What is the fundamental difference between a gene and an allele?

A gene is a section of DNA that codes for a general trait, while an allele is a specific version of that gene that determines a particular variation of the trait. For example, a gene might code for flower color, whereas different alleles specify red or white flowers.

How does the genotype differ from the phenotype of an organism?

The genotype is the internal genetic makeup or combination of alleles (e.g., $Bb$), whereas the phenotype is the observable physical characteristic (e.g., brown eyes). The phenotype results from the genotype's interaction with environmental factors.

Contrast homozygous and heterozygous genotypes.

A homozygous genotype consists of two identical alleles for a gene (either $AA$ or $aa$), while a heterozygous genotype contains two different alleles (e.g., $Aa$). Homozygous individuals are 'pure-breeding' for that trait, whereas heterozygous individuals can produce offspring with different phenotypes.

Why is it a common mistake to assume that dominant alleles are the most common in a population?

Dominance refers only to how an allele is expressed in a heterozygote, not its frequency in the population. Some dominant alleles are extremely rare (e.g., Huntington's disease), while certain recessive alleles are very common due to evolutionary advantages.

What error occurs if you assume a recessive allele has 'disappeared' in a heterozygous individual?

Recessive alleles are not destroyed or altered in a heterozygote; they are simply 'masked' in the phenotype. The recessive allele remains part of the genotype and can be passed on to future generations, where it may reappear in a homozygous recessive offspring.

How does confusing the allele symbol with the phenotype lead to incorrect genetic predictions?

Symbols like $B$ or $b$ represent alleles in the DNA, not the trait itself. If a student assumes $B$ means 'the trait is present' without checking the zygosity, they may fail to account for the inheritance of recessive traits in offspring from $Bb \times Bb$ crosses.

Define the term 'allele' in a biological context.

An allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place (locus) on a chromosome. Alleles are responsible for the specific variations in inherited characteristics among individuals.

What is meant by 'codominance' in inheritance?

Codominance is a pattern of inheritance where both alleles in a heterozygous individual are fully and equally expressed in the phenotype. This results in an organism showing traits from both alleles simultaneously, such as the AB blood group in humans.

What is a 'locus' in genetics?

A locus is the specific, fixed position on a chromosome where a particular gene or one of its alleles is located. Because homologous chromosomes come in pairs, a diploid organism has two loci for every gene, one on each chromosome of the pair.

Why do diploid organisms possess exactly two alleles for every gene?

Diploid organisms have two complete sets of chromosomes, one inherited from each parent. Since genes are located on these chromosomes, the organism inherits one version (allele) of every gene from the mother and one version from the father.

Alleles | Pearson Edexcel IGCSE Biology

3. Reproduction & Inheritance

Alleles

Summary

Alleles are variant forms of a gene that arise by mutation and are found at the same place on a chromosome. They determine the specific expression of traits in an organism through various inheritance patterns such as complete dominance, codominance, and polygenic interaction.

1. Definition & Core Concepts

Allele Fundamentals: An allele is a specific version of a gene that codes for a particular variation of a trait. While a gene might control eye color in general, different alleles determine the specific color, such as blue or brown. These variations result from minor differences in the base sequence of DNA within the gene.
Chromosomal Locus: Alleles exist in pairs within diploid organisms, occupying the same position (locus) on homologous chromosomes. One allele is inherited from each biological parent, creating a dual-genetic blueprint for every inherited characteristic. This pairing is the basis for genetic diversity and inheritance patterns.
Allele Origin: New alleles are created through the process of mutation, which involves random changes in the DNA sequence. While many mutations have no observable effect, some alter the resulting protein's structure or function, leading to a new phenotype that may be advantageous, harmful, or neutral.

Diagram showing a pair of homologous chromosomes with corresponding allele loci marked as A and a.

2. Genotype and Phenotype Relationships

3. Principles of Dominance

Complete Dominance: In many traits, one allele is dominant and masks the presence of a recessive allele in a heterozygote. The dominant allele is expressed in the phenotype even if only one copy is present (genotype $Aa$ ). The recessive allele is only expressed when the organism is homozygous recessive ( $aa$ ).
Codominance: Codominance occurs when both alleles in a heterozygote are expressed equally in the phenotype. Neither allele masks the other; instead, the organism displays traits associated with both alleles simultaneously. A classic example is the $AB$ blood type, where both $A$ and $B$ antigens are produced on the red blood cells.
Incomplete Dominance: This pattern occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For instance, if a red-flowered plant and a white-flowered plant produce pink offspring, the alleles are showing incomplete dominance rather than one being strictly dominant.

4. Key Distinctions

5. Polygenic Inheritance

6. Exam Strategy & Tips

Alleles

Summary

1. Definition & Core Concepts

Allele Fundamentals: An allele is a specific version of a gene that codes for a particular variation of a trait. While a gene might control eye color in general, different alleles determine the specific color, such as blue or brown. These variations result from minor differences in the base sequence of DNA within the gene.
Chromosomal Locus: Alleles exist in pairs within diploid organisms, occupying the same position (locus) on homologous chromosomes. One allele is inherited from each biological parent, creating a dual-genetic blueprint for every inherited characteristic. This pairing is the basis for genetic diversity and inheritance patterns.
Allele Origin: New alleles are created through the process of mutation, which involves random changes in the DNA sequence. While many mutations have no observable effect, some alter the resulting protein's structure or function, leading to a new phenotype that may be advantageous, harmful, or neutral.

Diagram showing a pair of homologous chromosomes with corresponding allele loci marked as A and a.

2. Genotype and Phenotype Relationships

Genotype Definition: The genotype refers to the specific combination of alleles an individual possesses for a particular gene. It is typically represented by letters, such as $AA$ , $Aa$ , or $aa$ , reflecting the underlying genetic composition. The genotype serves as the internal 'blueprint' that determines the potential traits of the organism.
Phenotype Expression: The phenotype is the observable physical or functional characteristic of an organism, such as hair texture or blood type. It is the result of the genotype's interaction with the environment. While the genotype is fixed at fertilization, the phenotype may change over time due to external factors like nutrition or sunlight.
Zygosity States: An organism is homozygous if it possesses two identical alleles (e.g., $BB$ or $bb$ ) and heterozygous if it has two different alleles (e.g., $Bb$ ). In a heterozygous state, the relationship between the two different alleles determines which trait is physically expressed in the phenotype.

3. Principles of Dominance

Complete Dominance: In many traits, one allele is dominant and masks the presence of a recessive allele in a heterozygote. The dominant allele is expressed in the phenotype even if only one copy is present (genotype $Aa$ ). The recessive allele is only expressed when the organism is homozygous recessive ( $aa$ ).
Codominance: Codominance occurs when both alleles in a heterozygote are expressed equally in the phenotype. Neither allele masks the other; instead, the organism displays traits associated with both alleles simultaneously. A classic example is the $AB$ blood type, where both $A$ and $B$ antigens are produced on the red blood cells.
Incomplete Dominance: This pattern occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For instance, if a red-flowered plant and a white-flowered plant produce pink offspring, the alleles are showing incomplete dominance rather than one being strictly dominant.

4. Key Distinctions

Comparison of Zygosity: Understanding the behavior of different allele combinations is crucial for predicting inheritance outcomes.

Feature	Homozygous	Heterozygous
Allele Pairing	Two identical alleles (e.g., $AA$ , $aa$ )	Two different alleles (e.g., $Aa$ )
Phenotype	Expresses the trait of the identical alleles	Usually expresses the dominant trait
Breeding	Pure-breeding (produces consistent offspring)	Non-pure-breeding (offspring will vary)

Dominance vs. Codominance: | Aspect | Dominance | Codominance | | --- | --- | --- | | Heterozygote Appearance | Shows only the dominant phenotype | Shows both phenotypes simultaneously | | Allele Interaction | One allele masks the other | Both alleles are fully functional | | Example | Pea plant height (Tall vs. Short) | Human Blood Groups ( $A$ , $B$ , $AB$ ) |

5. Polygenic Inheritance

Multi-Gene Control: Many complex characteristics, such as human height or skin tone, are polygenic, meaning they are controlled by the interaction of multiple genes rather than a single gene pair. Each gene involved may have several alleles that contribute incrementally to the final phenotype.
Continuous Variation: Polygenic traits typically show a continuous range of phenotypes (a bell curve distribution) rather than distinct, discrete categories. Because so many allele combinations are possible across different loci, individuals exhibit a wide spectrum of variations between the extremes of a trait.
Environmental Influence: Polygenic characteristics are often highly sensitive to environmental factors. For example, while genes set the potential range for an individual's growth, nutrition and health during development determine where within that range the final phenotype will fall.

6. Exam Strategy & Tips

Notation Standards: Always use a single letter to represent a gene, using the capital form for the dominant allele and the lowercase form for the recessive allele. Avoid using different letters for the same gene (e.g., do not use $T$ for tall and $s$ for short; use $T$ and $t$ ) to prevent confusion during Punnett square analysis.
Predicting Ratios: When crossing two heterozygotes ( $Aa \times Aa$ ), expect a phenotypic ratio of 3:1 (dominant to recessive) and a genotypic ratio of 1:2:1 ( $AA:Aa:aa$ ). Recognizing these standard ratios can help you verify your work quickly during exam questions.
Common Verification: Always double-check if a question mentions 'pure-breeding' or 'true-breeding,' as this implies the individual is homozygous. If a recessive phenotype appears in the offspring of two dominant-looking parents, both parents must be heterozygous carriers of the recessive allele.

Feature

Homozygous

Heterozygous

Allele Pairing

Two identical alleles (e.g.,

AA

aa

)

Two different alleles (e.g.,

Aa

)

Phenotype

Expresses the trait of the identical alleles

Usually expresses the dominant trait

Breeding

Pure-breeding (produces consistent offspring)

Non-pure-breeding (offspring will vary)