Genes, inheritance and selection Revision Notes

Genetic Variants: Definition and Impact

Genetic variants are differences in the DNA sequence within a population. These variations can range from a single nucleotide change to larger insertions, deletions, or rearrangements. Not all genetic variants have an observable effect on an organism's characteristics (phenotype). However, those that do can significantly influence traits, disease susceptibility, and other biological processes. The location of the variant within the DNA is crucial in determining its potential impact on phenotype.

Variants in Coding DNA (Higher)

Coding DNA regions are sequences that provide the blueprint for constructing proteins. When a genetic variant occurs within coding DNA, it can directly alter the instructions for protein synthesis. This may lead to changes in the amino acid sequence of the protein. Such changes can modify the protein's three-dimensional structure, which, in turn, can affect its function. For example, a variant might cause a protein to lose its ability to bind to other molecules, reduce its enzymatic activity, or even render it completely non-functional. The consequence is an altered protein activity, which then manifests as a change in the organism's observable phenotype.

Variants in Non-Coding DNA (Higher)

Non-coding DNA regions do not code for proteins directly, but they are vital for regulating gene expression. These regions include regulatory sequences like promoters, enhancers, and silencers, which control when, where, and how much of a gene is transcribed into mRNA. A genetic variant in a non-coding region can alter the binding sites for regulatory proteins or change the structural properties of DNA that affect gene accessibility. This can lead to increased or decreased expression of a gene. For instance, a variant might cause a gene to be expressed at the wrong time, in the wrong tissue, or at an inappropriate level. By altering the quantity or timing of protein production, variants in non-coding DNA can indirectly influence protein activity and consequently impact the organism's phenotype, even though the protein's amino acid sequence itself might remain unchanged.

The Fossil Record

Fossils are the preserved remains or traces of organisms from the distant past. They are typically found in sedimentary rock layers. The fossil record provides a visual timeline of life on Earth. Older rock layers contain simpler organisms, while more recent layers show a progression to more complex forms. This demonstrates that life has changed over geological time. Crucially, fossils also show transitional forms, which are species that have characteristics of two different groups. For example, fossils like Archaeopteryx show features of both reptiles and birds, providing strong evidence for evolutionary links between these groups. The discovery of new fossils continuously adds to our understanding of evolutionary pathways and relationships between species.

Comparative Anatomy

Comparative anatomy involves comparing the body structures of different species. One significant piece of evidence comes from homologous structures. These are structures that have a similar underlying anatomical plan but are adapted for different functions in different species. A classic example is the pentadactyl limb (five-fingered limb) found in all mammals, birds, reptiles, and amphibians. Despite being used for walking in a dog, flying in a bat, swimming in a whale, and grasping in a human, the basic bone arrangement is remarkably similar. This shared underlying structure strongly suggests that these species share a common ancestor from which this limb plan evolved. The modifications arose through adaptation to different environments and lifestyles.

Molecular Evidence (Biochemistry and Genetics)

Advances in molecular biology provide powerful evidence for evolution. This includes comparing the DNA and protein sequences of different organisms. All living organisms share a common genetic code, which points to a common origin of life. The degree of similarity in DNA sequences or amino acid sequences of proteins directly reflects how closely related two species are. For instance, humans and chimpanzees have very similar DNA sequences, indicating a recent common ancestor. More distantly related species will show greater differences in their molecular makeup. This molecular clock concept allows scientists to estimate when different species diverged from a common ancestor. The more differences there are, the longer ago they diverged.

Early Ideas of Inheritance

Before the late 19th century, the prevailing idea was that characteristics were inherited through a 'blending' process, where offspring would show an intermediate mix of their parents' traits. This would mean that variation would be lost over generations, which was a major problem for understanding evolution. There was no scientific understanding of discrete units of inheritance.

Gregor Mendel's Groundbreaking Work

Gregor Mendel, an Austrian monk, conducted extensive experiments with pea plants in the mid-19th century. He carefully crossed plants with different distinct characteristics and observed the patterns of inheritance across multiple generations. He correctly deduced that characteristics are passed on as discrete 'factors' (now called genes), which come in dominant and recessive forms. His work was revolutionary but was largely ignored for many years due to a lack of understanding and appreciation by the scientific community of his time.

Rediscovery and the Birth of Modern Genetics

Around the turn of the 20th century, several scientists independently rediscovered Mendel's work, recognising its profound significance. This led to a rapid growth in the field of genetics. Thomas Hunt Morgan's work with fruit flies further confirmed the chromosomal theory of inheritance, placing genes on specific locations on chromosomes. This era marked the beginning of modern genetics.

The Discovery of DNA Structure

A pivotal moment in genetics was the discovery of the structure of DNA (deoxyribonucleic acid) in 1953 by James Watson and Francis Crick, based on crucial X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins. They proposed the double helix model, which explained how genetic information could be stored, copied, and passed from one generation to the next. This provided a physical basis for Mendel's abstract 'factors' and explained the chemical nature of genes. Since then, the Human Genome Project and advanced gene technologies have further expanded our knowledge.

Asexual Reproduction: Definition and Advantages

Asexual reproduction involves a single parent producing offspring that are genetically identical to itself. This process does not involve gametes or the fusion of genetic material from two parents. Key advantages include rapid reproduction, which allows for fast colonisation of new habitats or quick recovery after population decline. It is also very energy-efficient as there is no need to find a mate, court, or invest energy in complex reproductive structures. Furthermore, all beneficial traits of the parent are passed directly to all offspring without dilution.

Asexual Reproduction: Disadvantages

The main disadvantage of asexual reproduction is the lack of genetic variation among offspring. Since all offspring are clones of the parent, they share the same susceptibilities. If environmental conditions change, such as the introduction of a new predator, disease, or a shift in climate, the entire population could be vulnerable and potentially wiped out because no individuals possess unique traits that might help them survive. This makes adaptation to changing environments very difficult for asexually reproducing organisms.

Sexual Reproduction: Definition and Advantages

Sexual reproduction typically involves two parents, each contributing half of the genetic material to form offspring. This usually occurs through the fusion of male and female gametes (sperm and egg). The most significant advantage is the creation of genetic variation within the offspring population. This genetic diversity increases the chances of some individuals possessing advantageous traits that allow them to survive and reproduce in changing environments, such as new diseases or shifts in resource availability, promoting adaptation and evolution of the species over time.

Sexual Reproduction: Disadvantages

Sexual reproduction generally requires more time and energy compared to asexual reproduction. Organisms must expend energy to find a mate, engage in courtship rituals, and then invest in the reproductive process itself, which can be prolonged. There is also a risk associated with finding a mate, such as exposure to predators or competition. Additionally, only half of an individual's genes are passed on to each offspring, meaning a successful parent's specific combination of advantageous genes might not be fully conserved or passed on effectively to the next generation.