Introduction
In the realm of genetics, the study of genes and their functions forms the cornerstone of understanding biological inheritance and variation. This document delves into the topic of genes as outlined in the International Baccalaureate (IB) Biology syllabus, specifically focusing on key concepts such as the nature of genes, alleles, mutations, and genomes.
3.1 Genes & Chromosomes
3.1.1 Genes
Definition and Function
A gene is a segment of DNA that contains the necessary information to produce a specific polypeptide. These polypeptides, in turn, influence various characteristics of an organism. For instance, the gene coding for the enzyme trypsin affects digestive processes.
NoteA gene is a heritable factor, meaning it can be passed from parent to offspring during reproduction.
Characteristics
- Polypeptide Production: Genes code for polypeptides, which are chains of amino acids that fold into functional proteins.
- Influence on Traits: These proteins can affect specific traits, such as blood type in humans or height in pea plants.
For example, the gene responsible for blood group in humans has different alleles that result in blood types A, B, AB, or O.
Chromosomal Location
Each gene occupies a specific location on a chromosome known as a locus (plural: loci). This precise positioning ensures that the gene for a particular characteristic is consistently found at the same location across individuals of a species.
3.1.2 Alleles
Definition
Alleles are alternative forms of a gene that exist at the same locus on homologous chromosomes. While they code for the same trait, different alleles can produce variations of that trait.
NoteAlthough alleles are different forms of the same gene, they still occupy the same locus on the chromosome.
Formation and Variation
- Mutations: New alleles arise through mutations, which are changes in the DNA sequence.
- Multiple Alleles: A single gene can have multiple alleles, each contributing to different variations of a trait.
The ABO blood group system in humans is determined by three alleles: $I^A$, $I^B$, and $i$. These alleles combine to produce four possible blood types: A, B, AB, and O.
Homozygous and Heterozygous Conditions
- Homozygous: When both alleles at a locus are identical, the organism is homozygous for that gene.
- Heterozygous: When the alleles at a locus are different, the organism is heterozygous.
A common misconception is that heterozygous individuals always exhibit a blend of traits from both alleles. In reality, the dominant allele often masks the effect of the recessive allele.
3.1.3 Mutation
Definition
A mutation is a change in the DNA sequence that can lead to the formation of new alleles. These changes can occur spontaneously or be induced by environmental factors.
Types of Mutations
- Point Mutations: Substitution of a single nucleotide.
- Insertions and Deletions: Addition or loss of one or more nucleotides.
- Frameshift Mutations: Insertions or deletions that alter the reading frame of the gene.
Sickle cell anemia is caused by a point mutation in the gene coding for the beta chain of hemoglobin. This single nucleotide change results in the substitution of valine for glutamic acid.
