Genetics Study Guide

There is usually extensive genetic variation within a population of a species. This means individuals have different versions of genes (alleles), resulting in differences in their traits. These genetic differences mainly arise through mutations, which are random changes in the DNA sequence. Mutations can happen spontaneously (such as errors during DNA replication) or be caused by environmental factors like radiation or chemicals. Most mutations have no effect on the phenotype (neutral), some can be harmful, and rarely, some can be beneficial. Only those mutations that cause changes in traits can be acted on by natural selection.

Alleles are distinct versions of a gene that determine specific characteristics. Humans, like many organisms, inherit two alleles for each gene, one from each parent. The combination of these alleles, known as the genotype, dictates the observable characteristic, or phenotype. Differences in inherited traits arise because individuals receive varying combinations of these alleles. For instance, if an individual inherits two identical alleles (homozygous), they will express the trait associated with that allele. If they inherit two different alleles (heterozygous), one allele might be dominant and mask the effect of the recessive allele, or both might contribute, leading to a blended or co-expressed trait. This vast array of allele combinations across a population leads to the diversity of characteristics observed.