Genes are made up of DNA, which provides instructions for different traits such as type of blood, hair color, height, etc. Genes come in different versions, each of which is known as an allele. For each gene, we inherit two alleles - one from each parent. These alleles together make up the genotype.

A heterozygous genotype for a particular gene is when there are two different versions of the gene. For example, a heterozygous genotype for eye color means that an individual carries one allele for black and one allele for brown. This combination of alleles has an effect on the expression of the traits and determines the traits that are carried.

Heterozygous - Possessing two different alleles of a particular gene or genes, and therefore two different forms of a trait.

Heterozygous is a state of inheriting different versions of a specific gene from each of the parents. These versions can differ in sequence, from small segments of the gene to the most important sections.

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, the gene could be different or could vary at different junctions in the same gene. This is in contrast to the Homozygous condition, where 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: A heterozygous organism has two different alleles for a particular gene. For example, a heterozygous organism for the gene that determines eye color might have one allele for blue eyes and one allele for brown eyes.

If both biological parents have different genes for eye colour, it is an example of a heterozygous condition. This heterozygous genotype is also present when a person has one allele for brown hair and the other allele for red hair. The equation between the two alleles determines which trait is expressed and which is suppressed, and is an indicator of what traits are passed on.

Heterozygous Alleles

Alleles can be described as either heterozygous or homozygous. Diploid organisms possess two copies of each gene, one of which is dominant and expressed, while the other is recessive and unexpressed. Both the recessive and dominant alleles are found on the same locus of the paired chromosomes, and a heterozygous individual carries both 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 are different.

The Inheritance of Heterozygous Alleles

1. Autosomal Dominant
2. Autosomal Recessive
3. X-Linked

Codominance: The expression of both alleles in the heterozygous pair, such that both phenotypes are expressed simultaneously. 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 completely dominates the other. This means that the dominant allele is the only one expressed and determines the individual’s phenotype.

In incomplete dominance, neither allele is completely dominant over the other, resulting in a combination of phenotypes in the individual. For example, pink flower color is seen in snapdragons, with the dominant flower color being red and the recessive flower color being white.

Heterozygous Genotype

The genotype of an entity differs from its phenotype, which are the observable traits of an individual arising from the interaction between the genotype and the environment. The relationship between genotype and phenotype is complex. As the phenotype is a result of the interaction between the genes and their environment, changes in the environment can lead to diverse characteristics with the same genotype.

Additionally, the same phenotype can result from different genotypes. This is due to genes having different alleles, where some of the alleles are recessive and some are dominant. The dominant characteristics are expressed in an individual even if they only have one allele, the one responsible for 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 an 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 is represented as “Tt”.

A heterozygote advantage is observed when a heterozygote expresses only the characteristic which the dominant allele codes, while the characteristic coded by the recessive allele is not found. This type of genotype may be more fit than either the homozygous recessive or homozygous dominant genotype when the desired trait is found by simple dominance.

Some examples of genotypes include:

• Height
• Hair Colour
• Eye Colour
• Skin Tone
• Facial Features

Heterozygous Phenotype

The phenotype is the outwardly expressed characteristics or traits, while the genotype is the internal hereditary instructions that comprise the genetic code. An individual’s observable traits are their phenotype. Phenotypes and genotypes differ in 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 directly equated with the phenotype. Some of the environmental aspects that can influence the phenotype are 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).

Genes regulate the amount and kind of melanin produced by us; however, when exposed to UV light, it can cause already existing melanin to darken and lead to increased melanogenesis, resulting in a darker skin color.

The heterozygous phenotype is the same as the dominant phenotype when the alleles demonstrate complete dominance, as different phenotypes are produced in a heterozygous individual due to the various relationships between alleles.

In contrast to incomplete dominance, co-dominance results in the expression of different alleles in separate body parts. In this case, the heterozygous individual will display the phenotype of one allele in one body part, and the phenotype of the other allele in another body part, rather than a phenotype that lies between the recessive and dominant phenotypes.

Heterozygous Dominant and Heterozygous Recessive

If an allele is faulty or mutated, the offspring can inherit a disease even if the parents do not show any signs of it. In heterozygosity, there are many forms of this phenomenon:

Mutated alleles would be recessive and hence suppressed and not expressed, making the person a carrier of the heterozygous recessive alleles.

Mutated alleles would be dominant, which means that the individual may or may not be affected by the heterozygous dominant alleles.

In a pea plant, red flowers can be either heterozygous (red-white) or homozygous dominant (red-red), and white flowers are homozygous recessive (white-white). Carriers of any of these alleles are said to be heterozygous. For Huntington’s disease, which is autosomal dominant, an individual can be homozygous dominant or heterozygous.

Heterozygous Disease

When there is a single faulty allele, it can result in a single gene disorder. If the faulty allele is recessive, the individual typically does not get affected. However, if the faulty allele is dominant, the mutated version can overpower the recessive version, leading to a less intense form of the disease or can even cause full symptomatic effects.

Familial Hypercholesterolemia (FH)

FH (Familial Hypercholesterolemia) is an inherited condition characterized by high levels of cholesterol, specifically LDL (low-density lipoproteins). It is estimated to affect 1 in 500 individuals and is a relatively common disorder.

Sickle Cell

The allele for sickle-cell is beneficial in a heterozygous environment, as it protects individuals from the condition of Malaria without causing them to suffer from sickle cell. This phenomenon is known as a heterozygote advantage, and is thought to be the reason it is seen in the human population.

Huntington’s Disease

The Huntingtin gene is the cause of an inherited condition which leads to the fatality of brain cells. This condition is a result of a dominant mutation in either one or both alleles.

In the event where both the parents possess a heterozygous recessive mutation, their offspring can have 1/4 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
• Cystic Fibrosis

Heterozygous vs Homozygous – Differences

The main difference between heterozygous and homozygous is that heterozygous individuals have two different alleles of a gene, while homozygous individuals have two identical alleles of a gene. Heterozygous individuals can produce offspring with either allele, while homozygous individuals will always produce offspring with the same allele.

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

Heterozygous	Homozygous
Two different alleles	Same two alleles
Genotype is usually represented as Aa or AB	Genotype is usually represented as AA or BB
Phenotype is usually a blend of both alleles	Phenotype is usually determined by only one allele

| | |

| — | — |

| Heterozygous |

| Homozygous |

Bold Text
Bold Text

| What are they? |

This is a table

This	Is	A	Table
1	2	3	4

| 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 identical, wherein either both are recessive or dominant.

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| This | is | a | cool | table |

Answer:| What are probable genotypes represented by? indicates table

Name	Age
John	18

Name	Age
John	18

| Rr |

Rr	RR
Rr	RR

Name	Age
John	20

Name	Age
John	20

| Number of Ways it Can Occur represents a table

Name	Age
Bob	20

Name	Age
Bob	20

| One |

| Two represents a table

Column 1	Column 2
Data A1	Data B1

Column 1	Column 2
Data A1	Data B1

How many types of gametes are produced?
Two types of gametes are produced: sperm and egg.

Column 1	Column 2
Cell 1	Cell 2

Column 1	Column 2
Cell 1	Cell 2

Two Types of Gametes

One type of gamete
Sperm

Name	Age
John	24

Name	Age
John	24

| The characteristic may not be expressed phenotypically until it is dominant, but it can still be carried.

The characteristics passed on an allele are always expressed as they appear on both chromosomes.

The teacher asked the student to read the book.

The student was asked by the teacher to read the book.

| Examples of Diseases |

This is an example table.

Column 1	Column 2
Value 1	Value 2

| Huntington’s disease can be present even if it is found on only one allele.

| Approximately 25% of cases of SRNS (Steroid Resistant Nephrotic Syndrome)

This table shows the number of books read by each student

Student	Number of Books
John	10
Jane	5
Dave	15

Table: Number of Books Read by Each Student

Student	Number of Books
John	10
Jane	5
Dave	15

| Variant of Allele |

This table shows the average temperature in different cities

City	Temperature
Miami	82°F
Tokyo	68°F

This table shows the average temperature in different cities

City	Temperature
Miami	82°F
Tokyo	68°F

| Can show Codominance, Complete Dominance, or Incomplete Dominance

| Can be either homozygous recessive or homozygous dominant