Inheritance

• We inherit:

  • Two genes for each trait
  • One from maternal origin, one from paternal origin
• Gene:

  • A section of DNA that carries the genetic code for a particular characteristic 
• Allele:

  • Different forms of the same gene / variation of a gene, due to different sequence of base pairs
• Genotype:

  • The combination of alleles that an individual has which is denoted by letters (e.g. AA, Aa, aa)
• Homozygous / pure breeding:

  • The individual has inherited two alleles for a trait that are the SAME (e.g. AA or aa)

  Heterozygous / hybrid:

  • The individual has inherited two alleles for a trait that are DIFFERENT (e.g. Aa)
• Phenotype:

  • The physical trait that an individual displays, that is determined by the genotype (but also by the environment)
• Dominant:

  • The trait that is observed in the phenotype of a heterozygous individual 

  Recessive:

  • The trait that remains hidden in the heterozygous condition and seen only in the homozygous condition
• Complete dominance: 

  • Complete dominance refers to the expression of a phenotype if at least ONE of the alleles are the dominant allele – can be homozygous or heterozygous
  • This is demonstrated when two homozygous parents of different phenotypes breed, all of the offspring will be only one phenotype

  
  

  • 2 alleles (A, a) – Dominant allele is capital. Recessive allele is same letter but lowercase
  • 2 phenotypes
  • Heterozygote (Aa): dominant allele (A) expressed in phenotype
• Incomplete dominance:

  • Incomplete dominance is where neither allele is dominant
  • In the heterozygote, neither allele is expressed, there is a blended or intermediate phenotype
  • The phenotype is completely different to both of the homozygotes

  
  

  • 2 alleles (B, W) – neither dominant therefore both capitals but different letters
  • 3 phenotypes
  • Heterozygote (BW): both alleles present lead to 3rd blended / intermediate phenotype
• Co-dominance:

  • Co dominance is where BOTH alleles are equally dominant
  • That is in the heterozygote, both alleles contribute and therefore are expressed in the phenotype

  
  

  • 2 alleles (B, W) – as both alleles co-dominant, both capitals and different letters
  • 3 phenotypes
  • Heterozygote (BW): BOTH alleles contribute to the phenotype – expressed together
• Sex-linked 

  • Genes present on the sex chromosomes
• Pedigree 

  • A flowchart that shows the family history of inheritance over several generations 
• Punnet square rules:

  1. State alleles 
  2. Write down the ‘cross’
  3. Draw punnet square
  4. Provide details asked in question