Genetic Diagrams and Terminology

Overview of Genetic Diagrams and Terminology

  • Genetic diagrams are used to predict the outcome of a specific cross or breeding experiment. They illustrate how genes and/or alleles segregate and combine.
  • Understanding genetic terminology is essential for making sense of these diagrams.

Key Genetic Terms


  • Genes are sections of DNA located on chromosomes that control specific characteristics or functions.
  • Genes can exist in different versions called alleles.
  • Some alleles are dominant, while some are recessive in terms of their expression.


  • Alleles refer to the different forms of a specific gene. For example, for the gene controlling eye colour, different alleles could account for blue, brown, or green eyes.
  • An individual can have two of the same allele (homozygous) or two different alleles (heterozygous) for a particular gene.
  • The presence of certain alleles leads to different phenotypes and genotypes.

Phenotypes and Genotypes

  • The phenotype is the observable characteristics or traits of an individual, resulting from gene expression.
  • The genotype is the specific set of genes an organism carries.
  • While the genotype is responsible for an individual’s phenotype, environmental factors can also influence the expression of certain traits.

Dominant and Recessive Traits

  • Dominant traits are expressed when at least one dominant allele is present.
  • Recessive traits are only expressed when both alleles for the trait are recessive.

Understanding and Constructing Genetic Diagrams

  • Genetic diagrams are tools used to predict the genotypes and phenotypes of offspring from a particular genetic cross.
  • These diagrams often use Punnett squares, a simple grid system that shows all possible combinations of offspring genotypes.
  • To construct a genetic diagram using a Punnett square, you first write the possible gametes for each parent along the top and side of the square. Then, you fill in the squares to show all possible gene combinations for the offspring.
  • Remember, each gamete only carries one allele for each gene because of the process called meiosis where chromosome number is halved.
  • Genetic diagrams can be used to work out probabilities of inheritance. However, keep in mind that they predict statistical probabilities, not exact outcomes.

Practical Uses of Genetic Diagrams

  • Among their uses, genetic diagrams are employed to trace and predict the inheritance of specific traits in pedigree analysis.
  • They’re also used to visualize Mendel’s laws of heredity, such as the law of segregation and the law of independent assortment.
  • In medicine, they can provide insight into potential genetic diseases an offspring may inherit.