Genetics

Spanish Version

    Since the dawn of civilization, humans have selected and crossed animals with good features, because they knew that the next generation would have the . But why does this happen? What are the mechanisms that enable paternal and maternal characteristics to be transmitted to the descendants?
    Some simple experiments in plant hybridization led to Gregor Johann Mendel, an Austrian monk, to enunciate theories proposing the existence of "hereditary particles" (or genes), which are responsible for the manifestation of a certain character. These theories, forgotten for decades, were rediscovered and are currently considered the cornerstone of genetics.

3.1.-genetics basics:

    -Allelic-genes: or alleles, are the pair of hereditary factors that come from each parent. Each gamete receives only one allele for each characteristic.

    -Dominance and recessive: the two different alleles for a character, which is expressed is called dominant allele, and that manifests itself only in the absence of the dominant is called recessive.
    -Pure and hybrid-Race: individuals homozygous or pure breeds have two identical alleles for a character. In genetics this is represented by two capital letters (AA, BB, CC ...) The heterozygous, or hybrid, have two different alleles for a character. This is represented by a capital letter and other letters (Aa, Bb, Cc ...)
    -Genotype and phenotype: genotype is the set of genes of an individual, received from the parents. The genotype is made up of visible characteristicss of an individual, controlled by genotype and influenced by the environment it inhabits.

3.2.-Mendel's laws:
    By scientific curiosity, Johann Mendel, Augustinian monk born in Austria, began to cross artificially pea plants homozygous for different physical characteristics in order to observe what was happening. When he was born the first generation of plants to study, which he called first filial generation (F1), scored their characteristics and allowed self-pollinating plant, obtaining the second filial generation (F2). Repeated self-pollination with the F2, obtaining the third filial generation (F3). Once all the results obtained, with the help of mathematics, Menel deduced the three laws that bear his name:

• Mendel's first law: says that by crossing two pure breeds with an opposite character, one of which is dominant, its progeny present this character.
         
  
• Mendel's second law: states that the recessive characteristics that didn't appear in the F1, do appear in the F2 in a ratio of 3:1 (three recessive per dominant)
         
  
• Mendel's third law:  each character is transmitted independently of the others.
  

    As scientists studied Mendel's theories, some exceptions were found that ,through a detailed study, allowed us to understand even better the mechanisms of biological inheritance. This is the case of intermediate inheritance, in which there is no dominance of characters, and the descendants have mixed characteristics of both parents.

3.3.-modern genetics:
3.3.1.-chromosome theory of heredity:
    Genetics evolved over the years, and in the twenties of last century, Thomas Hunt Morgan established the chromosome theory of inheritance, whose bases are:

• The genes are located on chromosomes. 
• The genes are arranged linearly.
• What is observed during the combination of genes in meiosis corresponds with the recombination of genes from a single chromosome.

3.3.2.-The inheritance of sex:
    In humans, the sex of the descendants depends on a pair of sex chromosomes represented by capital letters X and Y, which differ from the other pairs of homologous chromosomes, called autosomes. For this reason are also known by the name of heterochromosomes. Men present the combination XY and women, XX. During male meiosis, half of the gametes have one X chromosome and the other half, a Y chromosome, so the man is called heterogametic sex. In female meiosis, however, all the eggs have an X chromosome. For this, women are known as homogametic sex.

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