Key Genetic Principles: Heredity, Variation, and Technology
Mendel’s Laws of Inheritance
Gregor Mendel established fundamental principles of heredity, known as Mendel’s Laws:
- Law of Uniformity: When crossing two pure breeds (homozygous individuals with different alleles for a trait), all first filial (F1) generation descendants are genetically uniform and phenotypically identical, expressing the dominant trait if one exists.
- Law of Segregation: The two hereditary factors (alleles) for each character do not fuse but remain differentiated throughout an individual’s life. They segregate (separate) during gamete formation, so each gamete carries only one allele for each trait.
- Law of Independent Assortment: Hereditary factors (genes) for different traits assort independently of one another during gamete formation, leading to random combinations in the descendants. This applies when genes for different traits are located on different chromosomes or are far apart on the same chromosome.
Genetic Linkage
Linkage refers to the tendency of genes located close together on the same chromosome to be inherited together during meiosis.
Daltonism (Color Blindness)
Daltonism, or color blindness, is the inability to distinguish certain colors, commonly red from green. It is often an X-linked recessive condition. Different forms are governed by recessive genes affecting color vision: one gene variant can result in blindness to red, another can cause blindness to green, and others can affect perception of other colors or combinations.
Hemophilia
Hemophilia is an inherited disease characterized by the failure of blood to coagulate properly. It is typically due to recessive gene variants located on the differential segment of the X chromosome, making it an X-linked disorder.
The Genetic Code Explained
The genetic code is like a biological dictionary that establishes an equivalence between the language written in sequences of 4 DNA bases (nucleotides: Adenine, Thymine, Cytosine, Guanine) and the language of proteins, which are constructed from 20 common amino acids. The genetic code is nearly universal, meaning it is used by almost all known organisms and cells.
Understanding Genetic Mutations
Mutations are random, heritable alterations in the genetic material (DNA or RNA). While they can sometimes be neutral or beneficial, they often result in deficiencies and can even be lethal. Mutations are frequently recessive and may remain permanently hidden within a population. Despite potentially being negative for an individual, mutations are crucial for evolution as they provide the genetic variability upon which natural selection acts. This variability allows populations to adapt; if a significant environmental change occurs and conditions become adverse, genetic diversity increases the chance that some individuals will possess traits allowing the species to survive. This process is a cornerstone of natural selection.
Gene Mutations (Point Mutations)
Gene mutations, also known as point mutations, are alterations in the nucleotide sequence of a single gene. These can involve substitutions, insertions, or deletions of one or a few nucleotides.
Chromosomal Structural Mutations
Chromosomal mutations involve changes in the structure of one or more chromosomes. These include:
- Deletion: The loss of a chromosome fragment. If this fragment contains many genes, the deletion can be lethal or cause pathological conditions.
- Duplication: The repetition of a chromosome segment. The duplicated segment can be located on the same chromosome or, rarely, as a separate fragment.
- Inversion: A change in the orientation of a chromosome fragment within the chromosome. If the inverted segment includes the centromere, it is denominated a pericentric inversion; if it does not include the centromere, it is a paracentric inversion.
Genomic Mutations (Numerical Aberrations)
Genomic mutations, also known as numerical chromosomal aberrations, alter the total number of chromosomes characteristic of a species. These include:
- Aneuploidy: An alteration in the normal number of chromosomes, where there is an addition or loss of one or more individual chromosomes, without affecting the entire haploid set (e.g., having 45 or 47 chromosomes in humans instead of 46). This can lead to complex syndromes, for example, Trisomy 21 (Down syndrome), where there are three copies of chromosome 21.
- Euploidy: An alteration in the normal number of complete haploid sets of chromosomes.
- Monoploidy: The state of having only one set of chromosomes in an organism that is normally diploid.
- Polyploidy: The state of having more than two complete sets of chromosomes (e.g., triploidy with three sets, tetraploidy with four sets).
Principles of Genetic Engineering
Genetic engineering encompasses a set of techniques that involve the direct manipulation, modification, or introduction of genes within the genome of an organism. This is often carried out using molecular tools such as restriction enzymes, which cut DNA at specific, well-known recognition sequences or points of interest, and ligases, which join DNA fragments.
Recombinant DNA Technology
Recombinant DNA is a molecule of DNA that has been artificially created by combining genetic material from different sources. It is formed when a sequence of foreign DNA is integrated (intercalated) into a recipient DNA molecule (often a plasmid or viral vector), creating a new combination of genetic material.
Cloning Processes
Cloning, in a genetic context, can be defined as the use of methods to produce genetically identical copies (clones) of a DNA segment, cell, or entire organism. Organismal cloning can involve the use of pluripotent cells (like embryonic stem cells) or somatic cell nuclear transfer, where the nucleus of a somatic cell is transferred to an enucleated egg cell, which is then stimulated to develop, forming a new individual genetically identical to the donor of the somatic cell nucleus.
Microorganisms and Disease Spread
Microorganisms (often referred to colloquially as germs) are living beings of microscopic size, including bacteria, viruses, fungi, and protists. To observe them individually, a microscope must typically be used.
Epidemics
An epidemic occurs when a disease affects a significantly larger number of individuals within a population, community, or region at approximately the same time than is normally expected.
Pandemics
A pandemic is an epidemic of an infectious disease that has spread across a very extensive geographical area, often affecting multiple continents or even worldwide, impacting a large portion of the global population.
Endemic Diseases
An infectious disease is considered endemic when it is constantly present at a baseline or expected level within a specific geographic area or population group.