Bio 201 F11 Lect 4 (True) v2nr

Bio 201 F11 Lect 4 (True) v2nr - Biology in the News [see...

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Unformatted text preview: Biology in the News [see folder on BB] •  •  •  •  •  •  A study of nearly 50 sediment cores from Africa has shed light on the history of droughts and their linkage to polar ice melt events over the last 50,000 years In [email protected], a "megadrought" in Africa and South Asia was associated with a period of polar ice [email protected] 16,000‐17,000 years ago Most major rivers in Africa dried out, including the Nile and Congo rivers, as well as Lake Victoria (the world's largest tropical lake) and other major lakes [email protected] the drought was the southward movement of the tropical rain belt A major polar ice melt is going on right now It is yet unclear whether similar droughts will be seen cntn_id=118718&WT.mc_id=USNSF_51&WT.mc_ev=click Genes and alleles •  Alleles are alterna)ve forms of the same gene (can be single [email protected] or a whole locus) •  Each diploid individual has one allele of each locus from each parent –  (except for X‐ and Y‐linked loci in males) What is a polymorphism? hZps:// [email protected]/ members/post‐docs/jeremy •  At the phenotype level ‐ observable [email protected] in a [email protected] (e.g. plumage color in a bird species) –  Can be discrete –  or con)nuous •  At the genotype level ‐ any DNA sequence or allelic [email protected] at a gene –  e.g. AACTGGTA, AACAGGTA, AACCGGTA –  e.g. high or low activity alleles of a gene that codes for an enzyme hZp://www.ethno‐ Gene)cs: chalkboard to laboratory •  Simplest kind of polymorphism: one locus, two alleles (A,a) •  Individuals can be AA, Aa, aa •  Early empirical problem: dominance –  If A is dominant to a, then AA and Aa will have the same phenotype; aa will have the “recessive” phenotype –  (this is the completely dominant case, not the [email protected] case) •  Very common among laboratory mutants w‐/w‐ : white eyes female Drosophila melanogaster + superscript = normal ‐ superscript = mutant w+/w+ or w+/w‐ : red (“wild type”) eyes hZp://‐eyecolor.jpg History •  Darwin –  emphasized that [email protected] occurs among individuals of a species •  visible or physiological phenotypic traits •  survival and [email protected] success (fitness). –  recognized two broad classes of [email protected] •  “sports” •  “individual [email protected] –  thought this kind was more important –  He thought that most phenotypic [email protected] should be gradual • Darwin History – Incorporated well known [email protected]: • offspring tend to look like their parents •  However –  source of [email protected] unknown –  mechanism of inheritance unknown • Darwin proposed “pangenesis” as the hereditary mechanism – Each cell of the body buds off a small piece, called a “gemmule”. These pieces congregate in the [email protected] organs where they are incorporated into the gametes and transmiZed to offspring. – Pangenesis ‐> “blending inheritance” – there were only 11 published references to Mendel’s published work (in Proceedings of the Natural History Society of Brunn) before 1900 – two of them were books that were in Darwin’s library – hZp:// th century Early 20 •  Hereditary mechanisms [email protected] to be debated through the late 1800s [email protected] Mendel’s studies were rediscovered at the turn of the century • Two schools of [email protected] thought emerged – Biometricians, or “natural inheritance” school – concentrated on [email protected] variable traits – e.g. height, weight – Mendelian, or [email protected] – concentrated on large effect mutants and other traits that were inherited like the traits Mendel studied Mendelian/Biometrician debate •  raged for decades ([email protected] ~1900) •  had major [email protected] for [email protected] thinking •  Concept of dominance in Mendelian [email protected] posed major problem –  why doesn’t whole [email protected] have the dominant phenotype? •  Concept of “regression” = offspring traits being average of parental traits posed major problem –  why doesn’t whole [email protected] look the same eventually? [email protected] inheritance •  inheritance of one allele of each gene from each parent (except for sex chromosomes, which have special rules) –  solved both the dominance problem and the regression problem –  did not solve all of [email protected] •  just clarified the system so that more complex [email protected] could be asked Hardy‐Weinberg Principle •  the inheritance system by itself does not result in [email protected] •  independently discovered and published in the same year (1908) •  Weinberg was not credited for this [email protected] 35 years later – probably because he published in German and most [email protected] of that era published in English G. H. Hardy (1877‐1947) [Matheme)cian] W. Weinberg (1862‐1937) [Physician, Gene)cist] An example: A is dominant to a # individuals 16 48 36 Brown Blue Blue aa AA Aa Total Number of Individuals = 100 GENOTYPE FREQUENCIES Frequency of AA individuals = 16 ÷ 100 = 0.16 Frequency of Aa individuals = 48 ÷ 100 = 0.48 Frequency of aa individuals = 36 ÷ 100 = 0.36 Allele Frequencies # individuals 16 48 36 48 A 32 A 48 a Total number of alleles= 200 Total Number of A alleles = 32 + 48 = 80 Total Number of a alleles = 48 + 72 = 120 Frequency of A alleles = 80 ÷ 200 = 0.4 Frequency of a alleles = 120 ÷ 200 = 0.6 72 a Gamete frequencies # individuals 16 AA 48 Aa 50% 100% A A 50% a 36 aa 100% What different genotypes of gametes are produced? What are their frequencies? p = freq. of A: 16/100 + .5(48/100) q = freq. of a: 36/100 + .5(48/100) a Gamete frequencies # individuals 16 AA 48 Aa 50% 100% A A 50% a 36 aa 100% a Gamete frequencies are propor)onal to allele frequencies i.e. the gamete frequencies ARE the allele frequencies p = 0.4 q = 0.6 Random ma)ng (also: genotype frequencies equal in males and females) AA, Aa, aa EGGS p = 0.4 A q = 0.6 a AA, Aa, aa SPERM Reginald Crundall PunneZ (1875‐1967) p = 0.4 A 0.16 AA 0.24 Aa q = 0.6 a 0.24 Aa 0.36 aa Frequency of genotypes in Gen 2: AA=p2; Aa=2pq; aa=q2 0.16 0.48 0.36 Genotype frequencies do not change Punneg square The Hardy‐Weinberg principle In a freely interbreeding, sexually reproducing popula)on, the frequency of each allele remains constant Once at equilibrium, genotype frequencies remain constant …the whole country does not become brown‐eyed The Hardy‐Weinberg principle What if genotype frequencies start out differently? e.g. popula)on with 400 AA, 0 Aa, and 600 aa, also has p=0.4 and q=0.6 With one genera)on of random ma)ng, equilibrium genotype frequencies will be reached Condi)ons for non‐evolving popula)on Hardy‐Weinberg principle assumes •  large popula)on •  random ma)ng •  no muta)on •  no migra)on •  no selec)on When H‐W does not apply: Hardy‐Weinberg principle does not hold in any of these condi)ons: •  small popula)on •  non‐random ma)ng •  muta)on (e.g. changing A to a at high frequency) •  migra)on •  selec)on Causes of Evolu)on •  small popula)on •  non‐random ma)ng •  muta)on •  migra)on •  selec)on { These two processes involve [email protected] of new [email protected] [email protected] Back to “blending inheritance” •  an old idea, but it does have some basis in reality –  Offspring are oxen intermediate in trait value between parents •  How can Mendelian inheritance explain this? –  These traits are determined by several or many genes, inherited independently [email protected] example chromosome Allele [email protected] large size Allele [email protected] small size Large male Genotype of sperm (haploid) Genotype of egg (haploid) But keep in mind: genes aren’t the whole story! Small female Medium‐sized offspring Genotype of offspring (diploid) Causes of Evolu)on •  small popula)on •  non‐random ma)ng •  recurrent muta)on •  migra)on •  selec)on { These two processes involve [email protected] of new [email protected] [email protected] Origin of gene)c varia)on •  Gene)c change –  Muta)on –  Recombina)on •  Sexual organisms: meiosis •  Bacteria: DNA uptake from environment and/or exchange of species‐specific plasmids (circular DNAs) •  Gene Flow (migra)on) Causes of Evolu)on •  small (effec)ve) popula)on size •  non‐random ma)ng •  recurrent muta)on •  migra)on •  selec)on EFFECTIVE population size •  Refers to the number of individuals who successfully reproduce •  This number may be substantially smaller than the total number of individuals in the population –  In a sexually reproducing species, there may be 50% males and 50% females in the population •  BUT that does not mean that every individual reproduces [email protected] [email protected] defi[email protected] of [email protected] •  a change in allele frequency in a [email protected] over @me Causes of Evolu)on (i.e. viola)ons of Hardy‐Weinberg equilibrium) •  small (effec)ve) popula)on size –  diff[email protected] [email protected] success (horse example in previous lecture –  gene)c dril •  •  •  •  non‐random ma)ng recurrent muta)on migra)on selec)on Gene)c Dril Fig 23.7 Gene)c Dril Fig 23.7 Gene)c Dril Fig 23.7 ...
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This note was uploaded on 12/16/2011 for the course BIO 201 taught by Professor True during the Fall '08 term at SUNY Stony Brook.

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