Homozygous - The Definitive Guide | Dictionary of Biology (2023)


Homozygous is a term in the field of genetics that describes two identical copies of alleles in the DNA gene sequence that encodes a particular trait. Because we get our genetic material from a father and a mother, not all of the double strands of our DNA are exactly the same. Homozygosity can occur throughout DNA in any organism that reproduces through two parental cells.

What does homozygous mean?

is homozygousa Greek word– the prefix homo means equal; zygous comes from zugos - a word meaning togetherness.

The terms heterozygous and hemizygous indicate that two alleles composed of the sequences of two parents are different (hetero) and half (hemi), respectively.

Homozygous genotype

To understand what a homogeneous genotype is, we first need to know the absolute basics of genetics.

All of our cells contain DNA - a double-stranded spiral of molecules that code for the structure of the organism in which it resides.

DNA is the recipe book for protein synthesis. Our bodies are the result of these proteins – even when it comes to synthesizing fatty acids and sugars, proteins are essential. From the enzymes that help us digest food to our muscles, organs, and skin, they're all built on the chains and bundles of amino acids we call proteins.

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While each cell (with the exception of mature red blood, skin, hair, and nail cells) contains the complete recipe book, not all of this information is expressed. Cells express specific genes depending on their function. For example, a beta cell in the pancreas expresses the genes that produce the hormone insulin, and this function is turned off in other cells.

All of the DNA-carried information used to create an organism is called the genome. The information that the proteins produce for a single trait (trait) — such as insulin production — is called a gene. Because a very long strand of DNA is more likely to be damaged, portions of the genome are packaged into tightly coiled chromosomes. These chromosomes divide the genome into sections. One could say that the genome is the recipe book, the chromosome is a specific chapter and each gene is an individual recipe.

A gene is not always located at a single location on a chromosome. In addition, polygenic traits can be the result of more than one gene. Another word for a trait is a genotype. Where a trait is visible, it is referred to as a phenotype.

Identical twins start with the same genotypesince they come from the same sperm and ovum; However, environmental conditions can change some DNA sequences during their development in the womb and also after birth. The genotype will eventually differ. When these changes are visible, identical twins also differ in phenotype.

Two other terms are important when considering the homozygous genotype: allele and locus. An allele is a version of a gene. Our DNA has one allele from our father and one from our mother. Where these alleles are located on a chromosome is called the locus.

Chromosomes come in pairs - one maternal and one paternal. However, during the fertilization process, parts of these individual chromosomes can cross over in a process called recombination. This mechanism increases diversity within a species or its genetic variation.

Exactly where an allele is found on a chromosome is called a locus. Because the sequences that control traits can be distributed across different alleles, many genes have multiple loci and many traits are the result of multiple genes.

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In terms of the DNA recipe book, the locus can be viewed as a set of instructions and the allele as the page number of each instruction. The information that tells you how to convert gas marks to degrees Fahrenheit and the information that tells you what ingredients to use are in different places within a chapter (chromosome). If multiple genes are involved, you may even need to refer to different chapters. This page (allele) can contain more than one recipe. For example, a locus that containsseveral genes linked to type 2 diabetesis found at 20q12 -13.1. This number refers to the long arm (q) of the twentieth chromosome, in region one and band two. The number after the dash indicates an even more specific sub-band position.

Finally, we need to look at allele dominance, although this makes no difference to the outcome for homozygous alleles. If a puppy is born to a wirehaired father and a straighthaired mother, that puppy is more likely to have wirehaired hair. This is because the wirehair allele is dominant in dogs. A dog can only be smooth-coated if no wirehair alleles are present.

We can best view dominant and recessive traits with aPunnett Square. In the example below, W denotes the inherited dominant wirehair allele and w denotes the inherited recessive smoothhair gene.

The father of a litter of puppies has two dominant W alleles and the mother has two recessive ww genes. These alleles are passed on to their puppies, all of whom will be wirehaired like the father. However, all also have the recessive allele for straight hair. Only WW and ww are considered homozygous. All puppies produced in this litter are heterozygous (described in more detail below).

Only when two recessive alleles are present in a puppy does that animal exhibit the smooth phenotype. And that is only possible if both parents – including those with rough hair – have at least one recessive allele in their genome. On the Punnett square below, two ww parents produce three wirehaired puppies and one smoothhaired.

In terms of homozygous genotypes and phenotypes, both alleles must be the same. In the Punnett square above, two puppies have homozygous alleles - WW (wire coated) and ww (smooth coated).

The two wire-haired puppies with one of each allele - Ww - are not homozygous but heterozygous for the alleles that determine their hair growth pattern. Here the dominant gene determines the hair type.

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Homozygous versus heterozygous

Homozygous and heterozygous, as we saw in the Punnett square example above, do not necessarily determine which phenotype is present. Both homozygous and heterozygous alleles can produce a dominant phenotype; however, only one homozygous pair of alleles in an individual can produce a recessive phenotype.

A heterozygous pair of alleles (gene) can result in either complete dominance (Ww in the example above), codominance, or incomplete dominance. Codominance and incomplete dominance are similar - the former means that two different alleles have the same dominance. The best known example are the alleles that determine our blood type. People with heterozygous alleles for blood types A and B are more likely to have blood type AB than either A or B, depending on which is the dominant allele.

Incomplete dominance describes a mixing of two different phenotypes, e.g. B. a straight-haired father and a curly-haired mother who give birth to wavy offspring. Neither codominance nor incomplete dominance can occur in a homozygous allele.

Alternatively, just because offspring show a dominant phenotype, it is impossible to tell if they are the result of homozygosity unless you look closely at their DNA. Only when a recessive phenotype occurs do we have very strong evidence that the paired alleles are homozygous.

Self-pollinated plants are homozygous and repeat their genotype for countless generations. Despite this, environmental and biological factors can alter gene sequences over time through genetic mutations. If a heterozygous plant can fertilize itself, there will be more variation in the characteristics of its seedling.

Homozygous dominant examples

Homozygous dominant examples are countless; However, we cannot judge whether a phenotype is truly homozygous dominant without looking closely at the DNA. As we have already learned, a heterozygous dominant specimen would produce the same phenotype as the homozygous dominant genotype.

Animals bred for a particular trait over generations are more likely to be homozygous dominant or homozygous recessive. Over time, two similar alleles become more common and there is less chance of producing heterozygous pairs, except in cases of incomplete or codominance.

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Huntington's disease and Marfan syndrome are autosomal dominant disorders that only require one dominant allele to have an effect. However, this does not guarantee that the affected individual is a homozygous example of a dominant allele.

Any visible phenotype—whether it's how a body looks, behaves, moves, or functions—could be a homozygous dominant example; However, to be absolutely sure, we need to look at the gene locations.

Homozygous recessive examples

Homozygous recessive examples are much easier to spot than homozygous ones. This is because a recessive trait (which is neither codominant nor incomplete) can only be seen if two recessive pairs of alleles make up the gene that controls that trait. New technologies have brought us homozygosity mapping, which helps predict recessive traits in individual pedigrees and animal pedigrees.

If both parents have brown eyes but one child has blue eyes, we have an example of a homozygous recessive phenotype. In this case, brown eyes dominate. If brown eyes were recessive, all offspring would have brown eyes. Instead, two heterozygous parents produced a homozygous recessive child for eye color.

Many diseases are the result of homozygous recessive alleles. Recessive inheritance is associated with sickle cell anemia and cystic fibrosis. These are called autosomal dominant conditions because the affected alleles are only found in paired autosomal chromosomes (autosomes). When sex chromosomes produce abnormal inheritance patterns, the results are called sex-linked traits.

Because a mother has two X chromosomes and a father has one X and one Y chromosome, only X-linked homozygous dominance occurs in males. In females, this can be heterozygous or homozygous. Despite this, homozygosity in X-linked recessive disorders is very rare in females - most of which affect female fertility and very few offspring will carry the disorder into future generations. Y-chromosomal disorders are only possible in men.


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  • Khatib H (ed.). 2015. Molecular and Quantitative Animal Genetics. New Jersey, Wiley Blackwell.
  • Aguilar L. 2019. Gene, Genome, Genetics and Chromosomen. Waltham Abbey, ED-Tech Press.
  • Anestis M, Ploeger Cox, K. 2019. 5 steps to a 5: AP biology. New York, McGraw Hill.

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