Genes, inheritance and selection Study Guide

Genetics is the study of heredity, and it involves several fundamental terms that explain how traits are passed from parents to offspring. A gamete is a reproductive cell, like a sperm or an egg, which carries genetic information. Each gamete contains a set of chromosomes, which are thread-like structures found in the nucleus of cells and made of DNA. These chromosomes contain many genes, which are specific sections of DNA that code for particular traits, such as eye colour or blood type. For each gene, there can be different versions called alleles (or variants). For example, a gene for eye colour might have an allele for blue eyes and another allele for brown eyes.

Alleles can be dominant or recessive. A dominant allele expresses its trait even if only one copy is present. A recessive allele only expresses its trait if two copies are present, meaning no dominant allele is masking it. An individual's genetic makeup for a particular trait is called their genotype. When an individual has two identical alleles for a gene (e.g., two dominant alleles or two recessive alleles), they are described as homozygous for that gene. If they have two different alleles (e.g., one dominant and one recessive), they are heterozygous. The observable characteristics resulting from an individual's genotype and environmental factors are known as their phenotype. For instance, if an individual has one allele for brown eyes (dominant) and one for blue eyes (recessive), their genotype might be 'Bb', and their phenotype would be brown eyes because the brown allele is dominant.

The development of an organism's phenotype, which refers to its observable characteristics, is a complex interplay between its genetic makeup (genome) and the environment it experiences. The genome is the complete set of genetic material in an organism, encoded in DNA. This DNA contains genes, which are specific sequences that code for proteins, and these proteins carry out many essential functions, ultimately determining traits. For example, genes determine eye colour or the potential height an individual could reach. However, the environment plays a crucial role in how these genes are expressed and how the phenotype ultimately manifests. Environmental factors such as diet, temperature, light, lifestyle choices, and exposure to chemicals can switch genes on or off, or influence the degree to which they are expressed. For instance, a person might have genes for tall height, but if they suffer from malnourishment during childhood, their actual height (phenotype) might be shorter than their genetic potential. Similarly, skin pigmentation is influenced by genes, but exposure to sunlight will cause the skin to darken, demonstrating an environmental effect on a genetically determined trait. Therefore, the phenotype is not solely determined by genetics but is a dynamic outcome of the interaction between an organism's inherited genetic information and its surrounding environment throughout its development.