Mendel’s model of heredity - 5 main assumptions 

• parents don’t transmit traits directly to offspring
  • information about traits (factors) get passed down
  • factors encode how an individual expresses those traits
• 2 factors for each trait
  • factors carried on chromosomes
  • gametes (haploid) each carry a factor for each trait
  • random chance determines which factor goes into each gamete
• not all copies of factors are the same
  • alleles - alternative forms of a trait
  • homozygous - having the same 2 alleles for a certain trait
  • heterozygous - having different alleles for a certain trait
  • gene - factors that determine traits
  • locus - location of a gene on a chromosome
• alleles don’t influence/change each other
  • alleles stay the same, don’t blend w/ others
• an allele doesn’t guarantee that the trait will be expressed
  • genotype - all the alleles that the individual contains
  • phenotype - physical appearance/expression of those alleles

Punnet square - invented by Reginald Crundall Punnett 

• can predict the possibilities of mixed alleles
• shows a 3:1 phenotypic ratio and 1:2:1 genotypic ratio when hybrids bred
• testcross - procedure used to see if plant is heterozygous or homozygous
  • plant crossed w/ homozygous recessive plant
  • only homozygous dominant plant will guarantee that all offspring will have dominant trait

Mendel’s laws of heredity - 2 main laws 

• 1st law of heredity (segregation)
  • alleles for a trait separate and remain distinct
  • chromosomes align/split during meiosis
• 2nd law of heredity (independent assortment)
  • alleles don’t affect alleles for another trait
  • chromosomes align in homologous pairs during meiosis
  • dihybrids - heterozygous for 2 genes

problems w/ analyzing inheritance - scientists had problems getting same ratios as Mendel 

• continuous variation - range of small differences for a trait affected by multiple genes
  • polygeny - many genes affect 1 trait
  • not all phenotypes result from only 1 gene
  • quantitative traits - shows range of small differences
• pleiotropic effects - allele w/ more than 1 effect
  • single gene affects multiple traits
  • difficult to predict (side effects often unknown)
• incomplete dominance - not all alleles are totally dominant/recessive
  • allele pairs produce heterozygous phenotype either representative of both alleles or of an intermediate
  • codominance - representative of both parents
• environmental effects - alleles affected by the environment
  • some alleles heat-sensitive, code for traits that are more sensitive to temperature/light
• epistasis - 1 gene interfering w/ expression of another gene
  • occurs when genes act sequentially, one after the other
  • if enzyme defective early on in biochemical pathway, impossible to see if later steps work properly