4.6.1.6a - Genetic Inheritance Terms and Gametes

Chromosomes, genes and alleles

A chromosome is a long structure made of DNA. In plant and animal cells, chromosomes are found in the nucleus. Chromosomes carry many genes.

A gene is a small section of DNA on a chromosome. At GCSE level, a gene can be described as a section of DNA that controls a characteristic by coding for a particular protein or part of a protein.

Different forms of the same gene are called alleles. For example, a gene involved in fur colour could have one allele that gives black fur and another allele that gives brown fur. The important idea is that the alleles are versions of the same gene, not different genes in different places.

[DIAGRAM: asset_name: inheritance-terms-map - diagram 1; asset_slug: 075_4_6_1_6a_genetic_inheritance_terms_and_gametes_diagram1; file: diagram_assets/imagegen_regen_all/075_4_6_1_6a_genetic_inheritance_terms_and_gametes_diagram1_imagegen.png; recommended_method: codex_image_gen; description: Monochrome map linking chromosome, gene, allele, genotype and phenotype. Show a nucleus containing a chromosome pair with a highlighted gene position, two allele versions labelled F and f, and a simple flow from gene versions to genotype to phenotype.]

Genotype and phenotype

The genotype is the combination of alleles an organism has for a gene. In simple GCSE inheritance examples, letters are often used to represent alleles. A capital letter can represent a dominant allele, such as F, and a lower-case version of the same letter can represent a recessive allele, such as f.

The phenotype is the characteristic that is expressed or observed. The genotype works at a molecular level to help develop the phenotype. For a simple single-gene example, FF and Ff might both give the same visible fur colour if F is dominant, while ff gives the recessive fur colour.

Keep the two words separate: genotype is the allele combination, while phenotype is the characteristic. A genotype is written with allele symbols such as Ff; a phenotype is written in words such as black fur or brown fur.

Dominant, recessive and allele pairs

A dominant allele is always expressed if it is present. In allele notation, the dominant allele is usually shown with a capital letter. If F is dominant, an organism with genotype FF or Ff will show the dominant phenotype.

A recessive allele is expressed only when two copies are present, so there is no dominant allele present. In allele notation, the recessive allele is usually shown with a lower-case letter. If f is recessive, the recessive phenotype is shown only by genotype ff.

The words homozygous and heterozygous describe whether the two alleles are the same or different.

Homo- means same, which helps with homozygous: the alleles match. Hetero- means different, which helps with heterozygous: the alleles do not match.