Genetics and Inheritance

Genetics is the study of how characteristics are passed from parents to offspring. To describe inheritance you need a few key terms.A gene is a sequence of DNA bases that codes for a polypeptide (or a functional RNA). Each gene is found at a fixed position on a particular chromosome, called its locus.Diploid organisms have two copies of each chromosome, one inherited from each parent, so they have two copies of each gene. These copies may be the same version or different versions (different versions are called alleles, covered in the next lesson).The genotype is the genetic make-up of an organism: the alleles it has for a particular gene. It is written with letters, for example TT, Tt or tt.The phenotype is the observable or measurable characteristic of the organism, for example tall or short, blood group A, or the colour of a flower. The phenotype results from the genotype and its interaction with the environment:phenotype = genotype + environmentFor example, a plant may have the alleles to grow tall but stay short in poor soil, and identical twins can differ in body mass because of diet.Inheritance diagrams (genetic crosses) show the genotypes of the parents, the gametes they can make and the genotypes and phenotypes of the offspring. Because gametes are haploid, each gamete carries only one copy of each gene. These ideas are the basis of every genetic cross in this topic.