7.1.1 Key Terms in Genetics

Key Definitions in Genetics:

• Genotype: The complete set of alleles carried by an organism on its chromosomes.
• Phenotype: The observable characteristics of an organism, resulting from its genotype and environmental influences.

Alleles and Inheritance

Most organisms are diploid, meaning they carry two alleles for each gene, one inherited from each parent. Alleles can interact in different ways to influence the phenotype:

1. Dominant Allele:

• A single allele is enough to express the trait in the phenotype.
• Example: Heterozygous dominant (Aa) or homozygous dominant (AA) genotypes both show the dominant trait.

2. Recessive Allele:

• The trait is only expressed if there are two recessive alleles present.
• Example: Homozygous recessive (aa) is required for the recessive phenotype to appear.

3. Codominant Alleles:

• Both alleles are expressed equally in the phenotype.
• This can result in a blend of traits (e.g. red and white flowers producing pink flowers) or both traits being shown together (e.g. red and white patches on a flower).

Additional Key Terms

• Pure Breeding: Refers to a genotype that is homozygous, either dominant or recessive, ensuring the same trait is always passed on.
• Allele: An alternative form of a gene found at the same position on homologous chromosomes.
• Locus: The specific location of a gene on a chromosome.

Important Notes

• Homologous chromosomes carry the same genes at the same loci but may have different alleles.
• Genetic variation arises when different alleles combine during fertilisation.
• Codominance often results in a unique phenotype not seen with simple dominance/recessiveness.