Genes, inheritance and selection Revision Notes

Contributions of Darwin and Wallace

Charles Darwin and Alfred Russel Wallace independently developed the theory of evolution by natural selection. Darwin's extensive observations, particularly during his voyage on the HMS Beagle, provided massive amounts of evidence. His book, 'On the Origin of Species' (1859), systematically laid out the theory. Wallace, working in the Malay Archipelago, arrived at very similar conclusions through his own observations. He corresponded with Darwin, which spurred Darwin to publish his detailed findings. Both recognised that species change over time, and these changes are driven by environmental pressures.

Key Principles of Natural Selection

The core ideas of natural selection involve several key principles:

1. Variation: Individuals within a species show variation in their characteristics.
2. Overproduction: Organisms produce more offspring than can survive.
3. Competition: There is a struggle for existence due to limited resources.
4. Survival of the Fittest: Individuals with advantageous traits (better adapted to their environment) are more likely to survive and reproduce.
5. Inheritance: These advantageous traits are passed on to the next generation, gradually leading to evolutionary change in the population over time.

Impact on Modern Biology

The theory of evolution by natural selection profoundly impacted modern biology. It provided a scientific explanation for the diversity of life and the adaptation of organisms to their environments. This understanding is crucial in areas like:

• Genetics: Explaining how advantageous genes become more prevalent.
• Ecology: Understanding species interactions and ecosystem dynamics.
• Conservation: Informing strategies to protect biodiversity by recognising evolutionary processes.
• Medicine: Explaining the development of antibiotic resistance in bacteria and the evolution of viruses. It forms the unifying theory for all biological sciences.

Understanding Chromosome Sets

Chromosomes are structures found in the nucleus of eukaryotic cells that carry genetic information in the form of DNA. The number of chromosomes is characteristic of a species. When we talk about haploid and diploid, we are referring to how many complete sets of these chromosomes are present within a cell. Each set contains all the necessary genes for an organism.

Diploid Cells (2n)

Diploid cells contain two complete sets of chromosomes. This is often represented as '2n', where 'n' is the number of chromosomes in a single set. In humans, for example, 'n' is 23, so diploid cells have 2n = 46 chromosomes. These cells are somatic cells, which are all body cells except for gametes. One set of chromosomes is inherited from the mother and the other from the father, forming homologous pairs. Diploid cells are produced by mitosis, ensuring that daughter cells are genetically identical to the parent cell.

Haploid Cells (n)

Haploid cells contain only one complete set of chromosomes, represented as 'n'. In humans, haploid cells have n = 23 chromosomes. These are typically the gametes, or sex cells (sperm in males, egg in females). Haploid cells are produced through meiosis, a specialized cell division that reduces the chromosome number by half. When a haploid sperm fertilises a haploid egg, they combine to form a diploid zygote, restoring the 2n chromosome number for the new organism.

Single Gene vs. Multiple Genes

Traditional Mendelian genetics often focuses on traits controlled by a single gene, where clear dominant and recessive alleles determine a distinct phenotype. Examples include cystic fibrosis or Huntington's disease, where a single gene mutation has a profound effect. However, the vast majority of an organism's observable characteristics, or phenotypic features, are not determined in this simple manner.

Polygenic Inheritance

Most phenotypic features are the result of interactions between multiple genes. This is known as polygenic inheritance. In these cases, several different genes, each with a small additive effect, contribute to a single trait. This typically leads to a continuous variation of the trait within a population, rather than discrete categories.

Examples of Polygenic Traits

Common examples of polygenic traits in humans include height, skin colour, eye colour, and weight. For instance, human height is influenced by hundreds of genes, each contributing a small amount to the final physical characteristic. This explains the wide range of heights observed in the human population, which usually follows a normal distribution curve.

Gene Interaction and Phenotype

When multiple genes contribute to a phenotype, their alleles can interact in various ways, making the precise genetic basis of the trait complex. The combination of alleles from these different genes, along with environmental influences, all contribute to the final expression of the phenotypic feature. This complexity often makes it difficult to predict exact inheritance patterns for polygenic traits.

Specification Link

This digestive system diagram is used for: Communicate the key features of the development of our understanding of genetics.

What To Explain

Digestive organs and enzymes break large insoluble food molecules into smaller soluble molecules that can be absorbed.

Exam Answer Move

This is tied to: Communicate the key features of the development of our understanding of genetics. The learner should explain digestion, enzymes or absorption.