11.3 Genes and Alleles
Every chromosome carries numerous genes that determine specific characteristics of an organism. On a pair of homologous chromosomes, these genes occupy specific positions called loci and may exist in different versions known as alleles.
Learning Objectives
- Define the term locus in the context of chromosomes.
- Describe the characteristics of genes and alleles located on homologous chromosomes.
- Differentiate between homozygous and heterozygous gene combinations based on their alleles.
- Explain the relationship between locus, gene, and allele using diagrams and analogies.
Relationship Between Chromosome, Locus, Gene, and Allele
A pair of homologous chromosomes carries the same genes at the same loci. Alleles are alternative versions of a gene that occupy the same locus on both chromosomes.
Chromosome
A long structure carrying many genes.
Locus
The specific location of a gene on a chromosome.
Gene
A basic unit of inheritance that codes for a specific characteristic.
Allele
An alternative form of a gene at the same locus.
Short Explanation
Definition of Locus
A locus (plural: loci) is a specific location of a gene in a chromosome.
Genes in Humans
Each chromosome carries many genes. In humans, the number of genes that code for proteins in a set of haploid chromosomes is estimated to be 25 000.
Alleles and Homologous Chromosomes
An allele represents a specific version or alternative form of a gene. An allele representing a gene is located at the same locus as the gene on homologous chromosomes.
Homozygous
When a pair of homologous chromosomes carries similar alleles at a specific locus, the individual is homozygous for that gene. Examples include homozygous dominant \(FF\) or \(CC\), and homozygous recessive \(jj\).
Heterozygous
When a pair of homologous chromosomes carries different alleles at a specific locus, the individual is heterozygous for that gene. Examples include \(Aa\) or \(Mm\).
Analogy of Chromosome, Locus, Gene, and Allele
Think of a parking lot: the chromosome is the entire shopping mall X. The locus is a specific parking bay, such as Level 1 car park, parking boxes 212, 213, 214. The gene is the type of vehicle designated to park there, such as a sedan car. The alleles are the specific models of sedan cars that can occupy that bay, such as model P, model Q, or model R.
Flower Colour in Pea Plants
The gene that controls flower colour in pea plants occupies a specific locus on a pair of homologous chromosomes. The gene has alternative alleles at that locus: allele \(P\) for purple flower and allele \(p\) for white flower.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the definition of a locus?
What is the estimated number of protein-coding genes in a set of human haploid chromosomes?
How are homozygous and heterozygous states differentiated at a specific locus on homologous chromosomes?
Common Mistakes
- Confusing the terms gene and allele; a gene is the general unit or characteristic, such as flower colour, while an allele is the specific version or variant of that gene, such as allele \(P\) for purple flower or allele \(p\) for white flower.
- Thinking that alleles of the same gene can be situated at different locations on homologous chromosomes; alleles of the same gene must always occupy the exact same locus on both chromosomes of a homologous pair.
- Stating that homozygous only refers to dominant traits; homozygous also includes homozygous recessive where two identical recessive alleles are present.
Concept Misunderstandings
A single chromosome carries only one single gene.
Each chromosome carries many genes; in humans, an estimated 25 000 protein-coding genes are distributed across a haploid set of chromosomes.
A locus changes its position depending on whether an allele is dominant or recessive.
The locus is fixed for a particular gene; both dominant and recessive alleles of that gene are located at the exact same locus on homologous chromosomes.
Summary
- A locus is the specific location of a gene on a chromosome.
- Each human haploid set of chromosomes carries an estimated 25 000 protein-coding genes.
- Alleles are alternative versions of a gene and reside at the same locus on homologous chromosomes.
- Organisms with identical alleles at a locus are homozygous (\(FF\) or \(jj\)), while those with differing alleles are heterozygous (\(Mm\)).
Short Activity
Objective: check your understanding of locus, gene, allele, homozygous and heterozygous states.
A. Objective Quiz
Which statement correctly defines a locus?
In humans, what is the estimated number of genes that code for proteins in a set of haploid chromosomes?
B. Fill in the Blanks
An allele representing a gene is located at the same as the gene on a chromosome.
On a pair of homologous chromosomes, a locus with different alleles such as Mm is described as .
C. Matching / Drag and Drop
Match each component of the shopping mall car park analogy to its biological equivalent. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Analogy Component to Its Biological Concept
Write the correct biological concept next to each analogy component.
| Analogy Component | Biological Concept |
|---|---|
| 1. Shopping mall X | |
| 2. Level 1 car park, boxes 212, 213, 214 | |
| 3. Sedan car | |
| 4. Sedan car models P, Q, and R |
Answer choices: Chromosome • Locus • Gene • Alleles
