OCR GCSE Combined Science A Gateway revision notes
Genes, inheritance and selection Revision Notes
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Genes, inheritance and selection
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Understanding the Genome and its Importance Revision Notes
What is the Genome?
The genome is defined as the entire genetic material of an organism. This includes all of the DNA (or RNA in some viruses) packaged within its cells. It contains all the instructions needed for an organism to develop, survive, and reproduce. Each species has a characteristic genome, which is passed down from one generation to the next.
Components of the Genome
In most organisms, especially eukaryotes (like humans, animals, and plants), the genetic material is deoxyribonucleic acid (DNA). This DNA is meticulously organized into structures called chromosomes, located within the nucleus of cells. A chromosome is essentially a highly condensed strand of DNA, containing many genes. Genes are specific sequences of DNA that carry the instructions for making proteins, which are essential for virtually all cellular functions.
The Role of Genes
Genes are the functional units of heredity within the genome. Each gene provides the blueprint for a particular protein or a functional RNA molecule. These proteins perform a vast array of tasks, such as building cellular structures, catalyzing biochemical reactions, transporting molecules, and signaling between cells. The combination and expression of these genes determine an organism's traits, from eye colour to susceptibility to certain diseases.
Genome Size and Complexity
The size and complexity of a genome can vary significantly between different organisms. For instance, bacterial genomes are generally much smaller and simpler, often consisting of a single circular chromosome, compared to the human genome, which is vast and organized into 23 pairs of linear chromosomes. However, a larger genome size does not always correlate directly with the complexity of the organism. Understanding a species' genome provides valuable information about its biology and evolutionary relationships.
Understanding Evolution and Natural Selection Revision Notes
Definition of Evolution
Evolution is defined as the change in the inherited characteristics of a population over time. This means that the genetic traits passed down from parents to offspring can shift in frequency within a group of organisms across many generations. It is a slow, gradual process, not an individual organism changing during its lifetime. These changes accumulate over vast periods and are responsible for the vast diversity of life on Earth.
The Process of Natural Selection
Natural selection is the main driving force behind evolution. It operates on variations within a population. Individuals possess different characteristics due to genetic variations. Some of these variations provide an advantage in a particular environment, making individuals better adapted. For example, a faster predator or a plant resistant to disease. These individuals are more likely to survive, reproduce, and pass their advantageous traits to their offspring. Over generations, the frequency of these favourable inherited characteristics increases in the population.
Survival of the Fittest and Reproduction
The concept often referred to as 'survival of the fittest' describes how individuals with characteristics best suited to their environment are more likely to survive and successfully reproduce. It's not just about surviving, but also about passing on genes. Those who reproduce more successfully contribute more of their genes to the next generation, thus increasing the prevalence of their advantageous traits in the population's gene pool. This differential reproductive success is key to natural selection.
Formation of New Species (Speciation)
Over very long periods, continuous natural selection, particularly when populations become geographically or reproductively isolated, can lead to such significant changes in inherited characteristics that new species form. This process is called speciation. When two populations diverge so much that they can no longer interbreed to produce fertile offspring, they are considered distinct species. This represents the ultimate outcome of evolutionary change driven by natural selection.
Understanding Genetic Terminology Revision Notes
Basic Units of Inheritance
Genetic information is carried on chromosomes, which are thread-like structures found in the nucleus of most living cells. A gene is a segment of DNA on a chromosome that codes for a specific protein or characteristic. Each gene can exist in different forms called alleles or variants, which lead to different expressions of the same trait (e.g., an allele for blue eyes versus an allele for brown eyes). Gametes are specialised reproductive cells (sperm and egg) that carry only one allele for each gene, ensuring genetic diversity and the correct chromosome number upon fertilisation.
Genotype and Phenotype
An organism's genotype refers to the specific combination of alleles it possesses for a particular gene. For instance, if 'B' represents the allele for brown eyes and 'b' for blue eyes, a genotype could be 'BB', 'Bb', or 'bb'. The phenotype is the observable characteristic or trait that results from this genotype and environmental influences. So, an individual with 'BB' or 'Bb' genotype might have a brown eye phenotype, while 'bb' would result in a blue eye phenotype. The phenotype is what we can see or measure.
Dominance and Recessiveness
Alleles interact in different ways to determine the phenotype. A dominant allele is one that will always be expressed in the phenotype if at least one copy is present in the genotype. It masks the effect of a recessive allele. A recessive allele will only be expressed in the phenotype if two copies are present in the genotype (i.e., the individual is homozygous recessive). If a dominant allele is also present, the recessive allele's effect is not seen. For example, if 'R' is dominant for round seeds and 'r' for wrinkled seeds, an 'Rr' genotype will show the 'round' phenotype.
Homozygous and Heterozygous
These terms describe the combination of alleles an individual has for a specific gene. An individual is homozygous if they have two identical alleles for a gene (e.g., 'BB' or 'bb'). They are homozygous dominant if both alleles are dominant, or homozygous recessive if both alleles are recessive. An individual is heterozygous if they have two different alleles for a gene (e.g., 'Bb'). In a heterozygous individual, the dominant allele will determine the phenotype.
Impact of Biological Developments on Classification Revision Notes
Early Classification: Morphology
Historically, classification systems, like Linnaeus's, were based on observable physical features (morphology). Organisms were grouped by similarities in appearance, such as the number of limbs, body shape, or reproductive structures. This method was practical for cataloguing species but could sometimes misrepresent true biological relationships because appearance can be misleading. Convergent evolution, where unrelated species develop similar traits due to similar environments, is an example of why solely morphological classification can be insufficient.
Modern Classification: Molecular Biology
Advances in molecular biology have transformed classification. Techniques such as DNA sequencing allow scientists to compare the genetic material of different species directly. The more similar the DNA sequences, the more closely related the organisms are thought to be, reflecting shared evolutionary history. This shift led to phylogenetic classification, which focuses on evolutionary relationships and common ancestry, providing a more accurate 'tree of life'.
Biochemical Evidence
Beyond DNA, biochemical analyses also play a crucial role. Comparing proteins, enzymes, or metabolic pathways between species can reveal deep evolutionary connections. For example, similar protein structures in different species suggest a common genetic origin. This biochemical evidence complements genetic data to refine our understanding of how species are related.
Impact on Classification Systems
The integration of genetic and biochemical evidence has led to significant revisions in classification. For instance, the five-kingdom system gave way to the three-domain system (Bacteria, Archaea, Eukaryota) based on fundamental genetic differences, particularly in ribosomal RNA. This demonstrates how new biological discoveries continuously reshape our understanding and classification of biodiversity, moving from superficial similarities to deep evolutionary relatedness.
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