Meiosis -> Inheritance How do the events of Meiosis predict patterns of heritable variation?
Mendel s peas 1. Genes determine appearance (phenotype) 2. Genes vary and they are inherited 3. Their behavior explains patterns of variation 4. Genes lie on the chromosomes that recombine and segregate during meiosis Genes go wherever chromosomes go; genes are just along for the ride.
Mendel to the rescue 1822-1884; contemporary of Darwin* Monk; St. Thomas Abbey; pea plants Involved in Agricultural Society* Animal & Plant breeders knew that traits could be inherited Mendel sought to describe these patterns of inheritance
Competing Hypotheses Offspring inherit traits that parents acquire during their lifetime (acquired inheritance) Jean-Baptiste Lamarck Body-builders will have muscle-bound offspring Offspring traits are an average of their parent s traits. (blending inheritance) Nägeli Black sheep + white sheep = gray sheep
Advantages of peas Inexpensive to keep & propagate Short generation time Prolific Small (space economy) Tolerant of inbreeding (mutants survive to produce a phenotype) Simple phenotypic variation Matings (experiments) were COMPLETELY within his control Testcrosses!
Easy to pollinate Self Cross Generate truebreeding lines
Simple, informative traits
Terminology Phenotype genotype Diploid haploid gamete Homologs/homologous gene Locus/loci Allele Homozygous heterozygous True-breeding Hybrid P, F 1, F 2 generation Reciprocal cross Dominant/recessive Partial (incomplete) dominance Particulate inheritance Segregation of alleles Independent assortment of loci Recombination/Crossing over Sister/non-sister chromatids Linkage sex-linked Autosome sex-chromosome
What did Mendel DO? Created true- breeding lines for various traits When bred within lines, always produce same traits Round x round = all round Crossed pure wrinkled peas with pure round peas Genotype = genetic complement Phenotype = outward appearance M Trait determinants F
Whoa! All round? The wrinkled phenotype receded Recessive (r) Oh, peas from male plant were round; males dominant over females right?
Reciprocal cross
Self-pollinate 1. Variation (wrinkled) reappeared in F 2 ; round:wrinkled ~ 3:1 2. P generation phenotypes were unaltered Inheritance is particulate, not blending! Trait determinants, or alleles, (R & r) segregate between generations
Summary This same pattern recurred for all 7 traits: F 1 disappearance, F 2 reappearance, unaltered parental phenotypes, 3:1 ratio Trait determinants are distinct, unalterable units M & F contribute = to genotype More than 1 determinant per plant, and they segregate from each other between generations RR ---> R & R
What happened? Monohybrid cross Separation into gametes Union of gametes produces new genotype Punnett Square F 1 generation Let all Round plants self. Phenotype ratio? 3:1!!
What about 2 traits? How will two different traits behave? How will the alleles that produce them segregate relative to each other? Will they segregate together? Will they segregate independently?
2 Traits
2 trait dependent If alleles segregate together, we should expect only Parental phenotypes in F 2 Same 3:1 ratio as monohybrid cross
2 Traits
2 trait independent Dihybrid cross These were Mendel s results! 9:3:3:1 Principle of independent assortment
Testcrosses confirm predictions A testcross uses a parent with all recessive alleles to reveal the unknown genotype of another parent.
Mendel s work helped explain Charles Darwin s postulates 1. Variability among traits must exist 2. Some of that variability must be heritable 3. Due to battles for resources, individuals will vary in their ability to survive & reproduce 4. Those that reproduce the most, are those with the most favorable variations (adaptations)
Darwin s postulates require that: 1. Traits vary among individuals within a population 2. Some of this variation is heritable For natural selection to work there must be heritable variation in every generation
Where Darwin got stuck What produces variation? How is it propagated from one generation to the next? He (indeed, anyone) could demonstrate that traits are heritable, but not how.