Genetics study guide

Inherited characteristics, or traits, show variation among individuals due to differences in their genetic makeup. At the heart of this variation are alleles, which are different forms of a gene. For most genes, an individual inherits two alleles, one from each parent. These alleles can be identical (homozygous) or different (heterozygous). When alleles are different, their interaction determines the expressed characteristic. For instance, a dominant allele may mask the effect of a recessive allele. The combination of alleles an individual receives from their parents is a random process, occurring during sexual reproduction when gametes (sperm and egg) fuse. Each gamete carries only one allele for each gene. Because parents are often heterozygous, they can produce gametes with different alleles, leading to a unique combination of alleles in their offspring. This constant shuffling and combination of alleles, alongside new genetic variations introduced by mutation, ensures a wide range of inherited characteristics within a population, contributing to genetic diversity.

Protein synthesis involves two main stages: transcription and translation. In transcription, RNA polymerase binds to a non-coding region just before a gene, then synthesises a complementary mRNA strand from the coding part of the DNA. This newly formed mRNA strand exits the nucleus and attaches to a ribosome. During translation, the ribosome reads the codons—groups of three bases—on the mRNA, and tRNA molecules bring the corresponding amino acids. Each tRNA has an anticodon matching the mRNA codon, ensuring that amino acids are added in the correct order. The ribosome links the amino acids together to form a polypeptide, which then folds into its functional protein shape.