An individual is a result of the genes it is made up of as well as the environmental factors. It is important to know the genetic compositions as they differ from one person to another.

Homozygous vs heterozygous alleles in genetics


If you want to know the genotype or genetic composition of a gene, you will encounter two types of allele combinations – homozygous and heterozygous. These two types have many similarities and differences.
It is important to know the differences between the two to fully understand how each one works and their significance in the body. (1, 2)

Homozygous

What does homozygous mean?

It describes a gene that has two copies of a similar allele; each of the parents gave a similar allele for a gene.

It comes from the word “homo” which means “same” and “zygous” which means zygotes of a specified kind.

It is homologous in the sense that two identical copies of the genes are present on the two reciprocal homologous chromosomes.

Examples of homozygous

  • Color of the eyes
  • Appearance of freckles
  • Color of the hair
  • Having dimples on the cheeks
  • Curly hair
  • A full head of hair (1, 2, and 3)

Heterozygous

A person is heterozygous when it has two different alleles. The reason for different alleles is because each parent gave a different allele.

It comes from the word “hetero” which means different and “zygous” which means zygote of a specified kind. Heterozygous individuals demonstrate the characteristics of the dominant trait.

Examples of heterozygous

  1. Having curly hair
  2. Having a particular disease condition such as sickle cell anemia.
  3. Blood type
  4. Color of the hair such as when one of the parents has black hair and the other has blonde hair; the offspring’s hair color will exhibit the dominant gene.(3, 4, and 5)

Differences between homozygous and heterozygous :

Key differences/basis for comparisonHomogenousHeterozygous
Meaning/definitionA condition wherein an individual has the same alleles of a gene from both parents.A genetic condition in which an individual has different alleles of genes from both parents.
Other nameHomozygotesHeterozygotes
Genotype representationIt is represented as AA for homozygous dominant and aa for homozygous-recessive. (5)It is represented by Aa.
PhenotypesThere are two possible phenotypes with both dominant or recessive homozygous. (5, 6)It primarily depends on the dominant allele in heterozygous condition (6)
Traits/characteristicsThe traits they produced are the same over different generations.They produce different traits over generations.
GametesIt results in a single type of gamete.It leads to two different gamete types.
TypesThere are two types: homozygous-dominant and homozygous-recessive.There are three ways to express heterozygous genes. These are co-dominance, complete dominance, and incomplete dominance. (6, 7)
Hybrid vigorIt does not demonstrate hybrid vigor.It shows a hybrid vigor.
Where it is usually observed?It is observed in animals.It is observed in animals that reproduce through sexual means.
Medical conditions/diseasesMedical conditions usually associated with homozygous are the following: 
– Sickle cell anemia
– Fibrosis
– Phenylketonuria(7, 8)
Medical conditions commonly associated with heterozygous are the following: 
– Familial hypercholesterolemia
– Marfan’s syndrome
– Huntington’s disease(8)
ExamplesEye color – Brown is the dominant eye color and represented by BB genotype.
For homozygous-recessive, the color of the eyes varies; could be blue, green, or grey.

Presence of freckles – The tiny spots on the skin caused by melanin pigmentation are called freckles. It is a dominant trait and people with freckles have a homozygous-dominant genotype.

For homozygous-recessive, freckles are absent. (8, 9, and 10)
Curly hair – Having curly hair is an example of a dominant trait.
There is a specific protein code responsible for making the hair curly.  
For incomplete dominance heterozygous, the hair can be wavy, which is in between curly and straight.

Sickle cell anemia – it is a recessive trait causing the blood cell to form incorrectly.

For people with heterozygous genotype, the dominant trait is expressed, thus, preventing the possibility of having sickle cell anemia. (6, 7, and 8)
Examples of allelesTT, GG, tt, and ggGg and Tt
Mendel’s termTrue breedingHybrid
Mendelian GeneticsThe alleles are expressed phenotypically at all times because you can find it in both chromosomes. (4, 5)The alleles are not always phenotypically unless it is dominant.

Another thing, it should be carried as an unexpressed recessive allele. (5, 8, and 10)
Disease conditionsIt accounts for about 25% of cases of steroid-resistant nephrotic syndrome.

It is also the state wherein the alleles for the condition sickle cell anemia is expressed. (5, 6)
The heterozygous form protects the organism from certain diseases such as malaria.

However, some diseases are expressed in heterozygous such as Huntington’s disease. (8, 9, and 10)

Summary

  • Both homozygous and heterozygous refer to alleles that make up a specific gene.
  • Homozygous alleles are the same. They can either be both dominant alleles or recessive alleles, and have two identical copies of each gene.
  • Heterozygous is a state where each allele of a particular gene is different; one of the alleles is recessive and the other is dominant. The diploid organism has two different copies of each gene.
  • Some diseases are dependent on the type of alleles you have. Diseases associated with homozygous are sickle cell anemia and steroid-resistant nephrotic syndrome. For heterozygous, the most common condition is Huntington’s disease.
  • The result of homozygous is always the same; pure for a particular trait and breed. For heterozygous, an individual isn’t pure and the offspring usually has different genotypes.
  • In homozygous, an individual may carry a dominant or recessive allele, but one at a time. with heterozygous, the individual can have one dominant and one recessive allele.
  • Homozygous produces one type of gamete while heterozygous has the ability to produce two types of gametes. (4, 5, 8, 9, and 10)

References

  1. https://microbenotes.com/homozygous-vs-heterozygous/
  2. https://biodifferences.com/difference-between-homozygous-and-heterozygous.html
  3. https://www.expii.com/t/heterozygous-vs-homozygous-differences-similarities-10433
  4. https://sciencing.com/difference-between-homozygous-heterozygous-8606730.html
  5. https://byjus.com/biology/difference-between-homozygous-and-heterozygous/
  6. https://examples.yourdictionary.com/difference-between-homozygous-and-heterozygous-traits.html
  7. https://geneticeducation.co.in/a-comparison-between-homozygous-vs-heterozygous/
  8. http://www.differencebetween.net/science/difference-between-homozygous-and-heterozygous/
  9. https://www.verywellhealth.com/heterozygous-versus-homozygous-4156763
  10. https://www.technologynetworks.com/genomics/articles/genotype-vs-phenotype-examples-and-definitions-318446

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