Homozygous refers to the state or condition that occurs when an individual inherits the same DNA sequence for a certain gene from both biological parents. When a particular gene has identical alleles (versions) of chromosomes inherited from both parents, the gene is homozygous. A homozygous trait is referred to by two capital letters (XX) for a dominant trait, and two lowercase letters (xx) for a recessive trait.
Biological parents pass on versions of genes to their children. These versions are referred to as alleles. When the alleles are identical, the gene is said to be homozygous. This can lead to a person exhibiting homozygous traits. There are many examples of homozygous genes.
Homozygous genes can impact the appearance of a person’s eyes as well as medical conditions related to the eyes.
- The dominant trait for eye color is brown, represented by BB. All other eye colors - blue, grey, green, and hazel - are recessive traits, represented by bb. A homozygous brown-eyed person would have the BB gene, while a homozygous blue-eyed person would have the bb gene.
- An individual has the dominant SS gene, a homozygous gene that results in farsightedness. However, his friend has the homozygous gene ss for the recessive trait, which is normal sight.
- An individual possesses normal vision due to the homozygous VV gene. On the other hand, his friend possesses the recessive vv gene, which results in his colorblindness.
Homozygous genes can manifest in certain factors that impact a person’s appearance, including hair and skin.
- Having freckles is the dominant trait for melanin deposits on the cheeks and body. A homozygous freckled person would have the FF gene, while someone with the homozygous gene of the individual without freckles would be represented by ff.
- Having dimples on the cheeks is a dominant trait, with the homozygous DD gene causing them. However, the recessive trait is distinguished by the symbol dd; individuals with this homozygous gene will not have dimples.
- A full head of hair is a dominant feature, with the homozygous gene of HH. However, the recessive trait - baldness - can be manifested in an individual with the homozygous recessive gene, or hh.
- Curly hair is a dominant trait, with the homozygous HH gene causing the condition. Its recessive trait, represented by the homozygous gene hh, is straight hair.
Homozygous genes can predispose a person to develop or have immunity to certain medical conditions.
- One girl's blood clots normally, so her homozygous gene is the dominant CC. A boy in her class has hemophilia, which is present in the recessive gene cc.
- The dominant trait for hearing and speaking is being able to speak normally, which is the result of the SS homozygous gene. However, an individual with the homozygous recessive trait, ss, is unable to speak or hear properly and is instead deaf and mute.
- One person is immune to poison ivy, which is represented by the dominant PP trait; while another is not immune to poison ivy, which is represented by the recessive trait distinguished by the symbol pp.
Homozygous genes can impact a person’s musculoskeletal system, impacting factors related to their movement and appearance.
- One gentleman can bend his joints in funny ways because he possesses the dominant JJ gene for double-jointedness. Another gentleman does not possess this gene - instead, he has the recessive jj gene which means his joints bend normally.
- A boy is born with an extra finger on each hand due to the dominant gene EE, while another newborn with the recessive homozygous ee gene has the normal number of fingers on her hands.
- Unattached earlobes is a trait that is the result of a dominant gene UU, while an individual with the recessive homozygous gene uu actually has attached earlobes.
While the basic homozygous definition simply describes the word as being a descriptive term for a cell that has two identical copies of a gene, the scientific definition is a bit more complex:
“Having the same alleles at a particular gene locus on homologous chromosomes”
In order to interpret the scientific definition, a person will need to know the meaning of some other key scientific terms.
- allele - different forms or versions of genes
- chromosomes - structures that carry genetic information (passed from biological parents to offspring)
- homologous - similar in structure
- locus - a particular site or location
These examples of homozygous should give you a better understanding of chromosomes and genetics. Now that you have reviewed some examples of homozygous traits and conditions, consider expanding your knowledge of genetics. Start by exploring the difference between homozygous and heterozygous traits.