What happens if two heterozygous individuals are crossed?

updated: September 6, 2022

Definition

Heterozygous, as related to genetics, refers to having inherited different versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a genomic marker has two different versions of that marker. By contrast, an individual who is homozygous for a marker has identical versions of that marker.

Narration

Heterozygous. In diploid species, there are two alleles for each trait of genes in each pair of chromosomes, one coming from the father and one from the mother. An allele is one of two or more alternative forms of a gene, and they are found at the same place, or locus, on the chromosome. Heterozygous refers to having different alleles for a particular trait. If the two versions are different, you have a heterozygous genotype for that gene. The relationship between the two alleles affects which traits are expressed.

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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 every 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.

A homozygous genotype is the opposite of a heterozygous genotype.

If you’re homozygous for a particular gene, you inherited two of the same alleles. It means your biological parents contributed identical variants.

In this scenario, you may have two normal alleles or two mutated alleles. Mutated alleles can result in a disease and will be discussed later. This also affects which characteristics appear.

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

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. If you have one of each, you’ll 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

Incomplete dominance occurs when the dominant allele doesn’t overrule the recessive one. Instead, they blend together, which creates a third trait.

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

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

An example of codominance is the AB blood type. 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.

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 a “dominant disease” or “dominant disorder.”

If you’re heterozygous for a dominant disorder, you have a higher 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 dominant, a person with just one copy will develop Huntington’s disease. This progressive brain condition, which commonly shows up in adulthood, may cause:

• 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:

• abnormal curved spine, or scoliosis
• overgrowth of 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 250 people.

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

When you’re heterozygous for a specific gene, it means you have two different versions of that gene. The dominant form can completely mask the recessive one, or they can blend together. In some cases, both versions appear at the same time.

The two different genes can interact in various ways. Their relationship is what controls your physical features, blood type, and all the traits that make you who you are.