Genes and alleles Knowledge-Based Study

Study notes

1. What is a Gene?

   A gene is the fundamental physical and functional unit of heredity. In molecular terms, a gene is a specific sequence of nucleotides along a DNA molecule that carries instructions to synthesize a functional product, typically a polypeptide (protein) or functional RNA molecule.

   In diploid organisms, chromosomes occur in matched pairs called homologous chromosomes. One chromosome of each pair is inherited from the maternal parent, and the other is inherited from the paternal parent. A gene occupies a fixed physical position on a chromosome known as its locus (plural: loci). Homologous chromosomes have the same genes at the same loci, although the specific nucleotide sequences at those loci may differ.

   Distinguishing Genes from Alleles

   While a gene determines a general biological characteristic or trait, an allele is a specific alternative form or variant of that gene. Alleles arise through sequence variations or mutations in the DNA code.

   • Gene example: The gene controlling flower color in pea plants.
   • Allele example: The specific allele specifying purple pigmentation versus the alternative allele specifying white pigmentation.

   Because diploid individuals carry two homologous chromosomes for each autosomal gene, an individual possesses exactly two alleles for each gene locus—one inherited from each parent.

   Zygosity and Genetic Makeup

   The combination of alleles an organism carries for a specific gene constitutes its genotype. Depending on whether the two inherited alleles are identical or different, genotypes are classified as follows:

   • Homozygous: An individual carries two identical alleles at a given locus (for example, receiving the same purple-flower allele from both parents).
   • Heterozygous: An individual carries two different alleles at a given locus (for example, receiving a purple-flower allele from one parent and a white-flower allele from the other).

   In genetic notation, letters represent alleles. Uppercase letters traditionally denote dominant alleles (such as A), while lowercase letters denote recessive alleles (such as a). An organism can therefore be homozygous dominant (AA), homozygous recessive (aa), or heterozygous (Aa).

   From Genotype to Phenotype

   The observable physical, physiological, or biochemical manifestation of a genotype is termed the phenotype. The phenotype results from the interaction of alleles and their expression patterns:

   • Dominant Allele: An allele whose phenotypic effect masks the presence of an alternative allele when present in a heterozygous state. Both AA and Aa individuals express the dominant phenotype.
   • Recessive Allele: An allele whose phenotypic effect is masked in the presence of a dominant allele. A recessive phenotype is expressed only when an organism is homozygous recessive (aa).

   Understanding the distinction between genes, their allelic variants, and their dominance relationships forms the foundation of Mendelian genetics and inheritance patterns.