Genetics Study Guide

Genetic information in living organisms is stored within chromosomes, which are thread-like structures found in the nucleus of eukaryotic cells. Each chromosome is composed of DNA tightly coiled around proteins. Along these chromosomes are segments called genes, which are functional units of heredity that determine specific traits. Different versions of a gene are known as alleles. For any given gene, an individual inherits two alleles, one from each parent. The combination of these alleles dictates the organism's genotype. When an individual has two identical alleles for a specific gene, they are said to be homozygous. If they have two different alleles, they are heterozygous. One allele might be dominant, meaning its trait is expressed even if only one copy is present, while a recessive allele's trait is only expressed when two copies are present. The observable characteristics resulting from the genotype and environmental factors constitute the phenotype. Reproductive cells, such as sperm and egg, are called gametes and carry only one allele for each gene. When two gametes fuse during fertilisation, they form a zygote, which then develops into a new organism.

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.