Understanding Genetics: From Genes to Chromosomal Alterations
Genes: Fragments of DNA containing information related to one characteristic.
Gene pairs: Two genes, each located on a different chromosome of a homologous pair, that contain information about the same characteristic.
Dominant genes: Genes that manifest their expression in a hybrid individual, preventing another gene, called recessive, from expressing different information about the same characteristic.
Codominant genes: Genes in hybrid individuals where both characteristics have the same strength, and both are expressed.
Genotype: The set of genes that an organism possesses.
Phenotype: The expression or set of characteristics of the genotype.
Homozygous individuals: Individuals who possess identical gene pairs for the same characteristic.
Heterozygous individuals: Individuals who possess different gene pairs for the same characteristic.
Mendel’s work and laws: Mendelian genetics is the part of genetics that studies the transmission of characteristics from one generation to another through mathematical projections.
First Law (Law of Uniformity): All individuals descending from the cross of two purebred lines are equal among themselves.
Second Law (Law of Segregation): If the hybrids of the first generation are crossed, the characteristics present in these segregate and combine randomly in the offspring.
Third Law (Law of Independent Assortment): Different characteristics are inherited independently of each other and combine randomly in the offspring.
Chromosomal abnormalities: A chromosomal change in number (more or less) or arrangement results in a phenotypic variation or interruption of the organism’s development. Mutations can occur in both somatic cells and germinal cells, with the latter having greater significance as they are heritable. If mutations affect somatic cells, they are extinguished with the individual, generally in the case of an asexual organism. Mutations can occur at a natural rate (spontaneous) or be induced.
Locus: Refers to the position or location of a gene in the genome.
Allele: An allele is a version of a gene that is present at a given locus.
Variation in chromosome number: These are alterations in the number of chromosomes. They can be euploidies or aneuploidies.
When an organism gains or loses one or more chromosomes, but not a complete set, it results in aneuploidy.
Monosomy: Loss of a single chromosome.
Trisomy: Gain of a single chromosome.
Euploidies: Modification of a complete set of chromosomes.
Polyploidy: Three or more complete sets of chromosomes. Polyploidy can be caused in two ways:
Autopolyploidy: Addition of one or more extra sets of chromosomes.
Allopolyploidy: Combination of chromosome sets from different species as a consequence of interspecific crosses.
Gene expression: Transcription
A gene is a sequence of DNA that encodes a sequence of RNA. This RNA, in general, determines the amino acid sequence of a polypeptide. RNA is usually an intermediary between DNA and protein, but it can be functional on its own.
Monohybrid Crossings: Monohybrid crossings occur when the pair of parent plants is purebred and differs only in one feature.
In humans, the number of chromosomes of an individual is 46, or 23 pairs, of which 22 are equal in men and women. In women, there is also another pair of fairly large X chromosomes, while in the male, one X chromosome is paired with a smaller one: the Y chromosome.
In the formation of female gametes by meiosis, all carry 22 autosomes and one X chromosome. The man produces sperm, also by meiosis, from their spermatogonia. In this case, two types of gametes can be formed, each with 22 chromosomes, but about 50% with one X chromosome and the other 50% with a Y chromosome.
This means that the sperm determines the sex of the offspring. As we have seen, there is equal probability for both sexes, but statistics indicate that, in general, 106 boys are born for every 100 girls.
DNA Mutations
DNA duplication is one of the most faithful processes of reproduction, allowing daughter cells to inherit the instructions perfectly from their predecessors.
Each cell also has DNA repair mechanisms designed to reduce as much as possible alterations in the nucleotides.
Even with all these protections, due to the large size of DNA molecules, in every cell, there usually occur at least two or three nucleotide changes on average per duplication. These changes in DNA are called mutations.
Mutations are the source of variability on which selection mechanisms act; they are therefore indispensable for the occurrence of evolution.
Mutagenic Agents
Mutations can arise spontaneously. There may also be mutations induced by various substances (mutagens). Examples: X-rays, ultraviolet rays, base analogs, modifiers of bases in DNA, intercalating substances, tars of snuff, dyes, asbestos, colchicine, benzene.
Types of Mutations
Point: These are lesions that affect only one gene.
Chromosome: Affecting the structure of chromosomes, they are caused by breaks or changes in the chromosome arms. The most common are: Deletions (breakage with the loss of a chromosome segment, e.g., Cri du chat syndrome, an abnormality on chromosome 5) and Duplications (duplication of a chromosome segment, can be beneficial).
Genomic: Affect the number of chromosomes. They are caused by errors in meiosis. Example: Trisomies: Down syndrome occurs when the individual has three copies of chromosome 21, i.e., they have an extra chromosome. Individuals affected by Down syndrome have a short and massive body, thick neck, large tongue, and often have abnormalities in the heart or other organs. Mental retardation ranges from mild to severe.
It has long been known that there is a relationship between the possibility of occurrence of the defect and the age of the mother.
Abnormalities in the Number of Sex Chromosomes:
44 autosomes + X (45 chromosomes in total): Turner Syndrome. Individuals are sterile with a female phenotype.
44 autosomes + XXY: Klinefelter syndrome. Its frequency increases with parental age. Individuals with this disorder have sexual immaturity, small testes, and infertility. About 25% show mental retardation.