Transmission of Heritable Information

Transmission of heritable information from generation to generation and the processes that increase genetic diversity Mendelian Concepts (BIO) • Phenotype and genotype o Phenotype = what you observe (i.e. height, color, whether organism exhibits trait) o Genotype = the genetic makeup (i.e. homozygous dominant, heterozygous, homozygous recessive) • Gene o Stretch of DNA that codes for a trait (in molecular bio, a gene codes for a protein, which acts to bring about a trait) • Locus o Location of a gene on a chromosome • Allele: single and multiple o Allele is a variant of a gene (gene may have a number of alleles, all alleles of the same gene exist at the same locus) o Cell holds two alleles for each gene, one from mom and one from dad o 2 alleles = simple case (i.e. TT, Tt, tt) o When gene has more than 2 alleles, it is called MULTIPLE ALLELES ▪ e.g. Blood type (have IA, IB, i) • Homozygosity and heterozygosity o Homozygous = when two alleles that an individual carry are the same (i.e. AA, aa) o Heterozygous = when two alleles that an individual carry are different (i.e. Aa) • Wild-type o The “normal” allele or phenotype for an organism (usually most prevalent, but not always) • Recessiveness o The “weak” allele o Only expressed if both copies are present (aa) • Complete Dominance o Normal way of assigning alleles ▪ I.e. AA = dominant; Aa = dominant; aa = recessive • Co-dominance o When the heterozygous conveys both traits ▪ I.e. Blood type: AA = expresses A; AB = expresses A and B; BB = expresses B • Incomplete dominance, leakage, penetrance, expressivity o Incomplete Dominance: ▪ When the heterozygous conveys a mixture of the two alleles • I.e. AA = expresses A; AB = expresses a MIX of A and B; BB = expresses B • I.e. When crossing black and white chickens, you make blueish gray chickens o Leakage ▪ Gene flow from one species to another (through hybrid offspring) o Penetrance ▪ Frequency that a genotype will result in a phenotype ▪ e.g., 100% penetrance means that if you have the genes for being smart, you will 100% be smart • Less than 100% means that you could have the genes for being smart, but that doesn’t mean 100% that you’ll be smart o Expressivity ▪ The degree to which a penetrant gene is expressed ▪ Constant Expressivity = if gene for being smart manages to penetrate (show up as a trait), then IQ is 120 ▪ • • Variable Expressivity = your IQ does not have to be 120, it could be somewhat lower or higher Hybridization: Viability o Process of two complementary, single-stranded DNA or RNA combining together, producing a double-stranded molecule through base pairing o Technique is used for interbreeding between individuals of genetically distinct populations Gene Pool o All the alleles in a population o The sum of all genes/alleles in a population at a given time o High ratings have more genetic diversity and more fitness Meiosis and Other Factors Affecting Genetic Variability (BIO) • Significance of meiosis o Genetic division process of creating haploid sex cells o Occurs in two separate mitosis events o Final result = 4 cells with n number of singular chromosomes o Introduces genetic variability by genetic recombination ▪ Genetic recombination = product of independent assortment and crossing over, which introduces genetic variability • Important differences between meiosis and mitosis Mitosis Meiosis Tetrad formation (pairing of homologous chromosomes) and crossing over Daughter cells identical to parent cells Daughter cells different from parent cells Diploid (2n) daughter cells Haploid (n) daughter cells 1 division involved 2 divisions involved 2 daughter cells 4 sperm cells or 1 egg (with polar bodies) *A human cell has 46 total or 23 pairs of chromosomes. Following mitosis, the daughter cells would each have a total of 46 chromosomes. After meiosis I, the two daughter cells would have 23 chromosomes No tetrad • Segregation of genes o Independent Assortment ▪ Generates genetic variation ▪ Cell has 2 copies of each somatic chromosome – one from mom/dad • Independent assortment shuffles the chromosomes, and then places only one copy of each into the gamete • e.g., AaBb - A and B alleles assort independent of each other ▪ Mechanism: • During metaphase I of meiosis, homologous chromosome pair up along the metaphase line in random orientation – sometimes the mom chromosome is on the right, or sometimes on the left • During anaphase I of meiosis, the homologous chromosomes are pulled apart o Those on the left will go into one daughter cell, the one on the right will go into another daughter cell o Linkage ▪ Because of independent assortment, genes on different chromosomes can be randomized (HOWEVER, genes on same chromosome cannot be randomized by this mechanism) ▪ Genes on same chromosome are linked to a certain extent • Basically, genes located near one-another on the same chromosome are likely to be inherited together ▪ Crossing Over = mechanism that reduces linkage • Only efficient when the genes are physically apart from each other on the chromosome ▪ When genes are further apart on a chromosome, crossing over makes them less linked ▪ The physically closer the genes are, the more linked they are o Recombination (made up by independent assortment and crossing over, increase genetic diversity) ▪ Single Crossovers • Occurs during prophase I and suggests that chromatids exchange alleles at a locus (results in genetic recombination) • When homologous chromosomes are aligned and chromatids from two different chromosomes can exchange segments resulting in genetic recombination • Only affect the ends of a chromosome’s arms ▪ Double Crossovers • An event that has 3 outcomes: o Chromatids exchange alleles, then exchange back (no R) o Chromatids exchange alleles, then exchange with different chromatids (R) o Chromatids exchange alleles, then two different chromatids on the same chromosome exchange again • Chromatids from two homologous chromosomes come in contact at two points • • Can affect segments of the chromosome closer to the middle of the chromosome ▪ Synaptonemal Complex • Complex of proteins that are located between pairs of homologous chromosomes • Protein structure that mediates synapsis (the pairing of homologous chromosomes in Prophase I) o Consists of SYCP2 and SYCP3 which attach laterally to homologous chromatin structures and are attached via a central region (SYCP1 and other proteins) – interdependent with recombination ▪ Tetrad • A chiasma (point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands) between a pair of homologous chromosomes resulting in the formation of 4 chromatids o Sex-linked characteristics ▪ Gene for characteristic is on the X chromosome ▪ Mom always donates X chromosome o Very few genes on Y chromosome ▪ Y chromosome is very small and carries few genes of importance ▪ All the sex-linked alleles are carried on the X chromosome o Sex determination ▪ XX = female ▪ XY = male o Cytoplasmic/extra-nuclear inheritance ▪ Cytoplasmic inheritance = inheritance of things other than genomic DNA ▪ All cellular organelles, such as mitochondria, is inherited from the mother ▪ Extra-nuclear inheritance = situations where genes are inherited outside of the nucleus • Includes receiving all the information about the mitochondria from the mother’s egg Mutation o General concept of mutation – error in DNA sequence ▪ Mutation = change in DNA by means other than recombination o Types of mutations: random, translation error, transcription error, base substitution, inversion, addition, deletion, translocation, mispairing • • ▪ Random: • Random changes in the DNA sequence (could be due to radiation, chemicals, replication error, etc.) ▪ Translation Error: • Even if the DNA for a gene is perfect, errors during translation can cause expression of a mutant phenotype ▪ Transcription Error: • Even if the DNA for a gene is perfect, errors during transcription can cause expression of a mutant phenotype ▪ Base Substitution: • Mutation involving a base (A, T, G, C) changing to a different base ▪ Inversion: • A stretch of DNA (segment of chromosome) breaks off, then reattaches in the opposite orientation o Chromosome rearrangement in which a segment of the chromosome is reversed end to end (i.e. when a chromosome breaks and rearranges within itself) ▪ Addition: • Also called insertion → an extra base is added/inserted into the DNA sequence (can cause frameshift mutation) ▪ Deletion: • A base is taken out of the DNA sequence (can cause frameshift mutation) ▪ Translocation: • A stretch of DNA (segment of chromosome) breaks off, and then reattaches somewhere else ▪ Mispairing: • A not pairing with T, or G not pairing with C o Advantageous vs. deleterious mutation ▪ Advantageous = results in a benefit in the fitness of an organism ▪ Deleterious = results in harmful effects on the fitness of an organism o Inborn errors of metabolism ▪ Genetic diseases resulting in faulty metabolism ▪ Considered congenital (present since birth) metabolic diseases & inherited metabolic diseases o Relationship of mutagens to carcinogens ▪ Mutagen = something that causes a mutation ▪ Carcinogen = something that causes a mutation that causes cancer ▪ Carcinogens are almost always mutations (EXCEPTION: some are direct mitogens = increase mitosis) ▪ Not all mutagens are carcinogens Genetic Drift o Random changes in a population (random changes in allele frequencies) o Effect of genetic drift increases as the population size decreases Synapsis or crossing-over mechanism for increasing genetic diversity o Synapsis and crossing over help increase genetic diversity (genes recombine and create larger diversity in number of recombinants) Analytic Methods (BIO) • Hardy-Weinberg Principle o p+q=1 o (p + q)2 = 1 → p2 + 2pq + q2 = 1 • • • o 5 Assumptions of Hardy-Weinberg: ▪ Infinitely large population ▪ No mutation ▪ No migration ▪ Random mating (no sexual selection) ▪ No natural selection Testcross (Backcross; concepts of parental, F1, and F2 generations) o Test Cross ▪ So, you have something with dominant phenotype. It could either be Aa or AA. To find out, you cross it with the homozygous recessive aa. If Aa, half of the offspring will express the recessive phenotype. If AA, no offspring will express the recessive phenotype o Back Cross ▪ Matting between the offspring and the parent = preserve parental genotype o Parental Generation = P ▪ Generation of the parent. On a pedigree, this is the row that represents the parents o F1 Generation = Filial 1 = children ▪ On a pedigree, this is the row below the parents, and represents the children of the parents o F2 Generation = Filial 2 = grandchildren ▪ On a pedigree, this is the row below the F1, and represents the children of the F1 and the grandchildren of the parents Gene mapping: crossover frequencies o Genetic recombination occurs between maternal and paternal sister chromatids ▪ Can also occur with the identical chromatid within one chromosome, but that will not have any consequence o More likely for recombination to occur over a larger distance (can exchange genetic information anywhere along the long stretch of chromatid) ▪ Further apart that two genes are, the more likely it is that they recombine o Less likely for recombination to occur over a smaller distance (not that much space) o Centimorgen = unit of measurement we use to measure distance on a chromosome (or genetic map unit) ▪ Distance between genes for which one product of meiosis in 100 is recombinant ▪ e.g., 1 distance is 25 m.u. ; another is 6 m.u. • For 25 m.u., 25% of the time that meiosis happens, recombination will occur with respect for the two genes within the map unit distance • For 6 m.u., 6% of the time that meiosis happens, recombination will occur Biometry: statistical methods o Use of statistical methods to understand biological data o ID genes in the population that are bad Evolution (BIO) • Natural selection o Fitness concept ▪ Fitness is defined as the ability to pass your genes on (reproductive success) o Selection by differential reproduction ▪ Individuals who produce more VIABLE offspring are selected for ▪ Individuals who reproduce LESS VIABLE offspring are selected against o Concepts of natural and group selection ▪ Natural Selection: Survival and reproduction of the fittest ▪ Directional Selection: selects for a trait of one extreme (e.g., selection for tall canopy tree in rainforest because the taller tree can reach the sun) • • ▪ Stabilizing Selection: selects for a trait that is moderate, and selects against the extremes (e.g., just the right birth weight when a baby is born) ▪ Disruptive Selection: selects for the extremes (e.g., birds occupying habitat with 2 different niches, one bird with small beak, the other with big beak) ▪ Group Selection: natural selection acting on the group, not the individual (altruism = sacrifice the fitness of the individual to benefit the group or family, which shares similar genes with the individual) • When benefit outweighs the cost, altruistic behavior is selected for o Evolutionary success as increase in percent representation in the gene pool of the next generation ▪ If the frequency of an allele is increased, then that’s evolutionary success for that allele ▪ If the frequency of alleles increased in a population, then that’s evolutionary success for that individual Speciation o Polymorphism ▪ Different forms for alleles or traits (i.e. a yellow jaguar and a black jaguar) o Adaptation and specialization ▪ Adaptation = genetic change in a population caused by natural selection ▪ Specialization = adaption of traits to better fill a niche o Inbreeding ▪ Mating between relatives ▪ Increases the frequency of homozygotes, decreases heterozygotes, and decreases genetic diversity ▪ Inbreeding depression occurs because of the increase in frequency of homozygous recessive detrimental alleles o Outbreeding ▪ Mating with non-relatives, which is just the opposite of inbreeding ▪ Increases heterozygosity o Bottleneck ▪ Severe reduction in population size (e.g., can be caused by natural disaster that wipes out majority of the population) ▪ Genetic drift = random changes in allele frequencies ▪ Effect of genetic drift increases as population size decreases ▪ Bottlenecks increase the effect of genetic drift Evolutionary Time o Random genetic mutations (drift) that are not acted on by natural selection (neutral) occur at a constant rate o By measuring the amount of these neutral mutations, you can find out how much time has passed o You can compare genome differences between two species to find out how long ago they diverged o “Molecular Clock”