Mendelian concepts

Mendelian genetics explains how an organism’s observable traits (phenotype) are determined by its underlying genetic makeup (genotype). The phenotype is often thought of as what an organism or person looks like.

The genotype is composed of genes—specific segments of DNA that code for particular traits—located at defined positions, or loci, on chromosomes. Each gene can exist in different forms known as alleles, and each cell typically carries two alleles per gene—one inherited from each parent—resulting in either a homozygous state (both alleles identical) or a heterozygous state (alleles differ). The most common allele in a population is often referred to as the wild type, while alternative variants can be mutations.

Allele inheritance patterns

In simple cases where a gene has two alleles, classical patterns of inheritance are observed:

Complete dominance:

• The dominant allele masks the effect of the recessive one, resulting in individuals display the dominant trait:
  • Homozygous dominant (AA)
  • Heterozygous (Aa)

Co-dominance

• Both alleles contribute equally, resulting in phenotypes where both traits are visibly expressed, as in the AB blood type where both A and B antigens appear, and the reddish “roan” coat sometimes seen in horses and cows.

incomplete dominance

• The heterozygous condition produces a blend of the two alleles’ characteristics, leading to an intermediate phenotype—this is often seen in puppies who have wavy fur as an intermediate expression of one parents’ curly coat and the other’s straight coat.

Complete recessive

• Individuals express the recessive trait, as both alleles are the same recessive allele:
  • Homozygous recessive (aa)

Additional concepts such as penetrance, expressivity and leakage further refine our understanding of genotype-phenotype relationships.

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 simply occurs as cross-polination and results in some characteristics from one species appearing in the contaminated species. Ecological disruption can be a concern when leakage occurs, such as when herbicide-resistant crops cross-pollinate weeds, making the weeds very difficult to destroy.

Hybridization and gene pool

Another key concept is hybrid vigor (or heterosis), where the offspring of genetically diverse parents exhibit improved viability or performance. This increased fitness is partly due to the reduced likelihood of inheriting two copies of harmful recessive alleles. Finally, the entire collection of alleles within a population is known as the gene pool, which represents the genetic diversity that drives evolution and underpins the inheritance of traits across generations.