The long, twisty helix that is human DNA might be one of the greatest mysteries in the universe. It’s no surprise that the study of genetics is a complex field. One concept that is sometimes confusing is the difference between homozygous and heterozygous.
TL;DR (Too Long; Didn't Read)
Homozygous means that both copies of a gene or locus match while heterozygous means that the copies do not match. Two dominant alleles (AA) or two recessive alleles (aa) are homozygous. One dominant allele and one recessive allele (Aa) is heterozygous.
So Many Chromosomes
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.
Homozygous and Heterozygous
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 called an allele. If the alleles match, the person is homozygous for that trait. If the alleles are different, the person is heterozygous for that trait.
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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.
While it's true that genetics is a complicated topic, it is also a fascinating view into how humans inherit traits. While you are definitely more than your biology, your DNA is a significant part of who you are and is a seemingly endless source of investigation for geneticists.