Heterozygous - What is a heterozygous condition?

Genes comprise DNA which instructs, determining different traits such as type of blood, hair colour, height etc. Genes have different versions, each of which is referred to as an allele. For each gene, we inherit two alleles – one from each parent. These alleles together are the genotype.

It is a heterozygous genotype for a particular gene if there are two different versions of it. For instance, a heterozygous for eye colour corresponds to say that you have one allele for black and one for brown. This association between both the alleles has an effect on the expression of the traits. It also determines the traits you carry.

Definition of Heterozygous

Heterozygous is a state of inheriting various forms of a specific gene from each of the parents. Various forms correspond to saying that there are fragments of the genes wherein the sequence differs, which can range from trivial segments of the gene to the most important sections of the gene.

Heterozygous indicates when each of your biological parents contributed their copies of a specific gene, it was delivered in a way such that the sequences of DNA varied to an extent. At one junction, it could be different in the gene or could vary at different junctions in the same gene. The concept of the heterozygous condition is contradictory to that of the Homozygous condition. In the homozygous condition, identical forms of a specific gene are obtained from each of the biological parents. When this is read through the sequence of DNA obtained from both the parents, there is no difference in that gene or segment of a gene.

Example of Heterozygous

An example of a heterozygous condition is inheriting different genes for eye colour from both biological parents. If there are two different versions, it is a heterozygous genotype for that particular gene. Another example is heterozygous for the colour of hair, which corresponds to having one allele for brown hair and the other alleles for red hair. The equation between both alleles has an influence on which characteristic gets suppressed and which gets expressed. It is also an indicator of what traits are carried.

Heterozygous alleles

Allele pairs are described using the terms heterozygous and homozygous. Every diploid entity comprises two copies of each of the genes, wherein each could be recessive or dominant. Dominant alleles are expressed to give rise to a phenotype, whereas the recessive alleles remain unexpressed. Both the recessive and dominant alleles are found on the same locus of the homologous chromosomes. The heterozygous entities carry both the recessive and dominant alleles.

In an allele pair, heterozygosity can be seen in dominant and recessive alleles that go on to determine a specific characteristic of a diploid entity, which is indicated by Rr. This suggests that both the two alleles in a pair vary.

The heterozygous alleles are inherited in three ways:

  • Codominance: The expression of both alleles in the heterozygous pair. An example of this would be the AB blood group, wherein both antigens A and B are expressed independently in the RBCs.
  • Complete dominance: One allele dominates the other completely. As a result, the dominant allele only gets expressed and decides the individual’s phenotype.
  • Incomplete dominance: An allele is not dominant completely on the other as a result, a varied combination of phenotypes can be seen in the individual. For example, Pink colour flowers are seen in Snapdragon, dominant flower being red and recessive flower being white.

Heterozygous genotype

The genotype of an entity is in disparity with its phenotype, which are the observable traits of an individual as a result of interactions between the genotype and the surroundings. The association between genotype and phenotype is complex. As the phenotype is a yield of an interaction between the genes and their surroundings, variation in the surroundings can give rise to different characteristics with a specific genotype.

Additionally, varying genotypes can result in the same phenotype. This is a result of the genes having different alleles. For a few traits and genes, some of the alleles are recessive while others are dominant. Dominant characteristics are the ones that are expressed in an entity despite the individual having only one allele, the one generating the trait.

Heterozygous genotypes can have one normal allele and one mutated allele, or both mutated alleles that are different. These genotypes are indicated by the uppercase letter, which is indicative of the dominant allele, and a lowercase letter, which is indicative of the recessive allele for a gene, for instance – Tt, here the uppercase letter is placed before the lowercase letter and the gene “T” is taken to be “Tt”.

If the desired characteristic is found by simple dominance, a heterozygote expresses only the characteristic which the dominant allele codes while that which the recessive allele codes is not found. This type of genotype may depict a higher fitness relative to either the homozygous recessive genotype or homozygous dominant genotype. This is referred to as a heterozygote advantage.

Some examples of genotypes are – height, hair colour etc.

Heterozygous phenotype

The phenotype, in simple terms, is the expressed characteristics or traits on the outwards in contrast to the genotypes, which are the internal hereditary instructions that comprise the genetic code. An individual’s observable traits are its phenotype. Genotypes are different from phenotypes in the fact that genotypes are inherited from parents, while phenotypes are not.

While the phenotype is influenced by the genotype of an entity, a person’s genotype cannot be equated with the phenotype. Some of the environmental aspects influencing the phenotype can be temperature, nutrition, stress and humidity. A good example of this is seen in Flamingos, wherein the influence of its environment can be seen in its phenotype. Their natural colour is white, while they are known to be pink (as a result of the pigments in the entity).

Another example is the colour of the skin of an individual. Genes regulate the amount and kind of melanin produced by us, but when exposed to UV light, it can lead to the darkening of the already existing melanin encouraging increased melanogenesis and hence the darker skin.

Based on the different relationships between alleles, different phenotypes are produced in a heterozygous individual. The heterozygous phenotype is the same as the dominant phenotype, in case the alleles show complete dominance.

Contrastingly, in incomplete dominance, a heterozygous phenotype is produced, which lies between the recessive and dominant phenotype. Co-dominance is the dynamics in which different alleles are individually expressed across different body parts. In such a characteristic, the heterozygous person exhibits the phenotype of one allele on some body part, the phenotype of other alleles on a different body part.

Heterozygous dominant and Heterozygous recessive

In the event where an allele is faulty or mutated, a disease can be inherited by the offspring even if parents do not experience any sign of it. In heterozygosity, there are many forms of this:

  • Alleles are heterozygous recessive: Mutated alleles would be recessive and hence suppressed and not expressed. The person, in this case, would be a carrier.
  • Alleles are heterozygous dominant: Mutated alleles would be dominant, and hence the individual may or may not get affected by it.

For instance, in a pea plant, there can be red flowers and either be heterozygous (red-white) or homozygous dominant (red-red). If it has white flowers, it is homozygous recessive then (white-white). The carriers, in any case, are said to be heterozygous. Huntington’s disease is autosomal dominant; hence an individual with this condition can either be homozygous dominant or heterozygous.

Heterozygous disease

When there is a single faulty allele, it results in a single gene disorder. If the faulty allele is recessive, usually, the individual does not get affected. But, if the faulty allele is dominant in nature, the mutated version can overrule the recessive version causing a less intense form of a disease or can be completely symptomatic.

Familial hypercholesterolemia (FH)

An inherited condition, FH is distinguished by high levels of cholesterol, to be precise, the LDLs (low-density lipoproteins). It is seen to affect 1 in 500 individuals and is a common disorder.

Sickle cell

This characteristic can take place in a heterozygous environment. Here, the allele for sickle-cell is an advantage as it safeguards an individual against the condition of Malaria, however, does not cause them to fall sick with sickle cell. This, as discussed, is a heterozygote advantage and is suggested as the reason that character is seen in the population of humans.

Huntington’s disease

This is a condition that is inherited, leading to the fatality of the brain cells. The condition is a result of the dominant mutation in any one or both the alleles for a gene, referred to as the Huntingtin.

In the event where both the parents possess a heterozygous recessive mutation, their offspring can have 1/4th of the chances to develop that condition for each childbirth. In the event when both the parents possess a heterozygous dominant mutation, the offspring has a 50% chance of obtaining the dominant alleles, 25% chance of obtaining both the dominant alleles and 25% of obtaining both the recessive alleles.

Some of the diseases in which compound heterozygotes can be functional are Phenylketonuria (PKU), haemochromatosis, Tay-Sachs disease and cystic fibrosis.

Heterozygous vs Homozygous – Differences

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Some of the key differences between Heterozygous and Homozygous conditions are as tabulated below:

Heterozygous

Homozygous

What are they?

In this state, the individual has one recessive and one dominant allele for a particular gene

It is the state wherein the individual has two alleles for a gene that are exactly alike, wherein either both are recessive or are dominant

Probable genotypes are represented by?

Rr

Rr or RR

Number of forms in which it can occur

One

Two

How many types of gametes are produced?

Two types of gametes

One type of gamete

The characteristic could be carried and hence not expressed phenotypically until it is dominant

Characteristics passed on an allele is expressed always as it is seen on both the chromosomes

Disease example

Huntington’s disease is seen even if found on only one allele

Close to 25% of cases of SRNS (Steroid Resistant Nephrotic Syndrome)

Variation of allele shown

Can show codominance, complete dominance, or incomplete dominance

Can either be homozygous recessive or homozygous dominant

This was a brief on the Heterozygous condition. For important articles on NEET, visit BYJU’S.

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