The instruction manual for every living organism is contained within its DNA, organized into functional segments called genes. These genes provide the specific directions for building and maintaining the body, influencing characteristics from hair color to blood type. For each gene, an individual inherits two copies, one from each biological parent; these variant forms are known as alleles. A specific pairing of identical alleles defines the genetic state known as homozygous, which plays a significant role in determining an individual’s physical characteristics.
What It Means to Be Homozygous
The term homozygous describes a genotype where an individual possesses two identical alleles for a specific gene. This condition is the opposite of being heterozygous, which means inheriting two different versions of the gene. Geneticists use letter notation to represent this pairing, where a capital letter signifies a dominant allele and a lowercase letter signifies a recessive allele. A homozygous genotype can be represented in two ways: as two dominant alleles (AA), called homozygous dominant, or as two recessive alleles (aa), called homozygous recessive.
How Homozygosity Determines Traits
The consequence of being homozygous is that the genetic instructions for a trait are uniform, directly translating the genotype into an observable characteristic, or phenotype. When an individual is homozygous dominant, possessing two copies of the dominant allele, the associated trait is always expressed. For instance, a person homozygous for the dominant allele for a cleft chin will display that characteristic.
In the case of a homozygous recessive genotype, the trait associated with the recessive allele is expressed only because there is no dominant allele present to mask it. For example, the inability to taste the chemical PTC is a recessive trait, and only individuals with two recessive alleles will lack the taste receptors for it. This recessive pairing is also behind many single-gene inherited conditions, such as cystic fibrosis, which only manifests when an individual inherits two copies of the non-functional allele.
The Inheritance of Homozygous Conditions
An offspring acquires a homozygous condition when both biological parents contribute the same type of allele for a given gene during reproduction. During the formation of reproductive cells, the pair of alleles separates, so each sperm or egg cell carries only one allele for that trait. To inherit a homozygous dominant condition, the offspring must receive a dominant allele from the mother and a dominant allele from the father. To inherit a homozygous recessive condition, both parents must contribute the recessive allele to their offspring. If both parents are heterozygous, meaning they each carry one dominant and one recessive allele, there is a one-in-four, or 25%, chance that their child will inherit two recessive alleles and be homozygous recessive for that trait.