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

A **gene** is a segment of DNA that codes for a specific protein or functional RNA, determining a particular trait. An **allele** is an alternative form or version of that specific gene, leading to variations in the trait. For example, the gene for eye color has alleles for blue, brown, or green eyes.

How does a genotype differ from a phenotype?

A **genotype** refers to the specific combination of alleles an individual possesses for a particular gene (e.g., 'AA', 'Aa', 'aa'). A **phenotype** is the observable, physical expression of that genotype, which can also be influenced by environmental factors. The genotype is the genetic code, while the phenotype is the resulting characteristic.

Explain the distinction between a homozygous and a heterozygous genotype.

A **homozygous** genotype means an individual has two identical alleles for a gene (e.g., both dominant 'AA' or both recessive 'aa'). A **heterozygous** genotype means an individual has two different alleles for a gene (e.g., one dominant and one recessive, 'Aa'). This difference determines how a trait is expressed, especially with dominant-recessive inheritance.

What common error occurs when assigning allele symbols for a single gene?

A common error is using different letters for alleles of the same gene (e.g., 'A' and 'B' for eye color). Correct practice dictates using the same letter, with a capital for the dominant allele and a lowercase for the recessive allele (e.g., 'B' for brown eyes and 'b' for blue eyes). This ensures clarity and consistency in genetic notation.

What mistake might lead to misinterpreting the phenotype of a heterozygous individual?

A common mistake is assuming that both alleles in a heterozygous genotype will contribute equally to the phenotype, or that the phenotype will be a blend. In simple dominant-recessive inheritance, the dominant allele completely masks the expression of the recessive allele, so only the dominant trait is observed. Misunderstanding this can lead to incorrect phenotypic predictions.

What is a common pitfall when considering the number of alleles an individual has for a gene?

A common pitfall is forgetting that diploid organisms, like humans, possess two alleles for each gene, not just one. Each parent contributes one allele to their offspring. Incorrectly assuming only one allele or more than two alleles can lead to errors in genetic crosses and inheritance predictions.

An allele is an alternative form or version of a gene. These different versions arise from minor variations in the DNA sequence of the gene and are responsible for the different expressions of a particular trait, such as different hair colors or blood types.

Define a dominant allele.

A dominant allele is a version of a gene that expresses its associated trait in the phenotype even when only one copy is present in the genotype. It masks the effect of a recessive allele if both are present, meaning the dominant trait will be observed in both homozygous dominant and heterozygous individuals.

What characterizes a recessive allele?

A recessive allele is a version of a gene that only expresses its associated trait in the phenotype when two copies of that allele are present in the genotype. If a dominant allele is also present, the recessive trait is not observed, as it is masked by the dominant allele.

What is a genotype in genetics?

A genotype is the specific combination of alleles an individual possesses for a particular gene. It represents the genetic constitution for a trait, often denoted by letter pairs like 'BB', 'Bb', or 'bb', and serves as the underlying genetic information that influences the phenotype.

Alleles | Pearson Edexcel IGCSE Science

Double Award / Biology

3. Reproduction & Inheritance

Alleles

Summary

Alleles are fundamental units of heredity, representing alternative forms of a gene that determine variations in inherited characteristics. They are located at specific positions on homologous chromosomes, with diploid organisms inheriting two alleles for each gene—one from each parent. The interaction of these alleles, particularly through dominance and recessiveness, dictates an organism's observable traits (phenotype) based on its genetic makeup (genotype).

1. Definition & Core Concepts

Alleles are defined as alternative forms or versions of the same gene. While a gene specifies a particular trait, such as eye color, different alleles of that gene account for the variations of that trait, like blue, brown, or green eyes.
These variations in alleles arise from small differences in their DNA base sequences. Even a single nucleotide change within the gene's sequence can result in a different allele, leading to a modified protein or regulatory element that affects the characteristic.
Each gene occupies a specific physical location on a chromosome, known as a locus. Alleles for a particular gene are found at the same locus on homologous chromosomes, ensuring that the genetic information for a trait is consistently located.
The existence of multiple alleles for a single gene is the primary source of genetic variation within a species. This variation is crucial for adaptation and evolution, allowing populations to respond to changing environmental pressures.

2. Inheritance & Diploidy

In diploid organisms, such as humans and most mammals, cells contain two complete sets of chromosomes, one inherited from each parent. This means that for every gene, there are two copies present in each somatic cell.
Consequently, for each gene, an individual inherits two alleles: one allele from the biological mother and one allele from the biological father. These two alleles reside on the pair of homologous chromosomes.
During sexual reproduction, specialized cells called gametes (sperm and egg) are formed through meiosis, each containing only one set of chromosomes, and thus only one allele for each gene. Fertilization then restores the diploid state, combining one allele from each parent.
The specific combination of these two inherited alleles for a given gene constitutes an individual's genotype for that trait, which in turn influences the observable characteristic.

3. Genotype and Phenotype

The genotype refers to the specific combination of alleles an organism possesses for a particular gene. It represents the genetic blueprint for a trait and is typically expressed using letter symbols, such as 'AA', 'Aa', or 'aa'.
The phenotype is the observable characteristic or trait that results from the expression of the genotype. This includes physical attributes like hair color, physiological traits like blood type, and even behavioral patterns.
While genotype determines the potential for a phenotype, the actual expression of the phenotype can also be influenced by environmental factors. For example, a plant's genotype might confer the potential for tall growth, but poor nutrition (an environmental factor) could result in a shorter phenotype.
Understanding the relationship between genotype and phenotype is central to genetics, as it explains how genetic information translates into the diversity of life forms we observe.

4. Dominance and Recessiveness

5. Homozygous and Heterozygous States

6. Genetic Notation & Representation

7. Significance in Variation