7.2.1 Genes, Alleles and Loci

Introduction

Genetics is the study of how traits are passed from parents to offspring. To understand this process, we must first understand the basic units of inheritance: genes, alleles, and loci. These are the fundamental concepts that explain how characteristics are determined and inherited.

Definition

Gene: A specific segment of DNA, comprising a unique sequence of nucleotides that encodes the sequence of amino acids in a particular polypeptide. It is the unit of inheritance.

Allele: Different forms or variations of a gene that occupy the same position (locus) on a chromosome.

Locus (plural: loci): The specific position of a gene on a chromosome.

Main Idea

Each characteristic and function of the body is controlled by a factor called a gene. The parental traits are passed to their offspring in the form of genes. Therefore, a gene is the unit of inheritance.

Genes are found in chromosomes at specific positions called loci. At each locus on a chromosome, a gene can exist in two or more forms. These different forms or variations of a gene are called alleles.

Core Concept

Genes, Alleles, and Loci

Detailed Explanation

I) Genes

What is a gene?

• Each characteristic and function of the body is controlled by a factor called a gene.
• The parental traits are passed to their offspring in the form of genes.
• Therefore, a gene is the unit of inheritance.
• Basically, a gene is a specific segment of DNA.
• It comprises a unique sequence of nucleotides.
• It encodes the sequence of amino acids in a particular polypeptide.

Mendel's contribution:

• Mendel was the first person who proposed the idea of gene.
• He called them "factors" or "elements".
• He used to represent the genes of different characteristics with alphabetical symbols.

Simple explanation: A gene is a segment of DNA that controls a specific trait. It is the basic unit of inheritance passed from parents to offspring.

Why this is important:

• Genes determine all characteristics of an organism.
• They are passed from parents to offspring.
• They encode proteins that carry out all body functions.

II) Locus (Plural: Loci)

What is a locus?

• Genes are found in chromosomes at specific positions called loci (singular: locus).
• A locus is represented on both members of a homologous pair of chromosomes.

Simple explanation: A locus is the specific location or address of a gene on a chromosome.

Why this is important:

• The locus determines where a gene is located.
• Alleles of the same gene are found at the same locus on homologous chromosomes.
• Different genes are found at different loci.

III) Allele

What is an allele?

• At each locus on a chromosome, a gene can exist in two or more forms.
• These different forms or variations of a gene are called alleles.
• Alleles are versions of the same gene that occupy the same position on a chromosome.

Example:

• In humans, earlobes can either be free or attached.
• The gene for free earlobes is represented by E.
• The gene for attached earlobes is e.
• Both E and e are located at the same locus and are alleles of each other.
• Although E and e are different alleles, they are both considered genes as well.

Simple explanation: Alleles are different versions of the same gene. For example, the gene for earlobes has two alleles: E (free) and e (attached). Both are found at the same locus.

Why this is important:

• Alleles determine the different forms of a trait.
• One allele may be dominant, and the other may be recessive.
• The combination of alleles determines the phenotype.

Summary Table

For Your Information

The complete set of an individual's genes is called the genome. The number of chromosomes that carry the entire genome is referred to as a set of chromosomes, represented by "n" or monoploid. A cell with two sets of chromosomes is called diploid or 2n. In humans, the full genome is spread across 23 chromosomes, which make up one set. The term haploid is used to represent a cell that contains half the number of chromosomes of somatic cells.

Intermediate Level

Genes, alleles, and loci are fundamental concepts in genetics:

Key points:

• A gene is a specific segment of DNA.
• Genes are located at specific positions called loci.
• Alleles are different forms of a gene.
• Alleles occupy the same locus on homologous chromosomes.
• Mendel called genes "factors" or "elements".
• Genes encode the sequence of amino acids in a polypeptide.

Important Points

• - Each characteristic and function of the body is controlled by factor called gene.
• - The parental traits are passed to their offspring in the form of genes.
• - Therefore, a gene is the unit of inheritance.
• - Basically, a gene is a specific segment of DNA, comprising a unique sequence of nucleotides that encodes the sequence of amino acid in a particular polypeptide.
• - Mendel was the first person who proposed the idea of gene, he called them "factors" or "elements" and used to represent the genes of different characteristics with alphabetical symbols.
• - Genes are found in chromosomes at specific positions called loci (singular locus).
• - A locus is represented on both members of a homologous pair of chromosomes.
• - At each locus on a chromosome, a gene can exist in two or more forms.
• - These different forms or variations of a gene are called alleles.
• - Alleles are versions of the same gene that occupy the same position on a chromosome.
• - For example, in humans, earlobes can either be free or attached.
• - The gene for free earlobes is represented by "E" and the gene for attached earlobes is "e".
• - Both "E" and "e" are located at the same locus and are alleles of each other.
• - Although "E" and "e" are different alleles, they are both considered genes as well.

Remember

• - Gene = unit of inheritance
• - Gene = specific segment of DNA
• - Gene = encodes a polypeptide
• - Locus = position of a gene on a chromosome
• - Allele = different forms of a gene
• - Alleles occupy the same locus
• - Example: E (free earlobes) and e (attached earlobes)
• - Mendel called genes "factors" or "elements"
• - Genome = complete set of genes
• - Gene pool = all alleles in a population

Common Mistakes

• 1. Confusing gene and allele.
• - Gene = unit of inheritance.
• - Allele = different forms of a gene.
• 2. Mixing up locus and allele.
• - Locus = position of a gene on a chromosome.
• - Allele = different forms of a gene at that locus.
• 3. Thinking alleles are on different chromosomes.
• - Alleles of the same gene are at the same locus on homologous chromosomes.
• 4. Forgetting that genes encode polypeptides.
• - Genes contain the information for making proteins.
• 5. Believing genes are only found in humans.
• - Genes are found in all living organisms.
• 6. Confusing genome and gene pool.
• - Genome = complete set of an individual's genes.
• - Gene pool = all alleles in a population.
• 7. Thinking Mendel called them "genes".
• - Mendel called them "factors" or "elements".

Quick Revision

60-Second Revision

• Gene = unit of inheritance; segment of DNA that encodes a polypeptide
• Locus = position of a gene on a chromosome
• Allele = different forms of a gene at the same locus
• Example = E (free earlobes) and e (attached earlobes)
• Mendel called genes "factors" or "elements"
• Genome = complete set of genes
• Gene pool = all alleles in a population
• Warning: Do not confuse gene (unit of inheritance) with allele (different forms of a gene). Alleles of the same gene are at the same locus.