Key Terms in Genetics (SQA National 5 Biology): Revision Note

Key terms in genetics

Genes and alleles

• A gene is a short length of DNA found on a chromosome that codes for a particular characteristic (expressed by the formation of different proteins)

• Alleles are variations of the same gene

  • We have two copies of each chromosome; therefore, we have two copies of each gene and therefore two alleles for each gene

  • One of the alleles is inherited from the mother and the other from the father

  • This means that the alleles do not have to ‘say’ the same thing

    • For example, an individual has two copies of the gene for eye colour, but one allele could code for brown eyes and one allele could code for blue eyes

• Alleles can be dominant or recessive

  • A dominant allele only needs to be inherited from one parent for the characteristic to show in the offspring

  • A recessive allele needs to be inherited from both parents for the characteristic to show in the offspring

Genotype and phenotype

• Genotype is the combination of alleles that control each characteristic 

• The phenotype refers to the observable characteristics of an organism (seen just by looking - e.g.eye colour, or found – e.g. blood type)

  • Phenotype occurs as the result of both genotype and environmental influences

Homozygous and heterozygous

• Alleles are often given a lettered code, such as Bb or BB or bb, to easily represent the genotype 

  • When completing genetic diagrams, alleles are abbreviated to single letters

    • The dominant allele is given a capital letter

    • The recessive allele is given the same letter, but lowercase

• Homozygous is used to refer to genotypes where two alleles of a gene are the same (homo = same)

  • An individual could be

    • homozygous dominant (having two copies of the dominant allele): BB

    • homozygous recessive (having two copies of the recessive allele): bb

• Heterozygous is used to refer to genotypes where the two alleles of a gene are different (hetero = different): Bb

P, F₁ and F₂ generations

• In inheritance, the P generation (parental generation) refers to the original pair of organisms that are crossed in a genetic experiment

  • These are the true-breeding (pure-bred) parents with known genotypes

• The F₁ generation is the first generation of offspring produced from two parent organisms (P generation)

• The F₂ generation is the offspring produced when two individuals from the F₁ generation are crossed

• Family trees (also referred to as pedigree charts) can be used to identify phenotypes and their genotypes of different generations

  • They can be used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family

Interpreting a family tree

• Family trees, or pedigree charts, use standard genetic symbols to trace how a particular trait is inherited through several generations within different family groups

• They can be interpreted as follows:

  • Males are indicated by a square shape, and females are represented by circles

  • Horizontal lines between males and females show that they have produced children (which are shown underneath each couple)

  • In the diagram below, affected individuals are red, and unaffected individuals are blue

  • The family tree below shows:

    • both males and females are affected

    • every generation has affected individuals

    • one family group that has no affected parents or children

    • the other two families have one affected parent and affected children as well

Summary of key terms