Have you ever noticed how some diseases pass down generations even when parents don’t have it? Many individuals with thalassemia, for example, have perfectly healthy parents. The opposite also holds: parents with a trait don’t always pass it down to their children.
This curious case of genetics comes down to a power struggle between the copies of genes – called alleles – that you inherit from each parent. Some alleles mask other alleles’ effects, so traits tend to skip generations.
Depending on the type of alleles comprising the trait’s associated genotype, we classify them into homozygous and heterozygous individuals. Let us delve into this phenomenon in more detail:
Heterozygous Individual – Definition and Example
If you notice the word ‘heterozygous,’ you will understand what it means – the Greek word ‘hetero’ means ‘other’ or ‘different,’ and homozygous individuals have two different alleles.
If you recall my lectures, you may remember I talked about some traits being dominant and others being recessive. Dominant traits express if even one allele is present in an individual’s genotype, while recessive traits express only if both the alleles present in the genotype are recessive.
But what happens if different alleles, dominant and recessive, are present in an individual’s genetic makeup? In such a case, the dominant trait takes the driving seat and does not let the recessive trait be expressed. We say that the dominant allele has masked the effect of the recessive allele.
Understanding the concept of heterozygosity is fundamental in genetics, as it plays a crucial role in genetic inheritance, the expression of traits, and the diversity of genetic information in populations. An individual will express the same characteristic if they are dominant or heterozygous. You cannot tell if an individual is heterozygous or homozygous dominant just by the phenotype. You will have to analyze their pedigree and make punnett squares to assess it, or you can undergo genetic testing.
An Example of Heterozygous Specie
Let’s understand this concept with the help of an example. Let’s say that a particular gene encodes for the eye color in humans. The allele that codes for brown eyes is dominant, and we can express it with a capital ‘B,’ while the allele that codes for blue eyes is recessive, and we can denote it with a small ‘b.’
An individual with a ‘BB’ genotype will have brown eyes, while an individual with a ‘bb’ genotype will express blue eye color. But what about individuals with a ‘Bb’ genotype?
In such individuals, the gene for blue eyes tries its best to be expressed, but the gene for brown eyes blocks its expression. Consequently, the individual will have brown eyes.
If such a person marries another heterozygous individual with brown eyes, both parents will have a ‘b’ gene that is ‘hidden’ or masked. If both transfer their ‘b’ gene’ to one of their kids, they will have a ‘bb’ genotype and express blue eyes. That is how a blue-eyed kid can be born to parents who both have brown eyes!
Homozygous Individuals – Definition and Example
As the name suggests, homozygous individuals possess the identical alleles for a gene. In Greek, the word ‘homo’ means ‘same’.
Homozygous individuals can have both dominant alleles, which we call homozygous dominant. Similarly, if they have both recessive alleles for a gene, we call them homozygous recessive.
A Practical Example of Homozygous:
Take the example of eye color in humans. If a person has both copies of the allele that codes for brown eyes, their genotype will be ‘BB,’ and they will be homozygous dominant. If they instead have both alleles for blue eyes, their genotype will be ‘bb,’ and they will be homozygous recessive.
As you can see, there are two possibilities for an individual to have brown eyes: they can be homozygous dominant or heterozygous, while there is only one way an individual can have blue eyes. That is why you see many people with brown eyes, while blue ones are rare! This is true for every dominant and recessive trait.
Differences Between Homozygosity and Heterozygosity
Heterozygous and homozygous individuals differ in several ways. Here’s an overview of the differences:
| Homozygous Individuals | Heterozygous Individual | |
| Allele | Have only one type of allele | Have two different types of alleles |
| Genotype | Either ‘AA’ or ‘aa’ | Always ‘Aa’ |
| Expressed Phenotype | Homozygous dominant individuals express the dominant phenotype; homozygous recessive individuals express the recessive phenotype | Always express the dominant phenotype |
| Inheritance | Can pass only one type of allele to offspring | Can pass any kind of alleles |
Genetics is all about the intricate play between genes and alleles, and its consequences have monumental implications for our health.
The concept of homozygosity and heterozygosity is one example. Homozygous individuals have only one allele to pass down to their offspring, while heterozygous individuals can pass either the dominant or the recessive trait. By understanding their patterns of inheritance, we can track and understand how familial and hereditary disorders work.