11.3 - Genes and Alleles

Concept of Locus

• Locus: A specific location of a gene in a chromosome.
• 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.

Relationship Between Allele, Locus and Gene

• An allele represents a gene and occupies the same locus as that gene.
• Alleles for the same gene occupy the same locus on a pair of homologous chromosomes.
• A gene is homozygous at a locus when both alleles are identical:
  • Homozygous dominant, for example $FF$.
  • Homozygous recessive, for example $jj$.
• A gene is heterozygous at a locus when both alleles are different, for example $Mm$.