Vocabulaire de génétique

Genotype refers to the pair of alleles an organism inherits for a particular gene, while phenotype is the observable physical expression of those alleles, such as eye color. A diploid organism receives one allele from each parent at the same locus on homologous chromosomes. The phenotype results from the genotype, though multiple genotypes can produce the same phenotype, and environmental factors may also influence phenotypic expression.

A dominant allele requires only one copy to be expressed in the phenotype, while a recessive allele requires two copies for expression. When an organism inherits two different alleles, the dominant allele masks or overrides the recessive allele's instructions. For example, the brown eye allele is dominant over the blue eye allele, so an individual with one brown and one blue allele will have brown eyes.

A homozygous genotype occurs when an organism inherits two identical alleles for a gene, while a heterozygous genotype results from inheriting two different alleles. For instance, an individual with two brown eye alleles (BB) is homozygous, whereas an individual with one brown and one blue allele (Bb) is heterozygous. Both genotypes are possible for the same phenotype when the dominant allele is present.

Diploid organisms inherit two alleles for each gene—one from each parent—located at the same locus on homologous chromosomes. These two inherited alleles constitute the genotype for that gene. Since each parent contributes one allele, offspring can inherit different combinations of alleles, resulting in variation in genotypes and phenotypes across a population.

Yes, multiple genotypes can produce the same phenotype. For example, both homozygous (BB) and heterozygous (Bb) individuals express brown eyes because the dominant brown allele is present in both cases. Only individuals with the homozygous recessive genotype (bb) express blue eyes. This demonstrates why phenotype alone cannot always determine an organism's exact genotype.

Dominant mutations cause disease expression with just one mutated allele, while recessive diseases require two mutated copies. Huntington disease exemplifies dominant inheritance: a parent with one normal and one mutated allele has a 50% chance of passing the mutation to offspring, who will develop the disease. Understanding pedigree analysis and disease inheritance patterns helps predict disease risk and inform family planning decisions.

Eye color serves as a useful teaching example because it clearly illustrates dominant and recessive alleles, even though scientists discovered at least eight genes regulate eye color. The OCA2 gene accounts for nearly three-fourths of blue-brown variation, making the simplified model of brown dominance over blue effective for explaining how dominant alleles mask recessive ones, despite other genes occasionally modifying these effects.