Patterns of inheritance

Causes of variation

• Mutations

  • Random changes to the genetic code

  • Commonly caused during DNA replication

  • Or by mutagens

    • Chemical eg free radicals

    • Physical eg ionising radiation

    • Biological eg viruses

  • Can also be inherited by later generations if they occur in gametes or germ cells

• Sexual reproduction

  • Chromosome mutations during meiosis

  • Crossing over

  • Independent assortment

  • Fertilisation

• 

Genetic and environmental factors

• genetic only

  • Inheritance of cystic fibrosis

• Environmental only

  • Scars/tattoos

• Genetic and environmental

  • Effect of diet on growth and development

  • Etiolation

    • Dont make chlorophyll in the absence of light or Mg and grow long weak stems

    • Grown normally again once in light

Environmental variation - chlorosis

• Caused by lack of light

• Normally genes that produce chlorophyll are switched on

• When there is a lack of light plants turn off chlorophyll production to save energy

• Chlorosis thus occurs only in the dark

• Caused by a combination of genetic and environmental variation

• When light becomes available chlorophyll production restarts

• Other causes of chlorosis

  • Mineral deficiencies

    • Fe

      • Cofactor for some enzymes involved in chlorophyll production

    • Mg

      • Found in the centre of chlorophyll molecule

  • Virus infections

    • Interfere with plant metabolism and chlorophyll synthesis

• In all cases a combination of genetic and environmental factors

Inheritance of genes

• for most characteristics

  • 2 alleles are inherited

  • Alleles may be the same or different

    • Genotype

    • Observable characteristics due to the genotype is the phenotype

    • The phenotype may also be modified by the environment

      • Hair colour

      • Skin colour

      • Freckles

Types of variation

• the more genes that are involved in the expression of the phenotype the more variation there is

•