Genes, inheritance and selection Study Guide

Genetics is the study of heredity, and it involves understanding how characteristics are passed from parents to offspring. This field relies on a specific vocabulary. At the most fundamental level, characteristics are encoded in DNA, which is organized into structures called chromosomes within the nucleus of most cells. A gene is a specific section of a chromosome that codes for a particular trait. Each gene can have different forms, known as alleles or variants. These alleles determine the specific expression of a trait, such as eye colour. For many traits, an individual inherits two alleles for each gene, one from each parent. When these two alleles are the same, the individual is said to be homozygous for that gene. If the two alleles are different, the individual is heterozygous. The interaction between these alleles determines the observable physical or biochemical characteristics, known as the phenotype. The genetic makeup, or the combination of alleles an individual possesses, is called their genotype. In cases where an individual is heterozygous, some alleles are dominant, meaning they will be expressed in the phenotype even when only one copy is present. Other alleles are recessive, and they will only be expressed in the phenotype if two copies are present (i.e., the individual is homozygous recessive) and no dominant allele is present to mask their effect. Gametes, which are reproductive cells like sperm and egg cells, are haploid, meaning they carry only one allele for each gene, ensuring that when they fuse during fertilisation, the offspring inherits a complete set of two alleles per gene.

The genome is a fundamental concept in biology, referring to the complete set of genetic material within an organism. Think of it as the master instruction manual for building, operating, and maintaining an entire living being. For humans and most other complex organisms, this genetic material is DNA, organized into structures called chromosomes. Each chromosome contains many genes, which are specific sequences of DNA that code for proteins, critical molecules that carry out most of the work in cells and are necessary for the structure, function, and regulation of the body's tissues and organs. The human genome contains over 20,000 genes, along with vast stretches of non-coding DNA. In simpler organisms like bacteria, the genome is typically smaller and less complex. Understanding an organism's genome provides insights into its characteristics, evolutionary history, and susceptibility to diseases. The study of genomes, known as genomics, has transformed medicine, agriculture, and biotechnology, allowing for advancements such as genetic screening, personalized medicine, and the development of new treatments for genetic disorders. The genome is unique to each species, yet shares common features among all life forms, reflecting a shared evolutionary past. For example, all life uses the same basic genetic code, albeit with some minor variations.