Genes, Alleles & Other Key Terms in Genetics (WJEC GCSE Biology: Combined Science)

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
  • As we have two copies of each chromosome, 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

Genotype and Phenotype

• The phenotype refers to the observable characteristics of an organism (seen just by looking - like eye colour, or found – like blood type)
  • Phenotype occurs as the result of both genotype and environmental influences
• Genotype is the combination of alleles that control each characteristic 
  • Alleles can be dominant or recessive
    • A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
    • A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype
    • If there is only one recessive allele, it will remain hidden and the dominant characteristic will show

Homozygous and Heterozygous

• 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), or homozygous recessive (having two copies of the recessive allele)
• Heterozygous is used to refer to genotypes where the two alleles of a gene are different (hetero = different)
• 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 and the recessive allele is given the same letter, but lower case

Genotype key terms diagram

Alleles are different forms of the same gene. You can only inherit two alleles for each gene, and they can be the same (homozygous) or different (heterozygous)

F₁ and F₂ Generations

• The F₁ generation is the first generation of offspring produced from two parent organisms
• The F₂ generation is the offspring produced when two individuals from the F₁ generation are crossed
• We cannot always tell the genotype of an individual for a particular characteristic just by looking at the phenotype – a phenotype associated with a dominant allele will be seen in both a dominant homozygous and a dominant heterozygous genotype
  • If two individuals who are both identically homozygous for a particular characteristic are bred together
    • All of the F₁ generations will have exactly the same genotype and phenotype as the parents 
    • We describe them as being ‘pure breeding’
  • A heterozygous individual can pass on different alleles for the same characteristic each time it is bred with any other individual
    • It can therefore produce an F₁ generation with a different genotype and phenotype to the parents
    • Heterozygous individuals are not pure-breeding
  • If a homozygous dominant individual is crossed with a homozygous recessive individual 
    • All of the F₁ generations are heterozygous

Key Terms & Definitions for Genetic Inheritance Table

Key term	Definition
Gamete	Sex cells e.g. egg and sperm cells in animals or pollen and ovum in plants
Chromosome	Thread-like structures of DNA carrying genetic information in the form of genes. Found in the nucleus of cells
Gene	Short lengths of DNA are found on chromosomes. They code for specific proteins
Allele	Different versions of a gene
Dominant	An allele that is always expressed even if only one copy is present
Recessive	An allele that is only expressed if two copies (and no dominant alleles) are present
Homozygous	A genotype with two of the same alleles for a particular gene
Heterozygous	A genotype with two different alleles for a particular gene
Genotype	The combination of alleles that control a characteristic
Phenotype	The observable characteristics of an organism that result from genotype and environmental influences
F1 & F2 generation	F1 is the first generation of offspring from a cross. F2 is the generation of offspring from a cross between two of the F1 generation
Self-fertilisation or selfing	When the male and female gametes of the same organism fuse to make a zygote e.g. in plants