the crossing-over that results in the formation of parental-type (TP) gametes and recombinant-type (TR) gametes.
Crossing-over is a rare cellular phenomenon during which there is an exchange of fragments of chromatids of two homologous chromosomes, it results in the formation of four types of gametes of different proportions (%TP >> %TR). This phenomenon leads to a variety of genetic information in gametes and only occurs in the case of linked genes.
Intra-chromosomal shuffling
image 3: Intra-chromosomal shuffling

b-2 / Inter-chromosomal shuffling
At anaphase 1 of reductional division, homologous chromosomes separate in a random, a chromosome of a first pair can be associated indifferently with one or the other chromosomes of a second pair. Interchromosomal shuffling is a cellular phenomenon that is always carried out by a random distribution of the homologous chromosomes during anaphase 1, it leads to the formation of four types of gametes of the same proportions (50% TP =50%TR). This phenomenon leads to a diversity of genetic information in gametes, and it does not take place only in the case of independent genes.
Inter-chromosomal shuffling
image 4: Inter-chromosomal shuffling

the genetic importance of fertilization
Fertilization is a biological phenomenon that takes place by the union of a male gamete (n) with a female gamete (n) to give a diploid cell, called egg or zygote (2n), it carries paternal alleles and maternal alleles, and it will be at the origin of a new individual. This
phenomenon plays the following two roles:
- Recovery of the diploid phase in the chromosomal cycle of the living being (2n diploidy) by the union of the two haploid gametes (male and female).
- Deepening of the genetic diversity of living beings derived from eggs or zygotes by the inter-chromosomal mixing, which results from the random meeting of male and female gametes
(Random joining of paternal and maternal alleles).

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