Mendelian Genetics - Spring 2024 (ANTHRO 1)

Issue of Complexity William Paley: If you found a stone on a beach, it might have been there forever, or its presence could be explained by the operation of purely physical processes; but if you found a watch on a beach, it would demand an explanation in terms of design. Evolution of Complex Adaptat...

Issue of Complexity William Paley: If you found a stone on a beach, it might have been there forever, or its presence could be explained by the operation of purely physical processes; but if you found a watch on a beach, it would demand an explanation in terms of design. Evolution of Complex Adaptations Source of brand new variation comes from random mutation Two general kinds of variation Continuous Discontinuous Evolution of Complex Adaptations Each small step must be adaptive (beneficial)…but what good is 5% of an eye? = irreducible complexity …better than no eye! YouTube “Dawkins makes an eye” Natural selection is not equivalent to “random chance.” The variation upon which selection acts is generated randomly, but selection itself is non-random differential reproduction Problems Darwin couldn’t solve What maintains variation? The blending problem Problems Darwin couldn’t solve How can natural selection extend range of variation? The chihuahua problem Darwin did not understand the mechanisms of inheritance Darwin’s Unanswered Questions How are biological traits inherited? Mendel and particulate inheritance: genetics = system for the inheritance of biological information How does new biological variability arise? mutation = generation of new biological variability Hidden variation epigenetics=alteration in the phenotypic expression of genes without altering the structure of the DNA 7 Gregor Mendel (1822-1884) Rules Governing the Inheritance of Traits: Principle of Segregation Principle of Independent Assortment To resolve Darwin’s difficulties we need to understand the nature of inheritance: it is particulate, not a blending process ▪ Gregor Mendel, Augustinian monk ▪ Conducted systematic experiments on garden peas, 1856-63 ▪ Published results 1865 ▪ Importance of Mendel’s work not recognized until 1900, 16 years after his death (and 18 years after Darwin’s death) Pea Plant Polymorphisms Phenotype: outward physical expression of a trait Genotype: genetic makeup of a trait Gene: basic unit of heredity Alleles: variants of a gene Gametes: sex cells (eggs /sperm) that pass genetic information from one generation to another Mendel’s experiments  Focused on specific traits.  Green/yellow seeds  Smooth/wrinkled seeds  Cultivated true breeding lines and then studied results of crosses (hybrids)  Monohybrid crosses vs. dihybrid crosses  Kept careful records Mendel crossed two true-breeding lines (monohybrid cross) Cross green X yellow parents All the offspring (hybrids) were yellow Then crossed F1 Second generation (F2), 3:1 ratio When the F2 plants were allowed to self-fertilize: All the green individuals produced all green offspring (so they must be pure strains) 1/3 of the yellow individuals produced all yellow offspring (also pure strains) 2/3 of the yellow individuals produced offspring in the same 3:1 ratio as the F1 plants had (hybrids) Mendel realized that these results mean that:  The hereditary material must consist of discrete particles (genes)  Each plant has, for each trait, two “particles” of heredity (alleles), one from each parent  For each trait, which of its two genes a plant passes to its offspring is like a coin flip  A trait may not show up in the next generation but can still be passed down Terms to Know Dominant – trait expressed phenotypically Recessive – trait that seemed to be ‘lost’ Homozygous – having the same alleles for a trait Heterozygous – having different alleles for a trait Principle of Segregation Traits are controlled by pairs of alleles that segregate (separate) during the formation of gametes. For each trait, offspring inherit one allele from each parent Mendel’s F0: cross true breeding lines Yellow parents produce gametes with yellow alleles (A) Green parents produce gametes with green alleles (a) Mendel’s F1 cross Ratio of genotypes = 1 AA: 2 Aa: 1aa (1:2:1) Ratio of phenotypes = 3 yellow: 1 green (3:1) Punnett Square- can calculate all possible combinations of genotypes in children given the genotype of parents F1 cross Possible alleles from father Possible alleles from mother Phenotypic ratio…? Genotypic ratio…? Principle of Independent Assortment During the formation of gametes, segregation of one pair of alleles does not affect segregation of alleles for other traits; alleles sort independently* *only applies to humans under specific circumstances…. Principle of Independent Assortment F0 generation: Mendel establishes plots of peas that breed true for 2 traits (dihybrid) Phenotype: Genotype: Alleles: Round, Yellow RRYY R,Y Seed coat texture: R = round, r = wrinkled Seed color Y = yellow allele, y = green allele wrinkled, green rryy r,y What are the phenotypic and genotypic ratios of the F1 generation going to be? F1 generation: 1st offspring (hybrid) generation: Phenotype: Genotype: alleles: 100% Round, Yellow RrYy R or r Y or y Gametes: Y R Use path analysis to determine the allele combinations for gametes for F1 cross r y Y y RY Ry rY ry F2 generation: 2nd offspring generation There are exceptions to Mendel’s postulates! Mendelian Inheritance = Follows statistical inheritance patterns predicted by Mendel; Each trait is controlled by 1 gene, there are only two possible alleles The alleles for two different traits are not linked Phenotype is governed by genotype Human genetic variation is typically not this simple, but must understand DNA before you will understand why… Phenotypic variation produced by 1) genetic makeup of individual 2) environmental or developmental effects Genotype Phenotype Environment *Height in Japanese immigrants to Hawaii vs. their offspring *tooth development Simple (sort of) Mendelian inheritance in humans Achondroplasia, Brachydactyly, Polydactyly, Huntington’s disease, Marfan syndrome, cleft chin, PTC tasting, Cystic fibrosis, Tay-Sachs disease, Phenylketonuria (PKU), Albinism, Sickle cell anemia, Thalassemia… Misconceptions about dominance and recessiveness Ability to taste PTC Simple Mendelian gene system: 1 gene, 2 alleles T = taster, t = non taster Genotype TT Tt tt Phenotype taster non taster Frequency of tasting and non tasting alleles varies among human populations Evolutionary explanation…? Simple Mendelian Traits: 1 gene produces 1 trait % individuals Example PTC tasting 60 Small amount of phenotype variation due to environment 40 Each phenotype sorts discretely 20..sort of 0 Taster (TT, Tt) Non taster (tt) Phenotypic variation Red Blood Cells Blood Antigens Antigen – any substance which can trigger the immune response *self-antigens Antibodies – proteins of the immune system responsible for identifying and neutralizing foreign particles Human Blood Groups # alleles ABO 3 Rh 2 P 3 Lutheran 2 Kell-Celano 5 Lewis 2 Duffy 2 Kidd 2 Diego 1 Auberger 1 Xg 1 Dombrock 1 Stotzfuz 1 ABO blood group: Blood type incompatibility Mendelian trait: 1 gene, 3 alleles: A (IA), B (IB), O (i) A,B = codominant; O = recessive to A,B Genotype Phenotype Antigens Antibodies Donates to Receives from AA (IAA) A AO (IAi) A anti-B A AB A,O BB (IBB) B BO (IBi) B anti-A B AB B,O AB (IAIB) AB A,B none AB A,B, AB,O OO (ii) O None anti-A anti-B A,B, AB,O O Evolutionary significance of ABO blood groups Related to disease resistance Trace human migratory patterns and selective forces Rh factor and maternal-fetal incompatibility