Mendelian genetics explains how an organism’s observable traits (phenotype) relate to its underlying genetic makeup (genotype). You can think of phenotype as the traits you can observe (often including appearance), while genotype is the set of genetic instructions that help produce those traits.

The genotype is made up of genes - segments of DNA that influence particular traits - found at specific positions, called loci (singular: locus), on chromosomes. A gene can exist in different versions called alleles. In most cases, each cell carries two alleles for a given gene - one inherited from each parent. If the two alleles are the same, the individual is homozygous; if they differ, the individual is heterozygous. The most common allele in a population is often called the wild type, while alternative versions may be mutations.

Allele inheritance patterns

In simple cases where a gene has two alleles, several classical inheritance patterns can occur:

Complete dominance:

• The dominant allele masks the effect of the recessive allele, so individuals show the dominant trait:
  • Homozygous dominant (AA)
  • Heterozygous (Aa)

Co-dominance

• Both alleles contribute equally, producing a phenotype in which both traits are visibly expressed. Examples include the AB blood type (both A and B antigens are present) and the reddish “roan” coat sometimes seen in horses and cows.

Incomplete dominance

• The heterozygous condition produces an intermediate phenotype, often described as a blend of the two alleles’ effects. For example, puppies may have wavy fur as an intermediate expression between one parent’s curly coat and the other parent’s straight coat.

Complete recessive

• Individuals express the recessive trait when both alleles are the recessive allele:
  • Homozygous recessive (aa)

Additional concepts such as penetrance, expressivity and leakage further refine how we connect genotype to phenotype.

Penetrance refers to the proportion of individuals with a particular genotype who actually display the associated phenotype.

Expressivity indicates the extent or intensity of the phenotype when it is expressed.

Leakage occurs when there is inter-species transfer of genetic material. Commonly, this can occur through cross-pollination, leading to some characteristics from one species appearing in the contaminated species. Leakage can cause ecological disruption - for example, when herbicide-resistant crops cross-pollinate with weeds, making the weeds difficult to destroy.

Hybridization and gene pool

Another key concept is hybrid vigor (or heterosis), where offspring from genetically diverse parents show improved viability or performance. This increased fitness is partly due to a reduced chance of inheriting two copies of harmful recessive alleles. Finally, the complete collection of alleles within a population is called the gene pool. The gene pool represents the genetic diversity that drives evolution and supports the inheritance of traits across generations.