The long, twisty helix that is human DNA might be one of the greatest mysteries in the universe, so it’s no surprise that the study of genetics is a complex field. When we look at a genotype – a representation of genetic variance for specific locations in the genome– we can classify particular traits or genotypes is homozygous or heterozygous. This can also be useful in predicting the phenotype (the physical depiction of a genotype) of a particular gene.

Humans are diploid organisms, which means that each cell contains two copies of each chromosome. In order to conserve diploidy, the sperm and egg cells must each contribute only one copy of each chromosome at conception so that the resulting offspring receives the full diploid complement (and does not wind up with four copies). The process by which sperm and egg cells split chromosomal copies to become haploid (have one copy of each chromosome) is meiosis. These similar pairs homologous chromosomes, with copies of the gene from each parent.

When humans inherit genes from their parents, each genomic location gets these two different forms of a gene called alleles. Each allele of a gene influences the genetic makeup and expression for that gene. There are two different alleles – important to classifying homozygous and heterozygous traits: dominant alleles (usually denoted with a capital letter) and recessive alleles (usually denoted with a lowercase letter).

The composition and dominance of alleles influences genes that control everything from hair color to sickle cell anemia. If an allele is dominant, its presence at a locus will super-cede any recessive alleles. These systems are very common in Mendelian genetics with Punnett Squares, where – for example – the alleles ‌AA‌ and ‌Aa‌ would exhibit the dominant gene, while ‌aa‌ would exhibit the recessive gene.

When it comes to genetic traits, scientists look at genes and the locus where that gene or trait encodes on the chromosome. Since humans possess two copies of each chromosome, they also have two copies of each gene and locus on those chromosomes. Each of these trait-encoding genes (or loci) is an allele. If there are identical alleles, the person is homozygous for that trait. If the alleles are different, the person is heterozygous for that trait.

Dominant and Recessive Inheritance

For this type of inheritance, it is helpful to look at alleles using letters where a capital letter represents a dominant allele, and a lowercase letter represents a recessive allele: ‌AA‌, ‌Aa‌, and ‌aa‌.

Dominant traits like whether you can roll your tongue or have a genetic predisposition to developing Huntington's disease only require one dominant allele to express. This means that people with homozygous dominant alleles (AA) and heterozygous alleles (Aa) express those traits, but people with homozygous recessive alleles (aa) do not.

Recessive traits like having straight thumbs or cystic fibrosis require two recessive alleles to express. This means that only people with homozygous recessive alleles (aa) express the trait. People with homozygous dominant alleles (AA) will not express the trait or carry it, and people with heterozygous alleles (Aa) do not express the trait but are carriers for it.

These previous examples are examples of complete dominance, where a heterozygous individual will exhibit the dominant phenotype exclusively. Some chromosomes exhibit incomplete dominance (where a mixture of both allele expressions are found in the phenotype) and codominance (where both alleles are expressed side-by-side in the phenotype). Classic examples are eye color, blood cell type (ABO and MN blood types), and mixing the color of flowers through inheritance.