Genetic Concepts

DNA

In all cell types, is seen the presence of deoxyribonucleic acid (DNA), and in the case of eukaryotic cells, its presence can be evidenced within the nucleus and in organelles such as mitochondria and chloroplasts.

As a result of the interaction between the nucleotides, these DNA molecules are composed of a double-strand that has a helix shape. The number of molecules in the cell nucleus is different for each species. Besides the phosphate and a pentose grouping, the nucleotides that form DNA also have four varieties of nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). The sequence of nucleotides is what enables DNA to store information.

Inside the nucleus, the DNA molecules take the organized form of chromatin. And at specific periods in the cell cycle, they duplicate and assume the even more compact shape known as the chromosome.

In animal somatic cells, chromosomes can be grouped in pairs due to similar morphological characteristics and are, in this case, known as homologous chromosomes. In addition to these similarities, homologous chromosomes also have the same genes occupying the same specific regions, known as gene locus.

Each gene can have variations, called alleles, which can originate mutations. For example, the gene that determines the coat color of rabbits can manifest yellow or black.

Meiosis 

During the formation of gametes by meiosis, homologous chromosomes are paired (brought close together), making crossing-over, or permutation, possible. Later on, the homologous chromosomes migrate to opposite poles of the cell and, after cytokinesis, there is a reduction of the ploidy of the cell, characterizing meiosis.

In this process, the phenomenon of crossing-over and the separation of homologous chromosomes occurs randomly, creating numerous combinations of alleles in the gametes.

Phenotype and genotype

A set of structural or functional characteristics is called the phenotype. An example of a phenotypic characteristic is eye color. The determination of these characters depends, in turn, on the individual's set of genes, called genotype.

The determination of the phenotype is not given only by the action of the genotype. It also depends on the interaction between the genotype and the environment. And on molecular phenomena. These molecular phenomena may be related to the production of proteins by a gene or the interaction between two alleles of the same locus. Thus, the same genotype can be responsible for a diverse set of phenotypes when in different environments.

Some influences of the environment on genotype expression are known as reaction norms, which can be exemplified by what happens with the color of rabbit fur concerning temperature.

When the environment causes changes in the phenotype of an individual in such a way as to copy a characteristic that is usually genetically determined in other individuals, the copied phenotype is classified as phenocopy.

Regarding the determination of the genotype, it's necessary to know that a diploid individual presents two alleles of a gene: one coming from the mother and the other from the father. These alleles may or may not be identical, determining homozygosity or heterozygosity. Homozygous individuals are also called pure, while heterozygotes are known as hybrids. 

Dominance and recessiveness

In some cases, the allele only manifests itself when it is homozygous, that is, two copies of the allele are necessary. In this case, the allele is identified as recessive. When an allele manifests in both homozygosity and heterozygosity, it is known as the dominant allele.

The dominant or recessive allele can be represented in many ways. The most common is by the use of upper and lower case letters for the designation of dominant and recessive alleles, respectively.

Types of inheritance: autosomal or sex-linked

Chromosomes can be classified as autosomal or sex-linked, the last being responsible for determining the biological sex of the individual. 

The characteristics determined by genes arranged on autosomal chromosomes respect the inheritance known as autosomal. And are unrelated to the sex of the individual.

Inheritance of characteristics determined by genes found on sex chromosomes (X or Y) is called sex-linked inheritance.