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
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