What is an Allele | Types | Mutations | Examples | Genetics

What is an Allele

An allele is a variant form of a gene that determines specific traits in an organism. Genes are sections of DNA located on chromosomes, and alleles represent different versions of the same gene that can produce variations in physical or biochemical traits. 

Each individual inherits two alleles for each gene, one from each parent. Alleles can be identical or different in an individual, influencing how a trait is expressed.

Types of Alleles

1. Dominant Allele

A dominant allele is an allele that expresses its effect even when only one copy is present. If an organism has one dominant allele and one recessive allele, the dominant allele will mask the expression of the recessive allele. For example, in pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p).

2. Recessive Allele

A recessive allele is only expressed when an organism has two copies of it (homozygous recessive). If a dominant allele is present, it will overshadow the recessive allele. For instance, in humans, the allele for blue eyes (b) is recessive to the allele for brown eyes (B).

3. Co-Dominant Alleles

Co-dominant alleles are both expressed equally in a heterozygous individual. Neither allele is dominant or recessive. A well-known example is the AB blood type, where both A and B alleles are expressed together.

4. Incomplete Dominance Alleles

In incomplete dominance, the heterozygous phenotype is a blend of both alleles rather than one being dominant over the other. For example, in snapdragon flowers, a red-flowered plant (RR) crossed with a white-flowered plant (WW) produces pink flowers (RW).

5. Multiple Alleles

Some genes have more than two possible alleles, known as multiple alleles. A classic example is the human ABO blood group, which has three alleles: A, B, and O.

Homozygous and Heterozygous Alleles

• Homozygous: When an individual has two identical alleles for a gene (e.g., BB or bb).
• Heterozygous: When an individual has two different alleles for a gene (e.g., Bb).

Examples of Alleles in Humans

• Eye Color: Brown (B) is dominant over blue (b).
• Blood Type: A, B, and O alleles determine blood groups.
• Widow’s Peak: The presence of a widow’s peak (W) is dominant over a straight hairline (w).

Mutations and Alleles

Alleles arise due to mutations, which are changes in DNA sequences. Mutations can create new alleles, leading to genetic variation within populations. Some mutations may cause diseases, such as the sickle cell allele (HbS), which can lead to sickle cell anemia.

Effects of Mutations on Alleles

• Beneficial Mutations: Some mutations create advantageous alleles, leading to better survival (e.g., lactose tolerance in humans).
• Harmful Mutations: Certain mutations result in defective alleles, causing genetic disorders (e.g., sickle cell anemia, cystic fibrosis).
• Neutral Mutations: Some mutations do not affect an organism’s traits or survival.

Types of Mutations Affecting Alleles

• Point Mutation: A single nucleotide change that may create a new allele.
• Deletion or Insertion: Loss or addition of DNA bases, potentially altering gene function.
• Chromosomal Mutations: Large-scale changes affecting multiple genes.

Conclusion

Alleles are fundamental units of genetic variation that determine traits in all living organisms. They can be dominant, recessive, co-dominant, or exhibit other forms of inheritance. Understanding alleles helps in studying genetics, evolution, and hereditary diseases.

Some Questions and Answers

1. What is an allele?

A. An allele is a different version of a gene that determines variations in traits, such as eye color or blood type.

2. How do dominant and recessive alleles differ?

A. A dominant allele expresses its trait even if only one copy is present, while a recessive allele requires two copies to be expressed.

3. What is an example of a co-dominant allele?

A. The ABO blood group system, where both A and B alleles are expressed in individuals with AB blood type.

4. How do mutations affect alleles?

A. Mutations create new alleles by altering DNA sequences, which can lead to beneficial, harmful, or neutral changes in an organism.

5. What is an example of a genetic disorder caused by a mutation?

A. Sickle cell anemia is caused by a mutation in the hemoglobin gene, leading to misshapen red blood cells.