What is heterozygous and its examples? | Heterozygous

Heterozygous definition

Your genes are made of DNA. This DNA provides instructions, which determines traits like your hair color and blood type.

There are different versions of genes. Each version is called an allele. For each gene, you inherit two alleles: one from your biological father and one from your biological mother. Together, these alleles are called a genotype.

If the two versions are different, you have a heterozygous genotype for that gene. For example, being heterozygous for hair color could mean you have one allele for red hair and one allele for brown hair.

The relationship between the two alleles affects which traits are expressed. It also determines what characteristics you’re a carrier for.

Let’s explore what it means to be heterozygous and the role it plays in your genetic makeup.

Before we dive into a detailed explanation of Heterozygous condition first, we have to understand that

 how these heterozygous genes are formed?

Without hybridization the formation of heterozygous gene is impossible. Here we understand with the example.

For example, the mother have a gene for the expression for the blue eye color which is a recessive gene but in a homozygous condition so both alleles are should be (bb), whereas father has a gene for the expression of black color eye which is a dominant gene but also in a homozygous condition. So, both alleles should be (BB).

Now when they have child, they carry gene from the both parent so the child has the gene for eye color is (Bb). But there is the allele for the black color eye is dominant so the child has the black eye. But child also carries gene for the blue eye. This gene not expressed because of the dominant gene mask their effect.

Here, this child is having a heterozygous gene, now I think you understand how the heterozygous gene are produced.

Let’s move on to the how this heterozygous gene is expressed in the latter progeny?

Heterozygous Genotypic Ratios

When organisms that are heterozygous for certain traits reproduce, expected ratios of these traits can be predicted in the resulting offspring. The expected genotypic (based on genetic makeup) and phenotypic (based on observable characteristics) ratios vary depending on parental genes. Let’s discuss this using the above example, the allele for black color (B) is dominant to the blue eye color (b) trait. In a monohybrid cross between heterozygous people for black eye color (Bb), the expected genotypes are (BB), (Bb), and (Bb).

The expected genotypic ratio is 1:2:1. Half of the offspring will be heterozygous (Bb), one-fourth will be homozygous dominant (BB), and one-fourth will be homozygous recessive. The phenotypic ratio is 3:1. Three-fourths of the offspring will have Black eye (BB, Bp) and one-fourth will have blue eye (bb).

In a cross between a heterozygous mother with black eye  and a recessive father with blue eye, the expected genotypes observed in the offspring will be (Bb) and (bb). The expected genotypic ratio is 1:1

File:Punnett hetero homoblue.svg - Wikimedia Commons
Heterozygous Cross

Half of the offspring will be heterozygous (Bb) and half will be homozygous recessive (bb). The phenotypic ratio will also be 1:1. Half will exhibit the black eye (Bb) trait and half will have white-eye (bb).

What if the Genotype is unknown?

When the genotype is unknown, this type of cross is performed as a test cross. Because both heterozygous organisms (Pp) and homozygous dominant organisms (PP) exhibit the same phenotype (violet-colored petals), a cross with a plant is recurrent to observation (pp). Characterization (white) can be used to determine the unknown plant phenotype. If the genotype of the unknown plant is heterozygous, half of the offspring will have the dominant trait (purple), and the other half will show recurring traits (white). If the genotype of the unknown plant is homozygous dominant (PP), then all the offspring will be heterozygous (Pp) and have purple petals.

Examples of Heterozygouse condition

In a heterozygous genotype, the two different alleles interact with each other. This determines how their traits are expressed.

Commonly, this interaction is based on dominance. The allele that’s expressed more strongly is called “dominant,” while the other is called “recessive.” This recessive allele is masked by the dominant one.

Depending on how the dominant and recessive genes interact, a heterozygous genotype might involve:

Complete dominance of allele

In complete dominance, the dominant allele completely covers up the recessive one. The recessive allele isn’t expressed at all.

One example is eye color, which is controlled by several genes. The allele for brown eyes is dominant to the one for blue eyes. even you have one of each, you will have brown eyes.

However, you still have the recessive allele for blue eyes. If you reproduce with someone who has the same allele, it’s possible that your child will have blue eyes.

Incomplete dominance of allele

Incomplete dominance occurs when the dominant allele doesn’t overrule the recessive one. Instead, they are mixed together, which creates the third symptom.

This type of dominance is often seen in hair texture. If you have one allele for curly hair and one for straight hair, you’ll have wavy hair. The waviness is a combination of curly and straight hair.

Codominance expression of alleles

Codominance happens when the two alleles are represented at the same time. They don’t blend together, though. Both traits are equally expressed.

An instance of codominance is the AB blood group. In this case, you have one allele for type A blood and one for type B. Instead of blending and creating a third type, both alleles make both types of blood. This results in type AB blood.

Heterozygous genes and disease

A mutated allele can cause genetic conditions. That’s because the mutation alters how DNA is expressed.

Depending on the condition, the mutated allele might be dominant or recessive. If it’s dominant, it means only one mutated copy is needed to result in disease. This is called “major illness” or “major disorder”.

If you are heterozygous for a major disorder, you are at greater risk of developing it. On the other hand, if you’re heterozygous for a recessive mutation, you won’t get it. The normal allele takes over and you’re simply a carrier. This means your children may get it.

Examples of dominant diseases include:

Huntington’s disease

The HTT gene produces huntingtin, a protein that’s related to nerve cells in the brain. A mutation in this gene causes Huntington’s disease, a neurodegenerative disorder.

Since the mutated gene is predominant, only one copy will develop Huntington’s disease.

This progressive brain condition, commonly seen in adulthood, may be caused by:

  • involuntary movements
  • emotional issues
  • poor cognition
  • trouble walking, speaking, or swallowing

Marfan’s syndrome

Marfan’s syndrome involves the connective tissue, which provides strength and form to the body’s structures. The genetic disorder may cause symptoms like:

  • abnormally curved spine, or scoliosis
  • overgrowth of a certain arm and leg bones
  • nearsightedness
  • problems with the aorta, which is the artery that brings blood from your heart to the rest of your body

Marfan’s syndrome is associated with a mutation of the FBN1 gene. Again, only one mutated variant is required to cause the condition.

Familial hypercholesterolemia

Familial hypercholesterolemia (FH) occurs in heterozygous genotypes with a mutated copy of the APOB, LDLR, or PCSK9 gene. It’s quite common, affecting 1 in 200 to 250Trusted Source people.

FH causes extremely high LDL cholesterol levels, which increases the risk of coronary artery disease at an early age.

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