Inheritance

• Gene 


  The stored set of instructions for a protein
• Allele


  A form of a gene; different alleles code for the same trait but different version of the same trait
• Dominant Allele

  • always expressed in the phenotype
  • masks a recessive allele if paired with one
  • represented with a capital letter
• Recessive Allele

  • only expressed in the phenotype if homozygous
  • masked by a dominant allele
  • represented with lower case letters
• Homozygous 


  Possessing 2 identical alleles of a gene (AA, aa)
• Heterozygous 


  2 different alleles, dominant allele will always be expressed (Aa)
• 4 Main Modes of Inheritance

  • autosomal recessive
  • autosomal dominant
  • x-linked recessive
  • x-linked dominant
• Monohybrid Cross

  • involved fertilisation between 2 monohybrids (heterozygous parents)
• Test Cross

  • ratio of phenotypes in the offspring reveals the unknown genotype (homozygous dominant or heterozygous dominant)
• Multiple Alleles for 1 Gene

  • for most traits, there are more than 2 forms of alleles for a gene
  • multiple alleles result in discontinuous variation (eg, ABO human blood groups)
  • discontinuous variation is a set of discrete phenotypic categories controlled by a single gene and its set of alleles
• Polygenic Inheritance

  • a characteristic controlled by more than 1 gene is known as polygenic characteristic, and its transmission is polygenic inheritance
  • since there are many genes there are numerous phenotypes possible - continuous variation (bell curve)
• Continuous Variation

  • single-gene inheritance
  • caused by genetic and environmental factors
  • variation that shows gradual changed from 1 trait to another
  • eg. height, weight, skin colour
• Discontinuous Variation

  • polygenic inheritance
  • genetic factors
  • variation that shows clear and discrete changes between traits
  • eg. human blood groups, attached or detached earlobes
• Incomplete Dominance

  • 2 different alleles are present, but neither is completely dominant
  • mix of 2 phenotypes
  • eg. red + white = pink
• Codominance

  • 2 alleles are completely dominant
  • both alleles are expressed in the phenotype
• Autosomal Recessive

  • allele on a non-sex chromosome
  • people with only 1 defective allele = carrier
• Autosomal Dominant

  • a single dominant alleles is responsible for the occurence of a phenotype
• X-Linked Recessive

  • recessive phenotype is determined by an allele on the X chromosome
  • males with the recessive allele will always express the phenotype as they only have 1 X chromosome
  • females will only express the phenotype when both X chromosomes have the affected allele
  • heterozygous female=carrier
• X-Linked Dominant

  • heterozygous females will always show the phenotype
  • males showing the phenotype won't pass it onto their sons, but will pass it onto their daughters