Chapters 11 - Patterns of Inheritance

Mendelian Genetics

• Gregor Mendel laid the foundations for genetics

• experiments

  • used the garden pea

  • self-fertilization

  • cross-fertilization

  • true breeding

 

• Question 1: How are traits inherited?

  • Experiment 1: true-breeding plants with purple flowers X true-breeding plants with white flowers (P)

  • result: all plants had purple flowers (F₁)

  • Experiment 2: let F₁ self-fertilize

  • result: 3/4 purple and 1/4 white (F₂)

  • Conclusions:

    1. Traits are controlled by discrete units (called genes).

    2. There can be multiple versions of a gene (purple or white, here) (called alleles).

    3. Each individual has 2 alleles for each gene.

    4. Each parent gives 1 allele through the sex cells.

    5. Some alleles (dominant alleles) can mask other alleles (recessive alleles).

    6. The allele pairs are randomly separated during meiosis.

• other genetic terms to know:

  • homozygous

  • heterozygous

  • phenotype

  • genotype

• Punnett square method can be used to predict the outcome of a cross [see text for examples]

 

• Question 2: How are multiple traits inherited?

  • Experiment: make a cross and follow 2 traits

    • followed the traits of flower color and plant height

    • Cross 1: purple, tall X white, dwarf (P)

    • result: all purple, tall (F₁)

    • Cross 2: F₁ were self-fertilized

    • result: purple, tall and purple, dwarf and white, tall and white, dwarf

    • Conclusion: traits are not inherited as sets, there is independent assortment of alleles

Non-Mendelian Genetics

• one trait is not totally dominant over the other

  • incomplete dominance

  • heterozygous phenotype is a mix

  • e.g. snapdragon flower color

• one gene may have more than 2 alleles

  • codominance

  • more than 1 allele is expressed at the same time

  • e.g. blood type

• one gene affecting multiple traits

  • e.g. sickle-cell anemia

  • e.g. SRY gene for male development

• multiple genes affecting the same trait 

  • e.g. fur color

  • e.g. continuous variation

• environmental factors

  • e.g. Himalayan rabbit and Hydrangea

© Carol L. Wymer, 2004