Alleles
அல்லீல்கள்
Do not confuse alleles with genes. A gene is the functional unit; an allele is the variation. If you miss this distinction, you lose four marks on a basic Mendelian question.
The idea
Alleles are slightly different forms of the same gene that code for a pair of contrasting traits. They occupy the same locus on homologous chromosomes. A diploid organism carries two alleles for every gene. These alleles may be identical in homozygous or different in heterozygous conditions.
Why it matters
They exist to provide genetic diversity within a population, allowing for different phenotypic expressions of the same biological function.
NEET lens
NEET focuses on the 'same locus' concept. Expect questions matching alleles to homologous chromosomes. Do not memorize definitions; understand that alleles are the reason for phenotypic variation. If a question mentions 'contrasting traits', immediately identify the answer as alleles.
Clinical significance
Allelic mutations are the root cause of autosomal recessive disorders like Cystic Fibrosis or Sickle Cell Anemia. When a functional allele is replaced by a non-functional mutant allele, the protein product fails, leading to metabolic or structural pathology.
Introduction
Alleles are alternative forms of a gene that occupy the same locus on homologous chromosomes. They arise by mutation and may produce different phenotypes. A diploid organism has two alleles for each gene — which may be identical (homozygous) or different (heterozygous).
Core concept
Definition: alleles = different versions of a gene at the same locus. Locus: specific position of a gene on a chromosome. Diploid organisms: 2 alleles per gene (one on each homolog). Homozygous: both alleles same (TT or tt). Heterozygous: alleles different (Tt). Dominant allele: expressed in heterozygote. Recessive allele: only expressed in homozygous recessive. Multiple alleles: more than 2 alleles possible for a gene in a population (though any individual has only 2). Example: ABO blood groups — 3 alleles (IA, IB, i) in human population. Wild type allele: the most common allele in a population (often designated +). Mutant allele: arises by mutation from wild type. Allele frequency: proportion of a particular allele in a population — central concept in population genetics.
Go deeper
Molecular nature of alleles: alleles are alternative DNA sequences at the same chromosomal position. May differ by: single nucleotide polymorphism (SNP — 1 base change), indel (insertion/deletion of bases), microsatellite variation (variable number of repeats). Same gene, different alleles → same protein structure but different amino acids → different enzyme activity → different phenotype.
From Class 10
Alleles are different forms of the same gene. For example, the gene for flower colour may have a purple allele (P) and a white allele (p) — different versions of the same gene at the same chromosomal location.