What is homozygous?

In genetics, the definition of homozygous is when you inherit the same DNA sequence for a specific gene from each of your biological parents. If you inherit an identical version (allele) of a gene from both of your parents, you’re homozygous for that gene. Having homozygous genes can lead to homozygous traits, such as blue eyes or red hair. If you inherit mutated (not working the way they should) alleles, it can lead to homozygous genetic conditions.

What is an allele?

You inherit one copy of each gene from each of your biological parents. Almost all genes in the human body are the same in everyone. But a tiny fraction (less than 1%) of genes are slightly different among everyone. Alleles are versions or alternative forms of the same gene with small differences in their DNA sequences. These differences are what give you your unique traits.

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What’s the difference between heterozygous and homozygous?

Homozygous means you’ve inherited the same alleles of a gene from each of your parents. In contrast, heterozygous means you’ve inherited different alleles of a gene from each of your parents. In other words, when each of your parents passed on their copy of a gene to you, the DNA sequence was slightly different. The word “heterozygous” means different, and the word “homozygous” means the same.

What is a homozygous genotype?

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A genotype is the form or version of a DNA sequence that you inherit. A homozygous genotype means you inherit two identical alleles. With a homozygous genotype, you’ve inherited the exact same form of the DNA sequence for a particular gene from each parent. If you were to read along the DNA sequence that each parent gave you, you would see no differences in that gene or gene region whatsoever.

What are homozygous dominant and homozygous recessive traits?

Some alleles are dominant and others are recessive. In a heterozygous genotype, the dominant allele masks the recessive allele because it’s expressed more strongly.

But with a homozygous genotype, this doesn’t occur. You may have two dominant alleles for a particular trait (homozygous dominant). Or you may have two recessive alleles for a particular trait (homozygous recessive). The trait of the matching alleles, whether dominant or recessive, will be expressed. In the study of genetics, two capital letters (for example, BB) represent a dominant homozygous gene trait. Two lowercase letters (for example, bb) represent a recessive homozygous gene trait.

What are some homozygous examples?

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When your parents pass on identical alleles to you, they can lead to homozygous traits. Homozygous gene traits can include physical appearance, musculoskeletal characteristics and whether or not you develop certain health conditions.

Physical appearance

• Eye color: Homozygous genes can affect your eye color. Brown is the dominant trait for eye color. The symbol BB represents brown eye color. All other eye colors are recessive traits. The symbol bb represents all other eye colors. A brown-eyed person with homozygous genes has the BB gene. A blue-eyed person with homozygous genes has the bb gene.
• Freckles: Having freckles is a dominant homozygous trait. A person with freckles has more melanin in their body, represented by the FF symbol. A person without freckles has the recessive homozygous gene, represented by the ff symbol.
• Dimples: Having dimples is a dominant homozygous trait. The symbol DD represents having dimples. In people with the recessive homozygous trait, dimples won’t appear. This is represented by the symbol dd.
• Curly hair: Curly hair is a dominant homozygous trait. The symbol HH represents curly hair. The recessive homozygous trait is straight hair. The symbol hh represents straight hair.

Musculoskeletal characteristics

• Unattached earlobes: If you have unattached earlobes, you have the dominant UU gene. If you have attached earlobes, you have the recessive uu gene.

Health conditions

• Hearing and speech: The dominant trait for hearing and speaking is being able to hear and speak as most people do, represented by the dominant SS gene. If you have the recessive ss gene, you’re unable to speak or hear in the typical way. You’ll be deaf and nonspeaking.
• Poison ivy immunity: Immunity to poison ivy is another dominant homozygous trait. The PP symbol represents this dominant trait. If you have the recessive pp gene, you’re not immune to poison ivy.

What are examples of homozygous genetic conditions?

If you inherit identical mutated alleles from each of your parents, you’re homozygous for that genetic mutation. This means you’re more likely to develop the genetic condition that the gene is responsible for. Genetic conditions you’re more likely to inherit include:

• Cystic fibrosis: The CFTR gene makes a protein in your body that controls cell fluid movement. If you inherit two mutated copies of the CFTR gene, you’ll develop cystic fibrosis.
• Sickle cell anemia: The HBB gene helps part of the hemoglobin in your red blood cells deliver oxygen throughout your body. If you inherit two copies of the mutated HBB gene, you’ll develop sickle cell anemia.
• Phenylketonuria: The PAH gene tells your cells how to produce an enzyme that breaks down an amino acid in your body called phenylalanine. If you inherit two mutated copies of the PAH gene, you’ll develop phenylketonuria.

A note from Cleveland Clinic

You’re homozygous for a certain gene when you receive identical copies (alleles) of that gene from both of your biological parents. If you have a homozygous gene, then you’ll have a homozygous trait, which can code for one of the many physical aspects that makes you, you.

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