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Fundamental Concepts of Genetics
- Genes determine the physical and biochemical characteristics of every organism
- Are DNA sequences that code for heritable traits
- Chromosomes are organized bundles of the entire genome in order to ensure that the all DNA can be passed on to daughter cells.
- Alleles are alternative forms of a single gene.
- Genotype is the genetic combination possessed by an individual and the manifestation of this genotype as an observable trait is known as a phenotype.
- Homologues: humans possess two copies of each chromosome except for male sex chromosome (one X and one Y)
- Locus: Each gene has a specified location on the chromosome.
- A person will inherit two alleles for all genes since each chromosome comes with a homologues pair.
- Dominant Allele: alleles where only one copy is needed to express the given phenotype
- Recessive: alleles where two copies are needed for phenotype to be expressed
- Homozygous means that both alleles are the same type, heterozygous means that the alleles are different, and hemizygous is when there is only one allele present
Patterns of Dominance
- Complete dominance: one dominant and one recessive allele for specific gene
- Codominance: when more than one dominant allele exists (blood types A & B)
- Incomplete Dominance: heterozygote expresses a phenotype that is intermediate between the two homozygous genotypes
Penetrance & Expressivity
- Penetrance: population measure which is the proportion of individuals in the population carrying the allele who express the phenotype of that allele.
- Expressivity: Varying phenotypes for an identical genotype
- Can be constant or variable.
First Law: Law of Segregation
- Genes exist in alternative forms (alleles)
- Organism has two alleles for each gene, one from each parent
- Two alleles segregate during meiosis which results in gametes only having one allele.
- Only one allele will be fully expressed while other will be silent, except for when alleles are codominant or incomplete dominant.
Second Law: Law of Independent Assortment
- Inheritance of one gene does not affect the inheritance of another gene.
- Is due to recombination that occurs during prophase I of meiosis.
DNA as Genetic Material
- Was initially thought that protein was the material that genes were inherited through.
- Griffith discovered the transforming principle, which was the principle that bacteria were able to acquire genetic material and alter their genomes
- At Rockfeller institute, 3 scientists noticed that if DNA was destroyed, rat would survive even after undergoing transformation principle, but rat would still die if protein was destroyed by an enzyme
- Proved that genetic material was carried in DNA
- Hershey and Chase discovered that bacteriophages only injected DNA and protein through using radiolabeled sulfur and phosphorus.
Changes in the Gene Pool
- A change in the DNA sequence which results in a mutant allele. Can be compared to their wild-type These counterparts are the alleles which are considered normal or natural in the population
- Mutagens: are substances that can cause mutations (radiation, chemicals)
- Transposons: are elements that can insert and remove themselves from the genome. If inserted in the middle of a coding sequence, gene will be mutated.
- Can also arise during incorrect pairing of nucleotides during transcription/translation.
Nucleotide Level Mutations
- Point Mutations: when one nucleotide in DNA is swapped for another.
- Silent: change in nucleotide has no effect on the final protein synthesized from the gene. Most commonly occurs when changed nucleotide is the third one because there is a degeneracy (wobble) In the genetic code.
- Missense: Substituting one amino acid for another in the final protein
- Nonsense: substituting a stop codon for an amino acid in the final protein
- Frameshift Mutations: when nucleotides are inserted or removed from the genome.
- Can change the codon sequencing which would then shift the reading frame.
- Larger scale mutations where large segments of DNA are affected.
- Deletion Mutations: large segment of DNA is lost from a chromosome
- Duplication Mutation: Segment of DNA copied multiple times in the genome
- Inversion Mutations: Segment of DNA reversed in chromosome
- Insertion Mutation: Segment of DNA moved from one chromosome to another.
- Translocation Mutations: Segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome.
Consequences of Mutations
- Can be advantageous like heterozygotic sickle cell anemia which prevents malaria and has only minor symptoms
- Can be deleterious
- Inborn errors of metabolism: defects in genes required for metabolism.
- The flow of genes between species.
- Can sometimes produce hybrid offspring if two species are closely related.
- Usually cannot reproduce since they have odd number of chromosomes
- Changes in the composition of the gene pool due to chance.
- Founder Effect: extreme case where small population of species is in reproductive isolation as a result of natural barriers, catastrophic events or other bottlenecks that drastically and suddenly reduce the size of the population available for breeding.
- Inbreeding may occur in later generations which encourages homozygosity which subsequently decreases genetic diversity.
- Loss of genetic variation may cause reduced fitness of the populations which is a condition known as inbreeding depression
- Outbreeding/Outcrossing is the introduction of unrelated individuals into a breeding group.
Analytical Approaches in Genetics
- Diagrams that predict the relative genotypic and phenotypic frequencies that will result from the crossing of two individuals. Alleles of two parents are arranged on top and side of squares.
- Monohybrid is a cross in which only one trait is being studied. The P generation is the individuals being crossed and the offspring are the filial generation.
- Often called back crosses since they are used to determine the genotype of the parent based on the phenotype of its offspring
- Organism with an unknown genotype is crossed with an organism known to be homozygous recessive.
- Can extend Punnett square to account for two genes instead of one gene if the genes are unlinked (Mendel’s 2nd law).
- Females have two X chromosomes and thus can be homozygous and heterozygous for a condition on the X chromosome. Males are usually hemizygous for genes carried on X chromosome since there is only one X chromosome.
- Use different notations for these chromosomes: X is for x chromosome and is usually given a subscript if a diseased allele is present. Y is for Y chromosome and rarely carries a sex-linked trait.
- Heterozygous female means that one of her X chromosomes carries the recessive sex-linked trait
- Hemophiliac male means that the male expresses the trait
- Always assume that sex-linked traits are X-linked recessive
- The further apart two genes are, the more likely it is that there will be a point of crossing over (chiasma) between them.
- Recombination Frequency: likelihood of two alleles being separated from each other during crossing over. This is proportional to the distance between the genes on the chromosome.
- Tightly linked genes have recombination frequencies close to 0 percent.
- Weakly linked genes have recombination frequencies approaching 50%.
- A genetic map can be constructed using recombination frequencies. This represents the distance between genes on a chromosome
- Unit of measurement is a map unit or centimorgan which corresponds to a 1 percent chance of recombination to occur between genes.
- Allele Frequency: how often an allele appears in a population. Evolution results from the change in these frequencies in reproducing populations over time
- If frequency is not changing, then evolution is not occurring.
- Population needs to be very large so that there is no genetic drift.
- There are no mutations that affect the gene pool
- Mating between individuals in the population is random
- There is no migration of individuals into or out of the population
- The genes in the population are all equally successful at reproducing
- If these above conditions are met, the population is said to be in Hardy-Weinberg Equilibrium. Then you can use two equations to predict the allelic and phenotypic frequencies.
- These equations demonstrate that evolution is not occurring since parent allele frequencies go unchanged
- Termed survival of the fittest since it is the theory that certain characteristics or traits possessed by individuals may help them to have greater reproductive success.
- Organisms produce offspring, few survive to reproductive maturity
- Chance variation within individuals in a population are heritable, but if they give the organism a higher chance of survival, the variation is deemed favorable.
- Fitness is the level of reproductive success. Individuals with more favorable variances are more likely to have a higher fitness
- Modern Synthesis model or neo-Darwinism: when mutation or recombination results in a change that is favorable to the organism’s reproductive success, then that change is more likely to pass on to the next generation.
- Differential Reproduction: The traits passed on by the more successful organisms will become wide-spread
- Inclusive Fitness: measure of an organism’s success in the population. Based on the number of offspring, success in supporting offspring and the ability of the offspring to support others.
- Different from early descriptions since they were based solely on how many offspring could be created.
- Promotes the idea that altruistic behavior improves the fitness and success of a species.
- Punctuated Equilibrium: changes in some species occur in rapid bursts rather than evenly over time
Modes of Natural Selection
- Stabilizing Selection: keeps phenotypes within a specific range by selecting against extremes. E.g- fetus weights
- Directional Selection: adaptive pressure leads to the emergence of extreme phenotype. E.g. – bacteria who are resistant to antibiotics
- Disruptive Selection: Two extreme phenotypes selected over the norm. E.g – Darwins birds with small and large beaks
- Polymorphisms: naturally occurring differences between members of the same population. The driving force of this selection
- Adaptive Radiation: rapid rise of a number of different species from a common ancestor. Allows for various species to occupy different niches.
- Favored by environmental changes or isolation of small groups of the ancestral species.
- Species: largest group of organisms capable of breeding to form fertile offspring.
- Speciation: formation of a new species through evolution
- Isolation: progeny of two once-similar populations are unable to interbreed.
- Prezygotic mechanisms: prevents the formation of a zygote between two species
- Temporal isolation, ecological isolation, behavioral isolation, reproductive isolation or gametic isolation
- Postzygotic mechanisms: allows gametes to fuse but offspring is sterile.
- Hybrid inviability, sterility, hybrid breakdown (first gen is viable but second gen is not)
Patterns of Evolution
- Divergent Evolution: development of different characteristics between two or more lineages sharing the same common ancestor.
- Parallel evolution: related species evolve in similar ways for a long period of time in response to similar environmental selection procedures.
- Convergent Evolution: independent development of similar characteristics in two or more lineages not sharing a recent common ancestor.
Measuring Evolutionary Time
- Rate is measured by the rate of change of a genotype over a period of time and is related to the severity of the evolutionary pressures put on species.
- Molecular Clock model: analyzing when two related species split from each other by seeing the correlation between their genomes.
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