Unit 2: Biological basis of life, heredity, and genetics 1
Summary 1. Quiz info - Quiz next Wednesday, 3-22 2. Wrap-up Ch 3 - Cell Division 3. Abduction 4. Mendel's Principles of Inheritance 2
Mitosis and Meiosis - types of cell division Recap Mitosis -Somatic cells (i.e., body cells - skin, hair, muscle, etc.) are duplicated -ONE division produces cells with all 46 chromosomes Results: two daughter cells, genetically identical to parents and siblings Meiosis -Gametes (sex cells like sperm and ova in humans) -TWO divisions produces cells with only 23 chromosomes Results: four daughter cells, not genetically identical 3
Unlike mitosis, meiosis involves... -Gametes transmit genetic info from parent to offspring Crossing over: chromosomes break and reconnect onto different chromosomes which results in... Recombination: new combinations of genetic information is created -Every generation has new genetic combinations = additional variation 4
NEW genetic information? Recombination makes it so new combinations of genetic information appears from generation to generation But… Mutations - Changes in the nucleotide sequence of DNA -Only way NEW genetic variation is introduced 5
Natural selection and genetics Natural selection - Traits making reproductive success more likely given environmental pressures will appear in higher frequencies from generation to generation. Sources of Variation *Mutations - new genetic information *Meiosis ( recombination ) - new combinations of genetic information Result: new variation is found in every generation of sexually reproducing populations Now we know how the variation NS needs to act on is created. (Thanks science!) 6
Mendel - 1860s Monk experimenting with peas Recall: Cross-breeding - artificial selection, farmers...metaphor for natural selection Missing in Darwin's theory of NS: a mechanism governing how traits were inherited Background -Mendel cross-bred pea plants -Observed the traits individual plants possessed over thousands of generations Inferred the Principles of Inheritance 7
Mendel - Experiments Experiment: Selectively cross-breed common pea plants over many generations. Parental generation: Mendel cross-breed purebreds F1 generation: first offspring generation Later generations self-pollinated Mendel observed: Some traits seen in offspring w/out blending of parent traits E.g., Petals = either white or purple; seeds = either yellow or green - no inbetween Observation: Cross-breeding plants w/ yellow seeds and plants w/ green seeds ALWAYS produced offspring with yellow seeds for F1. BUT F2 = 3:1 ratio of yellow to green 8
Mendel - Cross-breeding pea plants Mendel inferred: There must be a regularity governed by a mechanism of inheritance; specifically, *Inheritance of each trait is determined by a 'unit' ( gene ) offspring receive from their parents unchanged *Individuals inherit one 'unit' from each parent for each trait *Traits might not be expressed in an individual but can still be passed on to the next generation 9
Mendel - more inferences Parental generation = Homozygous yellow seeds + homozygous green seeds F1 = Heterozygous yellow seeds I.e., Each offspring inherited two different alleles (one from each parent) Genotype - genetic makeup of an individual (e.g., YY, GG, YG, etc) Phenotype - physical expression of an individual's genotype (e.g., yellow, green, tall, short, smooth, wrinkled, etc.) Mendel inferred (some more): Whenever F1 breed each plant will have an equal chance of passing on either Y or G alleles 10
Mendel - overview -Cross-bred pea plants for thousands of generations -different trait expressions controlled by discrete units (genes) alleles - the different expressions of a gene The principles of inheritance 1. Segregation - for a trait, the pair of expressions from each parent separate and only one passes from parent to offspring. Meiosis - NOW we know this principle is Meiosis 2. Independent Assortment - different pairs of alleles are inherited by offspring independent from one another. 11
Mendel - inferences from observations Note: Parent plants = pureblood = homozygous for seed color I.e., each parent had identical expressions of the 'unit' (now gene) for this trait Allele - Alternative forms/expressions of a gene E.g., trait: seed color; expression: yellow or green. Y = yellow allele and G = green allele. Parent 1 = YY Parent 2 = GG 12
Mendel's Inferences Genotype - genetic makeup of an individual (e.g., YY, GG, YG, etc) Phenotype - physical expression of an individual's genotype (e.g., yellow, green, tall, short, smooth, wrinkled, etc.) Mendel observed: Some allele expressions dominated others. E.g., Pea seed genotype = YG resulted in phenotype yellow so the dominant expression/form/allele = yellow (green is recessive ) E.g., Trait: height; alleles: tall T, short t 13
Dominance and recessiveness Recessive - traits that are not expressed in heterozygotes Dominance - traits that are expressed in heterozygotes AND homozygotes -these traits prevent the expression of recessive alleles in heterozygotes. Alleles - the different expressions of a gene -genes = segments of DNA -> direct protein synthesis ->found at different locus or loci of a chromosome Since they are paired the dominant allele will be expressed Height example H = tall = dominant allele and h = short = recessive allele 14
Mendelian traits - discrete traits determined by alleles at a single genetic locus -they're either present or absent -allele frequencies of a trait in a given population ~20,000 Mendelian traits in humans - most biochemical Dominant Mendelian traits = cleft chin, dwarfism; Recessive = Tay-Sachs disease, Phenylketonuria (PKU), albinism, sickle-cell anemia Recessive disorders manifest if homozygous - if heterozygous, a person = unaffected but carrier 15
Mendelian traits Mendelian traits - discrete traits determined by alleles at a single genetic locus Dominant traits = cleft chin, dwarfism; Recessive = Phenylketonuria (PKU), albinism, sickle-cell anemia Recessive disorders manifest if homozygous - if heterozygous, a person = unaffected but carrier More clear with discrete Mendelian traits but gets hairy when we look at the next trait type 16
Polygenic Traits Polygenic - traits influenced by genes at 2 or more loci E.g., stature, skin, eye, and hair color Continuous traits - gradiation of difference in several expressions 17
Mendelian Traits vs Polygenic Traits Mendelian = discrete categories of variation Polygenic = continuous Both -determined by Mendelian principles at specific loci -Dominance and recessiveness still a factor NOTE: Mendelian traits = less likely affected by environmental factors Ex: ABO determined at fertilization and stays constant irrespective of environmental factors. 18
Modern Evolutionary Theory Modern synthesis in the later 1920s-early 30s. 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). -Both mutations and natural selection contribute to evolution Current definition of Evolution - Change in allele frequency from one generation to the next. Allele frequencies = indicators of a group/population's genetic composition -Described as proportions or percentages of a total 19
Things that produce and redistribute variation 1. Mutation - any change in DNA - bases, chromosome number &/or structure -Also any alteration of an allele into another/of the a gene -Random Ex. HbS is a different allele of hemoglobin -Only affect evolution if they occur in sex cells - mutations lead to change only if they're inherited Mutations are "the only way to produce new genes (that is, variation)" (86p). Evolution solely due to mutation = rare Only when combined with natural selection do we get significant/rapid evolutionary change Ex. Point mutations - substitution of one base for another -disrupt protein production or cause production of defective proteins 20
Things that produce and redistribute variation 2. Recombination - exchange of DNA segments b/w chrom. pairs during meiosis Like mutations - Doesn't cause change in allele frequencies/evolution alone BUT some genes are influenced by the alleles they're close to and recombination changes the composition of chromosome parts which further influences the ways certain genes function 21
Mendelian traits in humans - ABO blood system A, B, and O = alleles at the ABO locus on chromosome 9 Antigens - what the A/B/O alleles code for Antigen A = genotype; blood type = phenotype Only B, then blood type = B The O allele is recessive to both A and B -If type-O blood received - two copies of the allele from each parent (homozygous recessive) If blood type A then either genotype AA or AO If blood type B then either genotype BB or BO 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 22
Mendel's Experiments and observations Experiments: Cross bred pea plants with different physical traits and then let them generate many populations E.g., The trait of height of a pea plant can be expressed two ways: tall and short Cross breeding produced F1 - all plants = tall F2 = 3/4 tall and 1/4 short 3:1 ratio of tall:short, respectively. Mendel's inferences from the data different trait expressions controlled by discrete units (genes) 23
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