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Difference Between Homozygous and Heterozygous
Homozygous and heterozygous are terms used to describe the genetic makeup of an organism, specifically the alleles present in its genes. An allele is a version of a gene that controls a particular characteristic. For example, the gene for eye color has different alleles for blue, brown, green, and hazel eyes.
What is Homozygous?
When two identical alleles (recessive or dominant) of the same gene are present, the condition is called homozygous. Because there is no dominant allele to inhibit the expression of the gene in a homozygote person, the recessive features are phenotypically expressed in this case, providing Mendelian laws hold true.
Examples of homozygous genotypes − Assuming that A stands for dominant alleles and a for recessive alleles, a homozygous person might have either the AA or aa genotype.
Examples from Mendel’s work − Given the limited understanding of genetics at the time, Mendel referred to homozygous plants as true-breeding plants. Consistently reproducing the same characteristics in its progeny is a hallmark of a true-breeding plant, suggesting that the underlying causes are also consistent. A later generation of researchers deduced that these "factors" were, in fact, different alleles of the same gene.
Diseases or conditions that occur in the homozygous condition − Recessive alleles are responsible for a wide variety of human disorders. This is the situation with steroid-resistant nephrotic syndrome (SRNS), which is caused by a recessive mutation in the NPHS2 gene in roughly 14 percent cases, and is manifested only in the homozygous form. As the sickle-cell trait allele is recessive, sickle-cell anaemia and related illnesses only manifest in the homozygous situation.
What is Heterozygous?
When both copies of a gene, called "alleles," are distinct, a condition known as heterozygosity results. According to Mendelian genetics, a dominant allele will be expressed whether the individual is homozygous or heterozygous, whereas a recessive allele will be silenced in the heterozygous state.
Examples of heterozygous genotypes − Heterozygotes have the genotype Aa, where A denotes dominance and a recessive allele is denoted by a. Heterozygosity indicates that alleles are being secretly carried, but depending on the genotype of the offspring, they may not be shown in the phenotype.
Examples from Mendel’s work − Mendel referred to what we now know to be heterozygous plants as hybrids because he understood that they possessed a combination of components (alleles) rather than just one. Here is how the crossing of two purple-flowering plants resulted in some offspring with white flowers. Since both parents were heterozygotes, their offspring received the recessive gene for white blossom colour.
Diseases or conditions that occur in the heterozygous condition − Sickle-cell trait can also arise in a heterozygous state, where the sickle-cell allele is advantageous since it protects against malaria without causing sickle-cell disease. This phenomenon, known as a heterozygote advantage, has been put out as an explanation for the trait's continued presence in the human population. Due to the autosomal dominance of the disease-causing allele, heterozygosity can result in the development of diseases like Huntington's disease.
Differences: Homozygous and Heterozygous
The following table highlights the major differences between Homozygous and Heterozygous −
Characteristics |
Homozygous |
Heterozygous |
|---|---|---|
Definition |
Homozygous is the condition in which there are two alleles for a gene, both of which are exactly the same, either both alleles are dominant or both are recessive. |
Heterozygous is the condition in which there is one dominant and one recessive allele for a gene. |
How many forms it can occur in (assuming Mendelian genetics) |
The homozygous condition can occur in two forms. |
The heterozygous condition can only occur in one form. |
Potential genotypes |
Assuming A is dominant and a is recessive, in the homozygous condition the genotypes can be either AA or aa. |
In the heterozygous condition, the genotype can be Aa. |
Mendel’s term |
Mendel’s term for homozygous was true-breeding. |
Mendel’s term for heterozygous the term was hybrid. |
Mendelian genetics |
In the case of homozygotes, an allele is always expressed phenotypically because it is present on both chromosomes. |
In the case of heterozygotes, an allele is not always phenotypically expressed unless it is dominant, and it can be carried as a recessive allele that is not expressed. |
Diseases or conditions |
The homozygous condition is responsible for about 25 percent of cases of steroid-resistant nephrotic syndrome (SRNS) and is the state in which the alleles for sickle-cell anemia are expressed. |
The heterozygous condition can be an advantage in the case of sickle- cell which in the heterozygote form protects a person from malaria; some diseases like Huntington’s disease are expressed even in the heterozygous form. |
Conclusion
Homozygous and heterozygous refer to the genetic makeup of an organism and the alleles present in its genes. Homozygous organisms have two identical alleles, while heterozygous organisms have two different alleles.
The difference between homozygous and heterozygous is important in terms of inheritance and expression of traits, with homozygous organisms having a higher risk of passing on genetic disorders.
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