Key Differences Between Heterozygous and Homozygous Traits
Heterozygous means that an organism inherits different variants of the specific gene from both parents. Furthermore, a heterozygous genotype differs from a homozygous genotype. This is because a person with a homozygous genotype inherits identical copies of a gene from both parents.
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A heterozygote is represented by a capital letter as well as a lower case letter. As a result, Rr or Ss are used to represent it. Experts also consider that a heterozygote for gene R is Rr. The uppercase letter takes precedence in this case.
Heterozygote advantage is a circumstance in which the heterozygous genotype has a distinct benefit. The heterozygous genotype has greater relative fitness than the homozygous dominant or homozygous recessive genotypes in this case. Furthermore, heterozygote advantage is limited to a single gene in this scenario. Furthermore, this is a case of overdominance.
Heterozygous Example
The two different alleles in a heterozygous genotype interact with one another. This affects how their characteristics are expressed.
This encounter is frequently based on dominance. "Dominant" refers to the allele that is expressed more strongly, whereas "recessive" refers to the other. The dominant allele covers the recessive allele. Depending on how the dominant and recessive genes interact, a heterozygous genotype may include:
Complete Dominance
The heterozygous dominant allele entirely covers the recessive one in total dominance. The recessive allele does not have any effect.
Eye colour, for example, is influenced by a number of genes. The brown eye allele is more common than the blue eye gene. Brown eyes are the result of having one of each. You do, however, carry the recessive mutation for blue eyes. It's probable that your child will have blue eyes if you reproduce with someone who shares the same allele.
Incomplete Dominance
When the dominant allele doesn't overcome the recessive one, it's called incomplete dominance. Instead, they integrate together, resulting in a third characteristic.
Hair texture is a good example of this type of dominance. You'll have wavy hair if you have one allele for curly hair and one for straight hair. Curly and straight hair combine to create waviness.
Codominance
When both alleles are present at the same time, codominance occurs. They don't, however, blend in. Both features are present in equal measure.
Codominance is illustrated by the blood type AB. In this scenario, you have one allele for type A blood and one for type B blood. Both alleles produce both types of blood, rather than combining to form a third type. Type AB blood is the result of this.
Differences Between Heterozygous and Homozygous
Heterozygous Genes and Disease
A mutant allele can result in genetic disorders. Because the mutation changes the way DNA is expressed, this is the case. The mutant allele may be dominant or recessive, depending on the situation. If it's dominant, it suggests that sickness can be caused by just one mutant copy. A "dominant sickness" or "dominant disorder" is what this is referred to as.
You're more likely to get a dominant condition if you're heterozygous for it. If you're heterozygous for a recessive mutation, on the other hand, you won't acquire it. You become a carrier when the typical allele takes over. As a result, it's possible that your children will contract it.
The Following are Some Examples of Dominating Diseases:
Huntington’s Disease
Huntingtin, a protein associated with nerve cells in the brain, is produced by the HTT gene. Huntington's disease is a neurological condition caused by a mutation in this gene. An individual with just one copy of the mutant gene will develop Huntington's disease because the defective gene is dominant. This degenerative brain disease, which most usually manifests in adults, can lead to:
uncontrollable motions
emotional problems
low mental abilities
Having difficulty walking, speaking, or swallowing
Marfan’s Syndrome
Marfan's syndrome affects the connective tissue, which gives the body's structures strength and shape. Symptoms of a genetic disease include:
Scoliosis is an abnormally curved spine.
expansion of some bones in the arms and legs
nearsightedness
difficulties with the aorta, the artery that transports blood from your heart to your body
the rest of your body to your heart
A mutation in the FBN1 gene is linked to Marfan's syndrome. To cause the disease, only one mutant variation is required.
Familial Hypercholesterolemia
Familial hypercholesterolemia (FH) is caused by a mutant copy of the APOB, LDLR, or PCSK9 gene in heterozygous genotypes. It's quite prevalent, affecting 1 in 200 to 250 people, according to Trusted Source. FH raises LDL cholesterol levels to dangerously high levels, increasing the risk of coronary heart disease at a young age.
Assume that being heterozygous for a gene means that you have two different versions of that gene. The dominant form can totally obscure the recessive one, or they can merge. Both versions may display at the same time in some circumstances. The two genes can interact in a variety of ways. Their relationship is what determines your physical characteristics, blood type, and other characteristics that define who you are.
Do you know?
What is the probability of the heterozygous offspring? The following are the probabilities of the outcome: 50 % dominant allele x 50 % recessive allele = 25% chance that both offspring's alleles are dominant. Both of the offspring's alleles have a 50 % x 50 % = 25 % chance of becoming recessive. The probability of the kids being heterozygous is 50 % x 50 % + 50 % x 50 % = 25 % + 25 % = 50 %.
FAQs on What Does Heterozygous Mean in Biology?
1. What does the term heterozygous mean in genetics?
In genetics, heterozygous refers to a state where an organism has two different alleles (versions) for a particular gene, with one allele inherited from each parent. For example, if the gene for flower colour has a dominant allele for red (R) and a recessive allele for white (r), a heterozygous plant would have the genotype Rr.
2. What is the main difference between a heterozygous and a homozygous genotype?
The main difference lies in the alleles present for a specific gene. A heterozygous genotype consists of two different alleles (e.g., Aa), while a homozygous genotype consists of two identical alleles (e.g., AA or aa). This difference determines how traits are expressed; a heterozygous individual can pass on two different alleles to its offspring, whereas a homozygous individual can only pass on one type.
3. How can you identify a heterozygous genotype from its notation, for example, Tt vs TT?
You can identify a heterozygous genotype by observing the letters used to represent the alleles. A heterozygous genotype is always represented by a combination of a capital letter (for the dominant allele) and a lowercase letter (for the recessive allele).
- Tt is heterozygous because it has two different alleles (T and t).
- TT is homozygous dominant because it has two identical dominant alleles.
- tt is homozygous recessive because it has two identical recessive alleles.
4. What is the role of heterozygous genotypes in understanding genetic disorders like Huntington's disease?
Huntington's disease is an autosomal dominant disorder. This means an individual only needs to inherit one copy of the faulty gene to develop the condition. Therefore, a person with a heterozygous genotype (e.g., Hh) will have the disease. This is a crucial concept as it explains why the disorder can appear in every generation of a family, as a heterozygous parent has a 50% chance of passing the dominant faulty allele to each child.
5. Does a heterozygous individual always show the dominant trait?
No, not always. While in cases of complete dominance (as studied by Mendel), a heterozygous individual (like Tt) expresses the dominant phenotype, there are other patterns of inheritance:
- Incomplete Dominance: The heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, a cross between a red (RR) and white (rr) snapdragon flower results in a pink (Rr) flower.
- Co-dominance: Both alleles in the heterozygous genotype are fully and simultaneously expressed. A classic example is the AB blood group in humans, where both A and B alleles are expressed.
6. Can you explain the concept of 'heterozygote advantage' using the example of sickle-cell anaemia?
Heterozygote advantage is a situation where having a heterozygous genotype provides a survival benefit over both homozygous genotypes. The best example is sickle-cell trait in regions with high malaria rates.
- Individuals homozygous recessive (ss) have severe sickle-cell anaemia.
- Individuals homozygous dominant (SS) have normal blood cells but are highly susceptible to malaria.
- Heterozygous individuals (Ss) have the sickle-cell trait but are largely asymptomatic. Crucially, they show increased resistance to malaria. This survival advantage allows the 's' allele to persist in the population.
7. How does being heterozygous differ from being hemizygous?
These terms describe the number of alleles an organism possesses for a gene, but in different contexts. Heterozygous means having two different alleles for a gene on homologous chromosomes (e.g., Rr). In contrast, hemizygous means having only a single copy of a gene. This is common in males for genes on the X and Y chromosomes, as they have only one X and one Y chromosome (XY), not a homologous pair. Therefore, a male cannot be heterozygous or homozygous for most X-linked traits; he is hemizygous.
8. How does Mendel's Law of Segregation apply to a heterozygous parent?
Mendel's Law of Segregation is perfectly demonstrated by a heterozygous parent. The law states that during gamete formation (meiosis), the two alleles for a heritable character separate or segregate from each other so that each gamete ends up with only one allele. For a heterozygous parent with the genotype Rr:
- The two alleles, R and r, will segregate during meiosis.
- As a result, 50% of the gametes produced will carry the dominant allele (R), and the other 50% will carry the recessive allele (r).
