Index
It is a diploid organism with two alleles, each of a different type. People with alleles of the same type are known as homozygous individuals.
An allele is a variation of a gene that affects the functionality of the protein produced by the gene.
There can be many alleles in a population, but a heterozygous individual can only have two of them because each diploid organism receives one allele from each parent.
Although each organism can have only two alleles, there can be many variations of a gene present in a population.
The term heterozygous is a description of the genotype, or combination of alleles present in an organism. The genotype gives rise to the phenotype.
Depending on the different relationships between the alleles, different phenotypes are created in a heterozygous individual.
In alleles that show complete dominance, the heterozygous phenotype will be the same as the dominant phenotype .
Incomplete dominance, in contrast, produces a heterozygous phenotype that is somewhere between the dominant and recessive phenotypes.
A third type of relationship, codominance, exists when different alleles are expressed individually, in different parts of the body.
In a codominant trait, heterozygous individuals will show the phenotype of one allele in some parts of their body and the phenotype of the other allele in different parts of the body.
Examples of Heterozygotes
Heterozygous individuals and sickle cell trait
Sickle cell anemia is a recessive disease that causes blood cells to form incorrectly.
Instead of a wide, round disk, the cells form in the shape of a crescent moon or sickle. Cells do not work correctly in this way.
The anemia , or condition of the blood can not carry enough oxygen, it is a serious condition in people with sickle cell characteristics recessive.
The condition can also cause strokes, pain, and clots in the bloodstream . However, heterozygous individuals with sickle cell trait do not suffer from these conditions.
The only normal allele they have makes enough red blood cells to keep them healthy.
While it may seem that heterozygous individuals are still at a great disadvantage, there is actually a great benefit of sickle cell trait.
People with a single sickle cell allele still make enough sickle cells, but not enough to make them sick.
However, when the malaria parasite infests the body, it attacks and resides in the blood cells. Sickle cells are believed to collapse around the parasite and make it easier for the body to filter the parasite from the blood. More heterozygous individuals can survive malaria in this way.
Scientists believe that is the reason why the sickle cell allele has a fairly high frequency, especially in the tropics where malaria cases are high.
Curly hair
Interestingly, how curly your hair is can be related to genetics.
The gene that decides the phenotype for hair curl produces a protein that makes the locks grow curvy.
An individual with two recessive alleles, or two non-functional alleles, will have straight hair because there is no protein that causes the curly phenotype.
Homozygous individuals with two curly alleles will have very curly hair.
Heterozygous individuals have a phenotype somewhere in the middle, what people might call “wavy” hair.
In a heterozygous individual, only one curly allele is present, and only half the amount of protein can be made.
This makes the hair about half as curly as in a homozygous curly individual. This is an incomplete domain pattern.
Blood type
If you have ever given blood to a medical blood bank, you know that blood comes in different types.
Blood types correspond to proteins on the surface of blood cells that are recognizable by immune cells in the human body.
Put the wrong type of blood in the wrong person, and they could die from the immune reaction. There are several different alleles in the population that have different relationships with each other.
The different alleles are A, O and B. They produce the different genotypes shown in the Punnett classification below:
- A and B are dominant over O, but codominant with each other.
- AA and AO are known as the “A” blood type, because only the “A” proteins are present.
- BB and BO are known as the “B” blood type, because only the “B” proteins are present.
- OO do not make proteins in their blood cells, and they can donate blood to anyone.
In AB heterozygous individuals, some cells express “A” proteins, and some express “B” proteins.
This means that you can give an AB heterozygous individual an ‘A’, ‘B’ or ‘O’ blood type, and their body will not reject the donation.
This makes the heterozygous individual the easiest to donate blood.
Homozygous OO people can donate to anyone, but they cannot receive blood from anyone but another OO donor (just another advantage of being heterozygous).
Related Biology Terms
- Genotype: the combination of alleles that gives rise to a phenotype or physical trait.
- Diploid : cells that contain two copies of each gene, but may have different variants or alleles of each gene.
- Incomplete dominance : when two dominant alleles produce a third intermediate phenotype in the heterozygous individual.
- Codominance: when two dominant alleles are expressed at the same time, but in different parts of an organism.
