Inheritance: Terminology (HL IB Biology)

• A gene is a short length of DNA found on a chromosome that codes for a particular characteristic (by coding for the production of a specific protein)
• Alleles are variations of the same gene
  • As we have two copies of each chromosome, we have two copies of each gene and therefore every individual will have two alleles of each gene
  • One of the alleles is inherited from the mother and the other from the father
  • The alleles may be the same as each other, or they might be different. e.g. 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

• The combination of alleles that an individual organism inherits is its genotype
  • When the two alleles at a locus are the same/identical, an individual is said to have a homozygous genotype
  • When the two alleles at a locus are different the genotype is said to be heterozygous

Chromosome diagram

Chromosomes showing genes, loci and alleles

• The observable characteristics of an organism (seen just by looking - like eye colour, or found – like blood type) is called the phenotype
• The phenotype of all characteristics is determined by the following factors:
  • The genotype only - the combination of the two alleles for the gene for the characteristic, for example blood group is determined this way
  • The environment only - surroundings such as chemical or radiation exposure, diet or exercise can affect physical characteristics, for example scars and accent are determined this way
  • Interaction between both the genotype and the environment, for example height and skin colour are determined this way

• Alleles can be dominant or recessive
  • A dominant allele only needs to be inherited from one parent in order for the characteristic to be expressed in the phenotype
  • A recessive allele needs to be inherited from both parents in order for the characteristic to be expressed in the phenotype.
  • If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
• If the two alleles of a gene are the same, we describe the individual as being homozygous (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)
• If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different)

Different forms of allele pairs 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). Alleles can be dominant or recessive.

• Co-dominant alleles have a combined effect on the phenotype
  • The alleles are both expressed to an equal extent in the phenotype
• An example of codominance is in speckled chickens
  • Chickens can have different alleles for gene that determines the colour of their feathers
    • We can denote the gene for colour using the capital letter C
  • The two alleles for this gene are white for white feather colour, and black for black feather colour
    • We denote the two alleles using superscript letters, C^W and C^B
  • A chicken with the genotype C^WC^W has white feathers as their phenotype
  • A chicken with the genotype C^BC^B has black feathers as their phenotype
  • A chicken with the genotype C^WC^B has a combination of both feather colours, they are called speckled colour chickens
  • Because both alleles are expressed in the phenotype this is called codominance

Example of codominance in chickens diagram

Diagram showing the phenotypes and genotypes of white, black and speckled chickens, which is an example of codominance

• Incomplete dominance is similar to codominance because two alleles are expressed together instead of just one dominant allele being expressed
• However, instead of both alleles being expressed, both alleles are partially expressed leading to a phenotype which is a blend of both phenotypes or an intermediate phenotype between the two
• An example is incomplete dominance can be found in the four o'clock flower or marvel of Peru (Mirabilis jalapa)
  • Marvel of Peru can have different alleles for gene that determines the colour of their flowers
    • We can denote the gene for colour using the capital letter C
  • The two alleles for this gene are white for white flower colour, and red for red flower colour
    • We denote the two alleles using superscript letters, C^W and C^R
  • A plant with the genotype C^WC^W has white flowers as their phenotype
  • A plant with the genotype C^RC^R has red flowers as their phenotype
  • A plant with the genotype C^WC^R has a blend of both colours, which is expressed in the phenotype as a pink flower colour
  • Because the flowers are neither white or red, but an intermediate between this two, this is incomplete dominance

Example of incomplete dominance in marvel of Peru diagram

Diagram showing the phenotypes and genotypes of white, red and pink flowers of the marvel of Peru, which is an example of incomplete dominance