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An allele is a variant form of a gene. Humans, along with many other organisms, have two alleles for each gene: one inherited from each parent. These alleles can be identical or different and are located on corresponding positions on chromosomes.

Alleles contribute to the genetic diversity within a population and play a crucial role in determining an organism’s traits.

Types of alleles

Dominant alleles

A dominant allele is one that expresses its trait even when only one copy is present. If an individual carries one dominant allele and one recessive allele for a particular gene, the dominant trait will be expressed. For example, in the case of eye color, the allele for brown eyes is dominant over the allele for blue eyes.

Recessive alleles

Recessive alleles require two copies (one from each parent) to express their trait. If an individual carries only one recessive allele along with a dominant allele, the dominant trait will be expressed. For example, cystic fibrosis is caused by a recessive allele, and an individual must inherit two copies of this allele to exhibit the disease.

Codominant alleles

Codominant alleles occur when two different alleles are both fully expressed in a heterozygous individual. An example of codominance is the AB blood type in humans, where both A and B alleles are expressed equally, resulting in a distinct phenotype.

Incomplete dominance

Incomplete dominance is when the phenotype of heterozygous individuals is a blend of the two alleles. For example, if a plant with red flowers (dominant allele) is crossed with a plant with white flowers (recessive allele), the offspring might have pink flowers, showing a blend of both parental traits.

How alleles influence traits

Genetic variation

Alleles are fundamental to genetic variation, which is the diversity of gene frequencies within a population. This variation allows for adaptation and evolution as different alleles can be advantageous in changing environments.

Phenotype expression

The combination of alleles that an individual possesses determines their phenotype, which is the observable physical or biochemical characteristics of an organism. For instance, in pea plants, the allele for tallness is dominant, so a plant with at least one tall allele will be tall, while it will only be short if it has two short alleles.

Mendelian inheritance

Gregor Mendel’s contribution

The concept of alleles was first introduced by Gregor Mendel through his work on pea plants. Mendel’s laws of inheritance describe how alleles segregate and assort independently, leading to genetic variation in offspring.

Law of segregation

Mendel’s Law of Segregation states that during the formation of gametes (egg and sperm cells), the two alleles for a gene separate, so each gamete carries only one allele for each gene. This law explains why offspring inherit one allele from each parent.

Law of Independent Assortment

The Law of Independent Assortment states that alleles for different genes assort independently of each other during gamete formation. This means the inheritance of one trait generally does not influence the inheritance of another trait, allowing for genetic diversity.

Modern applications of allele research

Genetic testing

Allele analysis is a cornerstone of genetic testing, used to identify carriers of genetic diseases, understand ancestry, and even predict responses to certain medications. By examining specific alleles, scientists can determine the likelihood of an individual developing a genetic disorder or passing it on to their offspring.

Gene therapy

Gene therapy aims to treat or prevent diseases by introducing, removing, or altering genetic material within a person’s cells. By targeting specific alleles, gene therapy can potentially correct defective genes responsible for diseases.

Personalized medicine

Personalized medicine uses information about an individual’s genetic makeup, including their alleles, to tailor medical treatments. For example, knowing the alleles that influence drug metabolism can help doctors prescribe the most effective medication and dosage for a patient.

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