Genes, inheritance and selection Revision Notes

The Role of Chromosomes in Sex Determination

Humans have 23 pairs of chromosomes. One pair, called the sex chromosomes, determines an individual's sex. Females possess two X chromosomes, represented as XX. Males possess one X chromosome and one Y chromosome, represented as XY. The remaining 22 pairs of chromosomes are called autosomes and carry genes for other traits. This chromosomal difference is fundamental to understanding how sex is inherited.

Gametes and Sex Chromosome Contribution

During the formation of gametes (sperm in males, eggs in females), only one chromosome from each pair is passed on. Females produce egg cells, and every egg cell contains a single X chromosome. Males produce sperm cells, and approximately half of their sperm cells contain an X chromosome, while the other half contain a Y chromosome. This difference in male gametes is crucial for sex determination.

Genetic Crosses and Probability

A genetic cross, often depicted with a Punnett square, shows the possible combinations of sex chromosomes in offspring. When an egg (X) is fertilised by a sperm carrying an X chromosome, the offspring will be XX (female). When an egg (X) is fertilised by a sperm carrying a Y chromosome, the offspring will be XY (male). This results in an approximately 50% chance of a male and a 50% chance of a female offspring for each conception. The male's sperm ultimately determines the sex of the child.

Fossil Record

Fossils are the preserved remains or traces of organisms from the past. The fossil record provides a chronological sequence of life forms, showing how organisms have changed over millions of years. Older rock layers typically contain simpler fossilised organisms, while newer layers show more complex and diverse forms. This demonstrates a progression of life from early, less complex forms to more complex, later forms. Transitional fossils exhibit characteristics of both an ancestral group and a descendant group, acting as crucial links between different species and illustrating evolutionary pathways. For example, fossils like Archaeopteryx show features of both reptiles and birds.

Comparative Anatomy

Comparative anatomy involves studying the structural similarities and differences between species. Homologous structures are body parts in different species that have a similar basic structure but may have evolved to perform different functions. A prime example is the pentadactyl limb found in vertebrates (like humans, bats, whales, and birds). Despite their varied uses (grasping, flying, swimming), the underlying skeletal arrangement of these limbs is strikingly similar, suggesting a common evolutionary origin from an ancestor possessing this basic limb structure. This pattern supports the idea of divergent evolution from a common ancestor.

Molecular Evidence (DNA and Proteins)

Advances in molecular biology provide compelling evidence for evolution by comparing the genetic material (DNA) and protein sequences of different species. The more similar the DNA sequences or amino acid sequences of proteins between two species, the more closely related they are evolutionarily. This similarity indicates that they shared a common ancestor more recently. For instance, comparing the haemoglobin protein or specific gene sequences across various organisms reveals evolutionary trees that often align with evidence from fossils and comparative anatomy, reinforcing the concept of common descent. Molecular clocks can even estimate divergence times.

Observable Evolution (Antibiotic and Pesticide Resistance)

Contemporary examples of evolution in action include the development of pesticide resistance in insects and antibiotic resistance in bacteria. When populations of bacteria are exposed to antibiotics, individuals with genetic mutations that confer resistance are more likely to survive and reproduce. Over time, the proportion of resistant bacteria in the population increases, making the antibiotic less effective. This is a direct observation of natural selection leading to evolutionary change within a relatively short period. The same principle applies to insects developing resistance to pesticides, demonstrating ongoing evolutionary processes.

Variation Within a Population

Within any population of organisms, individuals show natural variation in their characteristics (phenotypes). This variation can be in features like size, colour, resistance to disease, or behaviour. The initial source of this variation is primarily random genetic mutations, which create new alleles. Sexual reproduction also shuffles existing alleles, contributing to diversity. Without this inherent variation, natural selection would not be able to act, as all individuals would be identical.

The Role of Environmental Pressures

Organisms often produce more offspring than can possibly survive, leading to competition for limited resources such as food, water, light, and space. There are also environmental challenges like predators, diseases, and extreme weather conditions. These factors create 'selection pressures' that determine which individuals are most likely to survive and reproduce successfully. Individuals with traits that give them an advantage in these conditions are 'selected for'.

Survival and Reproduction of the Fittest

Individuals that possess advantageous characteristics, making them better suited to their environment, are more likely to survive the selection pressures. For instance, a faster predator or a plant that tolerates drought better. These 'fitter' individuals have a higher chance of reaching reproductive age and producing offspring. They then pass on their beneficial genes, including those responsible for their advantageous phenotypes, to the next generation. This is often summarised as 'survival of the fittest'.

Accumulation of Advantageous Traits Over Time

Over successive generations, the advantageous traits become more prevalent in the population because the individuals possessing them have a higher reproductive success rate. Conversely, individuals with less favourable traits are less likely to survive and reproduce, so their genes become less common. This gradual accumulation of beneficial changes over long periods leads to significant evolutionary change, potentially resulting in the development of new species better adapted to their environment.

What The Diagram Shows

DNA Double Helix is a biology diagram for Genes, inheritance and selection. Use it beside the written explanation so the visual supports the exact idea being revised.

What To Explain

The answer should connect the visible structure or process to the biological function named in the specification point.

Syllabus Link

Private mapping: OCR GCSE combined-science-a-gateway J250, Genes, inheritance and selection.

Common Trap

Do not describe the diagram only as a picture. State the concept, then explain what the pattern, relationship, structure, or graph means.