Genes, inheritance and selection Study Guide

Genetic variants are differences in the DNA sequence among individuals. These variations can have a significant impact on an organism's phenotype, which refers to its observable characteristics. The influence of a genetic variant depends on where it occurs within the DNA and what function that region of DNA has.

One key way genetic variants affect phenotype is when they occur in the <b>coding DNA</b>. Coding DNA sequences provide the instructions for making proteins. If a variant changes the sequence of the coding DNA, it can lead to a change in the amino acid sequence of the resulting protein. This alteration in the protein's structure can then affect its function or activity. For example, a genetic variant might cause a protein to become less active, more active, or even completely non-functional. Since proteins carry out most of the work in cells and are responsible for cell structure, function, and regulation, an altered protein activity can directly translate into a change in the organism's phenotype.

The second key way genetic variants influence phenotype is when they occur in <b>non-coding DNA</b>. Non-coding DNA does not directly code for proteins. Instead, these regions often play crucial roles in regulating gene expression, meaning they control when and where genes are turned on or off, and how much protein is produced. Examples of non-coding DNA include promoter regions, enhancers, and silencers. A genetic variant in a non-coding region might alter how strongly a gene is expressed. For instance, a variant in a promoter region could reduce the binding efficiency of transcription factors, leading to less messenger RNA (mRNA) being produced from that gene, and consequently, less of the corresponding protein. Conversely, another variant might increase gene expression. By affecting the amount or timing of protein production, variants in non-coding DNA can still indirectly alter the activity of relevant proteins and ultimately influence the phenotype.

Single gene inheritance describes how traits are passed down from parents to offspring, determined by a single gene with usually two alleles. Each parent contributes one allele to their offspring for a particular gene. The combination of these alleles in the offspring determines their genotype, which then dictates their phenotype (observable characteristic). For example, if a gene controls eye colour and has alleles for brown (dominant) and blue (recessive), a child inheriting one brown allele and one blue allele will have brown eyes because the dominant allele masks the effect of the recessive one. Only if two recessive alleles are inherited will the recessive trait (blue eyes) be expressed. We represent alleles using letters, with capital letters for dominant alleles (e.g., 'B' for brown eyes) and lowercase letters for recessive alleles (e.g., 'b' for blue eyes). Genetic crosses can be analysed using Punnett squares to predict the probability of offspring inheriting specific genotypes and phenotypes. This fundamental concept is central to understanding how characteristics are passed through generations.