1/10. Lecture 6 Ch 3 & 4 Lecture Notes Heredity and Evolution

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1 1/10 Lecture 6 Ch 3 & 4 Lecture Notes Heredity and Evolution Chromosomes = packages of coiled DNA in the cell nucleus In humans, DNA is placed on to 46 chromosomes > 23 pairs. 22 = autosomes and the 23 = pairing of the sex chromosomes DNA Replication Copying DNA is required for both mitosis and meiosis. Meiosis is the production of cells those cells with only 23 single chromosomes (1/2 the genetic set). process is behind the diversity observed in the combination and genetic expressions Explains why sexually reproducing organisms >> asexual organisms re diversity. Recombination occurs during meiosis which shuffles maternal/paternal chromosomes in the gamete and resulting in gametes having a new combination of genetic material. results in unique sequences of existing genetic information Recombo explains why offspring from the same parents still differ. Crossing over is when genes found on the same chromosome are inherited by the offspring as a unit Introduction Mendel the papa of genetics Before the rediscovery of Mendel's work there were problematic models explaining how traits were inherited. Darwin believed in the blending model Mendel's research with hybrid pea plants generated principles and the beginnings of concepts like genes, alleles, etc. Explained how variation was maintained in spite of selection Mendel's research on pea plants developed principles that apply to the genetics of all biological organisms Looked at 7 different traits each with 2 expressions. Color (flowers): purple or white Location (flowers): axial or terminal Color (seeds): yellow or green Shape (seeds): smooth or wrinkled Color (pods): green or yellow Shape (pods): inflated or constricted Height of plant: tall or short ^dominant or recessive Mendel cross bred pea plants with different traits to create hybrid generations and documented the results. Research program First he cross bred pea plants with different traits and produced a first generation or 'parental generation' F1 F2 = pea plants self fertilized

2 2/10 F3...F4...Fn Consider crossbreed: short and tall pea plants F1 = all tall F2 = 3/4 tall and 1/4 short Pattern : whatever composed F1 were more common in F2 Ratio of expressions for a trait = 3:1 = three tall: one short Mendel inferred: different trait expressions were controlled by discrete units (genes) Mendelian Principles of Inheritance after his experiments Mendel identified several principles we can use to better understand heredity Principle of Segregation Genes (alleles) occur in pairs because chromosomes occur in pairs During Gamete formation the pair of alleles separate one into each gamete Now know that this principle is explained by meiosis Recall: during meiosis, paired chromosomes do what was described in the last three points They reunite in the zygote restore the full complement of chromosomes which are present in the offspring Dominance and recessiveness Recall trait: height; expressions: tall and short F1 = all tall F2 = 3:1 ratio tall to short Mendel figured out that the unit for shortness didn't go away, it just wasn't expressed. Recessive traits that are not expressed in heterozygotes Recessive traits or alleles are only expressed if an individual has two copies of it they're homozygous Dominant traits that are expressed in heterozygotes Prevent the expression of recessive alleles in heterozygotes Recall: Genes = segment of DNA directing the production of specific proteins, protein part, functional product Locus location of genes on a chromosome Loci have genes which come in variations or expressions what we have been calling alleles. Allele official definition: different versions of a gene with the ability to direct calls to produce slightly modified forms of the same unit though expressed differently. Height example the tall expression = dominant allele vs short recessive allele. Dominant alleles are always expressed Recessive allele is masked

3 3/10 H = dominant height allele = tall and h = recessive allele = short If Hh = heterozygous Recessive alleles are only expressed if the alleles are homozygous recessive = hh So an individual may be labelled the following HH = homozygous dominant = tall trait is expressed Hh + hh = 2Hh = heterozygous dominant = tall trait is expressed hh = homozygous recessive = short (no heterozygous recessive, btw) Note: F1 was must have resulted in heterozygous individuals = Hh Cross = HH x hh Genotype: an individual's combination of alleles = three = HH, Hh, hh; while... Phenotype: the observable trait/characteristic dictated by the genotype. So, HH and Hh = tall because dominant alleles mask recessive; and Hh = short phenotypic expression Thus we have 3 Genotypes but 2 Phenotypes BUT the phenotype is also influenced by environment Mendel controlled for these traits, though Simple Punnett Square Dominant/recessive workings multiple things produce either dom or rec One way both alleles code for two variants of the same protein... Principle of Independent Assortment genes coding for different traits sort out independently from each others during gamete formation. Now know this is because the genes controlling for two different traits are located at different, nonpartner chromosomes Random assortment During meiosis chromosomes travel to newly forming cells independently Mendelian inheritance and us M" traits = discrete traits controlled by alleles at only one genetic locus. Humans have ~20,000 different traits inherited this way but many of them are biochemical in nature and so lack any obvious phenotypic expressions. Either present or absent and described in terms of frequencies within populations. The frequencies of a trait in one population can be compared to those in another or from one generation to another generation. Because we know the approx loci for many M" traits we can look at the mechanisms/patterns of inheritance associated with these loci.

4 4/10 Notable examples Dominant traits Achondroplasia dwarfism Cleft chin dimple/depression in middle of chin Recessive traits Cystic fibrosis common Mt's associated with genetic disorders among Euro American males Patients develop obstructive lung disease and until recently 1/2 ppl with this never reached adulthood. Tay Sachs disease common among Ashkenazi Jews; degeneration of the nervous system starting at 6 months and lethal by age 2/3 Phenylketonuria (PKU) Mental impairment stemming from the inability to metabolize the am acid phenylalanine treat via strict diet Albinism inability to produce normal amounts of pigment melanin phenotype: very fair skin, light hair and light eyes. Sickle cell anemia Abnormal form of hemoglobin HbS resulting in collapsed rbcs clog capillaries reduce blood flow to organs and sans treatment could result in death. If a person inherits a harmful dominant allele she will also have the condition it causes even if the partner chromosome recessive allele is different A recessive disorder requires two copies of the recessive allele associated with the cause. If you only have one of the harmful recessive allele you're unaffected but a carrier. If mate with another carrier then possible to have the have an affected child for the cause. ABO blood system = another M" trait in humans A, B, and O = ABO locus on chromosome 9. Determine your ABO blood type code for antigen production Antigens on surface of red blood cells If only antigen A present, then bloodtype = A Antigen A = genotype; blood type = phenotype Only B, then blood type = B If A and B are both present, then bloodtype = AB If neither, then bloodtype = O The O allele is recessive to both A and B meaning a person with type O blood received two copies of the allele from each parent and it's homozygous recessive A and B are both dominant to O If blood type A then either genotype AA or AO If blood type B then either genotype BB or BO

5 5/10 Type AB = codominance two different alleles present and both expressed on the surface of red blood cells In this situation, both alleles influence the phenotype Caveats to Dominance/Recessiveness Dominant alleles are not "stronger" than recessive ones nor are they more common because natural selection favors them if this were true then the dominant genetic disorders above would be more common in the population. More clear with discrete Mendelian traits but gets hairy when we look at the next trait type Polygenic traits traits influenced by genes at 2+ loci. Examples: stature, skin, eye, and hair color. Many polygenic traits influenced by environmental factors nutrition, UV ray exposure, etc. Contra Mendelian traits, polygenic = continuous traits the alleles at two different loci each influence the phenotype such that we observe a graded difference between several expressions rather than one or more distinct differences Pigment coloration is due to the pigment melanin produced by melanocytes Influences coloration of skin, hair, and eyes (81p) Influenced by various interactions between different loci found on roughly five different 'pigmentation' genes Polygenic traits = influenced by multiple genes, we can't analyze like Mendelian traits Account for most easily recognizable phenotypic traits in humans + other species Also influence stature, facial structure, fingerprints Measured on a distribution of equal intervals hence continuous traits. Height measured in m/cm measure many individuals to result in a continuous distribution from short to tall. Genetics and the influence of environmental pressures Interaction between genotype and environmental factors determine phenotypic expression. Stature in humans heavily affected by nutritional factors during childhood Sunlight exposure, altitude, and temperature. NOTE: Mendelian traits = less likely affected by environmental factors ABO determined at fertilization and stays constant irrespective of environmental factors. More Mendelian vs Polygenic traits Mendelian = discrete categories of variation Polygenic = continuous Both determined by Mendelian principles at specific loci

6 6/10 Each loci will have different alleles whether a trait is influenced by one or several Dominance and recessiveness still a factor Combination of multiple loci interacting with loci to produce phenotype Mitochondrial inheritance the hundreds of organelles in a cell powering it like little engines (they convert energy from nutrients) They house ++ copies of ring shaped DNA called mitochondrial DNA or mtdna Similar structure and function to nuclear DNA Sequenced entire molecule 40 genes MORE when discuss mutations Modern Evolutionary Theory Modern synthesis in the later 1920s early 30s. Both mutations and natural selection contributed to evolution Evolution now defined in two stages 1. Variation inherited differences among organisms is produced and redistributed through various processes 2. Natural selection acts on variation resulting in differential reproductive success (85p) Current definition of Evolution Change in allele frequency from one generation to the next. Allele frequencies = signify a population's genetic makeup where individuals share a gene pool ABO blood type example (85p) Microevolution = short term effects from one generation to the next Macroevolution = long term over time synonymous with speciation Both ^ influenced by same basic evolutionary mechanisms need genetic variation produced by mutation to occur first which is then acted upon by natural selection Things that produce and redistribute variation Mutation any change in DNA Also any alteration of an allele into another/of the a gene Mutations any change DNA bases, chromosome number &/or structure Point mutations substitution of one base for another can disrupt protein production or result in the production of defective proteins affect function of cells as a consequence Can also cause alleles to change Only affect evolution if they occur in sex cells Evolutionary change can only arise from mutations if they're passed from one generation to the next Evolution solely due to mutation = rare Only a big deal in microorganisms

7 7/10 Only when combined with natural selection do we get significant/rapid evolutionary change IMPORTANT: mutations are "the only way to produce new genes (that is, variation)" (86p). Key role in the 1st stage of evo Gene flow exchange of genes between populations Occurs when the migrants interbreed Anthro ex: our mating patterns primarily determined by social factors Migration is a consistent feature of human evolution "and gene flow between populations (even though sometimes limited) helps explain why speciation has been rare during the past million years or so" (86p). Anthro Ex: African Americans today = largely W. African descent + significant genetic admixture with Eur. Americans Measure migration of Eur alleles into African American gene pool via measuring allele frequencies Study mentioned on 86 7p Genetic drift plus founder effect Genetic drift evolutionary change produced by random factors in small populations REALLY important due SOLELY to the fact that the population is small Rare alleles in small populations might disappear completely because they're not passed to later generations merely by chance Founder effect TYPE of genetic drift evident in numerous human/nonhuman popns Small band of 'founders' leaves larger popn to form separate group or colony elsewhere and/or small groups of survivors of natural disasters/in fragmented forests etc Over time, mating w/in the smaller group increases popn of individuals w/ genes stemming only from original colonists Rare alleles in the larger popn might bcm more common in colony b/c an individual carried it over. B/c large proportion of descendents connected to original colonist Smaller group only have a sample of a larger gene pool So influenced more significantly by chance Outcome = reduced genetic diversity, substantial differences in allele frequencies between later founder generations and the larger group/popn Also called genetic bottleneck

8 8/10 Ex cheetahs = genetically uniform past dramatic decline in numbers resulting in loss of many alleles Yield high counts of defective sperm decreases reproductive potential Anthro Ex: we are genetically uniform to chimpanzees All modern human popns descendent from small groups ~170kya Founder effect in isolated farming communities and island groups French Canadians = 6 million descending from 8, th century French founders significant difference in allele frequencies between popn of Quebec and France Diseases seen in French Canadians: Tay Sachs, PKU, etc Last 4 5my hominins lived in small groups making drift a significant force Genetic bottleneck in last ~150ky informs modern day genetic makeup Note Gene flow and Genetic Drift = microevolutionary changes = changes within a species over short term Natural selection is needed to have speciation but does not operate independent of mutation, gene flow, and genetic drift Recombination exchange of DNA segments b/w chrom. pairs during meiosis Doesn't cause change in allele frequencies/evolution by itself BUT some genes can be influenced just by the alleles they're close to and recombination changes the composition of chromosome parts which further influences the ways certain genes function The direction of natural selection and more on it's need for variation to act on Recall: mutation, gene flow, genetic drift, and recombination interact and produce variation + redistribute genes within and bw popns BUT HAVE NO EFFECT ON LONG TERM DIRECTION OF EVO CHANGE Adaptation and evo occurs only if popn's gene pool changes in a specific direction some alleles bcm more common versus other alleles Natural selection directs this relative to given environmental factors

9 9/10 Adaptation change or shift in allele frequencies in a given direction due to changes in selective pressures caused by changes in the environment Anthro Ex: natural selection in humans sickle cell trait Hemoglobin S (HbS) = abnormal form of Hb molecule Most are homozygous HbA (HbA/HbA) which produces normal hemoglobin Heterozygotes = HbA/HbS = sickle cell trait Some abnormal Hb, "enough is normal to allow them to function normally under most circumstances" (90p) Homozygous HbS/HbS = sickle cell anemia SCA reduces red blood cells' ability to transport oxygen in blood stream and during times of stress the rbcs collapse look like sickles Life expectancy = 45 years Causes ~100k deaths per year worldwide HbS occurs in all human populations but more common in w and central African popns ~20% frequency (in Greece and India too) Sickle cell trait (SCT) selected for b/c counters malaria Malaria = parasitic organism that invades rbcs obtaining the oxygen Spread by mosquitoes Kills ~2 million people per year Prominent in areas mentioned above SCT = 40% rbcs collapse and do not accommodate malarial parasite Malaria = selective agent favoring the HbA/HbS heterozygous phenotype due higher reproductive success of people with SCT Always have people with SCA because heterozygous

10 10/10 Cross to F1 H H h Hh Hh h Hh Hh F1: genotype = Hh = all heterozygous dominant = phenotype = all tall F1 to F2 H h H HH Hh h Hh hh F2: genotype = HH, Hh, hh = homozygous, heterozygous dominant, and homozygous recessive = phenotype = 3:1 = tall to short

Ch. 14 Mendel and the Gene Idea

Ch. 14 Mendel and the Gene Idea 2006-2007 Gregor Mendel Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used experimental method