Greek Genno, which means giving birth, genetics is a science that studies the genes. They are present on chromosomes and define the physical characteristics of a living being. It is through them that hereditary traits are passed from generation to generation. The study of genetics is one sure way that scientists have found for analyzing living beings and their functioning.

The gene begins with Mendel's work, identifies when, in 1865, the transmission of certain hereditary traits. His work is validated at the beginning of the twentieth century, after Walter Sutton discovered that chromosomes are the support of genes, and thus heredity, highlighted by several researchers. Meanwhile, many scientists have studied the origin of human physical characteristics and the system of cell division as the existence of DNA were discovered.

In practice, the genes are present on chromosomes, which number 22 in the human karyotype, which is added to the sex chromosomes X (female) and Y (male). The genes present in the nucleus are DNA sequences that encode information about the organization and operation. During fertilization, the chromosomes of male and female gametes combine to form a new karyotype. Thus, each gene is present in two copies. A gene may have several variations, called alleles. When two different alleles are present in a karyotype is the dominant version of the gene that expresses and defines a character. The version that is not expressed is called recessive then. Only genes present on the lower arm of chromosome X are not necessarily present in two copies, since they are absent from the Y chromosome

The embryonic development takes place through cell division: mitosis. In simplified terms, it occurs as a sequence of four stages: interphase (the genes are transcribed and the chromosomes are duplicated), prophase (the genetic material, DNA condenses to form chromosomes separate) metaphase (chromosomes come together) and anaphase (chromosomes separate to partition into two cells). Genetic information, or genome, is present in every cell division after the first egg-cell, established at fertilization. Thus the gametes, reproductive sex cells contain the genetic information of individuals who possess and transmit at fertilization.

Genetics has uncovered and prevent many inherited diseases. They are indeed very often caused by the expression of specific genes within a genome and can be detected or prevented. Thus, the blindness is due to the presence of a recessive allele, responsible for the disease, on the arm of the X chromosome missing the Y chromosome, which can not be canceled by a dominant allele, boys. Furthermore, in women, if both X chromosomes are carriers of the allele ill, blindness occurs. Genetic defects are the cause of diseases more or less severe and disabling, and can be detected from the first cell divisions of the fertilized cell. Thanks to the techniques of genetics that we can now know before birth whether a child will have a particular genetic disease. In some cases, doctors also suggest the therapeutic interruption of pregnancy, the fetus continues its development so that the mother did not give birth to a sick child. This practice is limited to some very serious diseases and is widely criticized.


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