Chapter 14

• Character


  A heritable feature that varies among individuals (such as flower color)
• Trait


  Each variant for a character, such as purple or white color for flowers
• Peas were available to Mendel in many different varieties
• Mendel's approach


  • Chose to track only characters that occurred in two distinct alternative forms
  • Started with varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
• Mendel's typical experiment


  1. Mated two contrasting, true-breeding varieties (hybridization)

  2. The true-breeding parents are the P generation

  3. The hybrid offspring of the P generation are called the F1 generation

  4. When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced
• Mendel's observation in the F1 generation

  All plants had purple flowers
• Mendel's observation in the F2 generation

  705 purple-flowered plants, 224 white-flowered plants
• Dominant trait


  The purple flower color
• Recessive trait


  The white flower color
• Alleles


  Alternative versions of a gene
• Each gene resides at a specific locus on a specific chromosome
• Homozygous


  An organism with two identical alleles for a character
• Heterozygous


  An organism with two different alleles for a gene
• Law of segregation


  The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
• Testcross


  Breeding the mystery individual with a homozygous recessive individual to determine the genotype
• Dihybrid


  An individual heterozygous for two characters
• Law of independent assortment


  Each pair of alleles segregates independently of any other pair of alleles during gamete formation
• Genes located near each other on the same chromosome tend to be inherited together
• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive, When a gene has more than two alleles, When a gene produces multiple phenotypes
• Complete dominance


  Phenotypes of the heterozygote and dominant homozygote are identical
• Incomplete dominance


  The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
• Genes located near each other on the same chromosome

  Tend to be inherited together
• Inheritance patterns more complex than simple Mendelian genetics


  • When alleles are not completely dominant or recessive
  • When a gene has more than two alleles
  • When a gene produces multiple phenotypes
• Incomplete dominance


  Phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
• Codominance


  Two dominant alleles affect the phenotype in separate, distinguishable ways
• Multiple Alleles


  Most genes exist in populations in more than two allelic forms
• Pleiotropy


  Most genes have multiple phenotypic effects
• Extending Mendelian Genetics for Two or More Genes

  • Epistasis: one gene affects the phenotype of another due to interaction of their gene products
  • Polygenic inheritance: multiple genes independently affect a single trait
• Polygenic Inheritance


  Quantitative characters vary in the population along a continuum due to the additive effect of two or more genes on a single phenotype
• Humans are not good subjects for genetic research due to long generation time, few offspring, and unacceptable breeding experiments
• Carriers


  Heterozygous individuals who carry the recessive allele but are phenotypically normal
• Huntington's disease is a degenerative disease of the nervous system with no obvious phenotypic effects until age 35-40