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IB DP Biology Study Notes

3.1.2 Alleles

Alleles are various forms of a gene, reflecting different versions of a particular trait. This page will delve into the concept of alleles, the difference between dominant and recessive alleles, and how they contribute to genetic diversity within a population.

Understanding Alleles

  • Alleles: Different versions of a gene that are located at the same position on a chromosome.
  • They correspond to various possible expressions of a particular trait that an individual organism may exhibit.

Types of Alleles

Dominant Alleles

  • Represented by a capital letter (e.g., A).
  • Traits coded by a dominant allele are expressed when at least one copy is present.
  • Dominant alleles mask the effect of any recessive alleles that might be present.
  • Example: If the dominant allele codes for brown eyes, having even one copy will result in brown eyes.

Recessive Alleles

  • Represented by a lowercase letter (e.g., a).
  • Traits coded by a recessive allele are only expressed when two copies are present.
  • Recessive traits are only apparent when no dominant alleles are present for that trait.
  • Example: If the recessive allele codes for blue eyes, two copies (aa) are required for blue eyes to be expressed.

The Role of Alleles in Genetic Diversity

Inheritance Patterns

  • Offspring inherit one allele from each parent for a specific trait.
  • This combination results in various genetic possibilities and is responsible for the unique genetic makeup of an individual.

Genetic Diversity Within a Population

  • Different combinations of alleles add to genetic diversity within a population.
  • The presence of various alleles can lead to many different phenotypes, adding to the complexity and variety of a population.
  • Genetic diversity is essential for a population's resilience and adaptability.

Homozygous and Heterozygous Alleles


  • Refers to having two identical alleles for a specific trait (e.g., AA or aa).
  • Homozygous Dominant (AA): Two dominant alleles; the dominant trait is expressed.
  • Homozygous Recessive (aa): Two recessive alleles; the recessive trait is expressed.


  • Having two different alleles for a specific trait (e.g., Aa).
  • The dominant trait is expressed, but the individual carries the hidden recessive trait.

Punnett Squares and Allele Prediction

Utilising Punnett Squares

  • Punnett Squares are diagrams used to predict the genetic outcomes of a particular cross.
  • They demonstrate how alleles segregate and assort independently according to Mendelian genetics.

Examples of Predicting Alleles

  • A cross between two heterozygous individuals (Aa x Aa) will result in various allele combinations.
  • Probabilities: 25% AA (homozygous dominant), 50% Aa (heterozygous), 25% aa (homozygous recessive).

Co-dominance and Incomplete Dominance


  • A scenario where both alleles for a gene are equally strong and both alleles are visible in the phenotype.
  • Example: In certain flower species, a cross between a red-flowered plant and a white-flowered plant may result in flowers with both red and white areas.

Incomplete Dominance

  • Neither allele is completely dominant over the other, leading to a phenotype that's a blend of the two alleles.
  • Example: In snapdragons, a cross between a red-flowered plant and a white-flowered plant might produce pink flowers.

Alleles and Evolution

  • Alleles are central to the process of evolution, with new alleles arising through mutation and recombination.
  • The frequency of different alleles within a population can change over time due to natural selection, genetic drift, and other evolutionary factors.
  • The study of allele frequencies helps scientists understand how populations are evolving and adapting to their environments.


In diploid organisms like humans, individuals typically possess two alleles for a single gene, one from each parent. However, in some cases, more than two alleles can exist in a population. This phenomenon is known as multiple allelism. While an individual may still only inherit two alleles, the presence of multiple alleles in a population contributes to increased genetic diversity.

Meiosis is a specialized form of cell division that occurs in the formation of gametes (sperm and egg cells). During meiosis, homologous chromosomes pair up and exchange genetic material through crossing over. This recombination of genetic material creates new combinations of alleles on each chromosome. When gametes fuse during fertilization, the resulting zygote inherits a unique combination of alleles from both parents, leading to genetic variation within a population.

Yes, the frequency of alleles in a population can change over time due to various factors. Natural selection is one of the primary drivers of allele frequency changes, where certain alleles confer advantages in survival and reproduction, leading to their increased prevalence. Genetic drift, the random change in allele frequencies due to chance events, can also play a role, particularly in smaller populations. Migration and gene flow between populations can introduce new alleles or alter existing allele frequencies. Additionally, mutation and recombination during reproduction contribute to genetic variation and may impact allele frequencies in a population.

Environmental factors can influence allele expression through a process called gene regulation. External cues, such as temperature, light, or nutrient availability, can activate or suppress specific genes. Epigenetic modifications, such as DNA methylation, can also be influenced by environmental factors and affect how genes are expressed. Additionally, environmental stressors can induce mutations, altering the DNA sequence and potentially leading to the formation of new alleles. The interplay between genetic factors and the environment contributes to the complex expression of traits in living organisms.

A gene mutation is a change in the DNA sequence of a gene. Mutations can be caused by various factors, such as environmental influences or errors during DNA replication. Mutations can lead to the formation of new alleles when they alter the genetic code, resulting in a different version of the gene. These new alleles may code for slightly different traits, contributing to genetic diversity within a population.

Practice Questions

Explain the difference between dominant and recessive alleles, and illustrate how they are represented in a Punnett Square with an example of a trait. Include how homozygous and heterozygous genotypes are expressed.

Dominant alleles mask the effect of recessive alleles and are represented by a capital letter (e.g., A), while recessive alleles are represented by a lowercase letter (e.g., a) and are only expressed when two copies are present. In a Punnett Square, a cross between two heterozygous individuals (Aa x Aa) can be represented, leading to genotypes 25% AA (homozygous dominant, dominant trait expressed), 50% Aa (heterozygous, dominant trait expressed), and 25% aa (homozygous recessive, recessive trait expressed).

Discuss how alleles contribute to genetic diversity within a population, and explain why this diversity is vital for a population's survival.

Alleles contribute to genetic diversity by allowing various possible expressions of particular traits within a population. The combination of different alleles leads to numerous phenotypes, increasing the complexity and variability of the population. This diversity is vital as it enables the population to adapt to changing environmental conditions, thereby enhancing resilience. A genetically diverse population is more likely to contain individuals with traits that allow them to survive in new or altered conditions, facilitating the survival of the population as a whole and potentially leading to evolutionary changes.

Dr Shubhi Khandelwal avatar
Written by: Dr Shubhi Khandelwal
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