Basic Knowledge Needed for Studying Inheritance

• Alleles


  • different forms of the same gene that lead to different traits
  • occupy the same relative position on a pair of homologous chromosomes
  • every gene, 2 alleles (one from mother one from father)
  • When the 2 alleles are the same, it is a pure-bred

  When there are two different alleles:

  • the expressed allele is called the dominant allele.
  • the unexpressed allele is called the recessive allele.
• Homologous Chromosomes


  
  

  • Same length same genes
  • Female : 23 pairs of homologous genes
  • Male : 22 pairs of homologous genes (One XY chromosome)
• Genotype and Phenotype 


  • The combination of alleles for a particular gene is called genotype
  • The expressed trait or outward appearance is called phenotype
• Homozygous and Heterozygous


  • Homozygous refers to having two identical alleles of a particular gene (e.g. DD or dd)
  • Heterozygous refers to having two different alleles of a particular gene (e.g. Dd).
• Genetic Diagram


  1. Parent phenotype
  2. Parent genotype
  3. Gametes
  4. F1 genotype
  5. F1 phenotype
  6. Ratio of phenotypes

  
• Punnet Square

  
• Test Cross

  
• Codominance


  • both alleles express themselves in the heterozygote, which has a phenotype intermediate between that of its pure-bred parents. The alleles are said to be codominant.