Genetics & Evolution Flashcards
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Genetics Terms
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- Genes: DNA sequences that code for heritable traits; basic unit of heredity.
- Chromosomes: Composed of genes and a large supply of non-coding DNA; allow genetic material to be easily passed to daughter cells during mitosis and meiosis.
- Alleles: Alternative forms of a gene, such as the three ABO blood antigen alleles for the same gene (Iᴬ, Iᴮ, i); person will inherit two alleles for all genes, because each chromosome is part of a homologous pair (except for MALE sex chromosomes).
- Gene Pool: All the alleles that exist within a species.
- Genotype: Genetic combination of a trait.
- Phenotype: Physical and observable manifestation of a trait coded by given genotype.
- Homologues: Two copies of each chromosome, one from father and one from mother (except for MALE sex chromosomes).
- Locus: Location on a specific chromosome in which a gene can be found.
- Dominant, Recessive, Homozygous, Heterozygous, Hemizygous (Genotype in which only one allele is present for a given gene, as is the case for parts of the X chromosome in males).
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Patterns of Dominance
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- Complete Dominance: Occurs when only one dominant and one recessive allele exists for a given gene in the gene pool.
- Codominance: Occurs when more than one dominant allele exist for a given gene in the gene pool, such as person with A blood antigen allele and B blood antigen allele will express both alleles simultaneously.
- Incomplete Dominance: Occurs when a heterozygote expresses a phenotype that is intermediate between the two homozygous genotypes, such as when pink Rr flower is produced from red RR and white rr parents.
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Penetrance and Expressivity
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- Penetrance: The proportion of the population with a given genotype who actually express the phenotype; the probability that, given a particular genotype, a person will express the phenotype. Full penetrance (100% of individuals with the allele will express it), high penetrance, low penetrance, nonpenetrance.
- Expressivity: The different manifestations of the same genotype across the population. Constant expressivity (all individuals with a given genotype express the same phenotype), Variable expressivity (individuals with the same genotype have different phenotypes). A disease shows variable expressivity because some individuals carrying the defective allele express tumors, while others carrying the same defective allele express cataracts, while others are asymptomatic.
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Mendelian Concepts
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- Mendel’s First Law of Segregation: (1) Genes exist in alternative forms called alleles. (2) An organism has two alleles for each gene, one inherited from each parent. (3) The two alleles segregate during meiosis, resulting in gametes that carry only one allele for any inherited trait. (4) If two alleles of an organism are different, only the dominant one will be fully expressed while the recessive one will be silent (codominance and incomplete dominance are exceptions). Correlates to the separation of homologous chromosomes during Anaphase I of meiosis.
- Mendel’s Second Law of Independent Assortment: The inheritance of one gene does not affect the inheritance of another gene. Correlates to the process of crossing over (recombination) that occurs in tetrads during Prophase I of meiosis. Complicated by the discovery of nonindependent assortment of linked genes.
- Both segregation of homologous chromosomes and independent assortment of alleles increase the genetic diversity of gametes and thus of offspring, which improves the ability of a species to evolve and adapt to environmental stresses.
5
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DNA as Genetic Material
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- Transforming Principle discovered through experimentation with mice and virulent and nonvirulent bacteria.
- Transforming principle determined to be DNA. When treated with enzymes known to degrade DNA, bacteria were not transformed and mice lived. When treated with enzymes noted great proteins, bacteria were transformed and mice died.
- DNA confirmed to independently carry genetic information. Bacteriophages with radiolabeled sulfur (which is found in proteins but not in DNA) and bacteriophages with radiolabeled phosphorus (which is found in DNA but not in proteins) were permitted to infect a group of nonlabeled bacteria; radiolabeled proteins in the viral capsid did not enter the cells, but radiolabeled DNA did.
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Mutations
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- Mutation: Change in DNA sequence that results in a mutant allele.
- Wild-Type Allele: Counterpart of mutant allele that is normal or natural and ubiquitous in the population.
- Mutagens: Agents such as ionizing radiation and chemical exposure that can cause mutations.
- Transposons: Genetic elements that can insert and remove themselves from the genome; if transposon inserts in the middle of a coding sequence, mutation will occur and disrupt the gene.
- Nucleotide-Level Mutations: Point Mutations (one nucleotide in DNA is swapped for another) can be subcategorized as Silent, Missense, Nonsense Mutations. Frameshift Mutations (causes shift in reading frame) can be subcategorized as Insertion or Deletion Mutations.
- Chromosomal Mutations: Large segments of DNA are affected that can be subcategorized as Deletion Mutations (large segment of DNA is lost from chromosome), Duplication Mutations (segment of DNA is copied multiple times in the genome), Inversion Mutations (segment of DNA is reversed within the chromosome), Insertion Mutations (segment of DNA is moved from one chromosome to another), Translocation Mutations (segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome).
- Some mutations can be Advantageous, confirming positive selective advantage that may allow the organism to produce fitter offspring (heterozygotes for sickle cell disease have selective advantage as they are less likely to die from malaria). Some mutations can be Deleterious. One important class of deleterious mutations is Inborn Errors of Metabolism (defects in genes required for metabolism).
7
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Genetic Leakage and Genetic Drift
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• Genetic Leakage: Flow of genes between species. Individuals from different but closely related species can mate to produce Hybrid offspring (such as mule, which is infertile hybrid of male horse and female donkey).
- Genetic Drift: Changes in the composition of the gene pool due to chance; tends to be more pronounced in small populations.
- Founder Effect: Extreme case of genetic drift in which a small population of a species finds itself in reproductive isolation from other populations 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 these small populations; encourages homozygosity and increases prevalence of both homozygous dominant and recessive genotypes.
- Genetic drift, founder effect, and inbreeding cause reduction in genetic diversity and increase in prevalence of certain traits and diseases, resulting in Inbreeding Depression (reduced fitness of population). Outbreeding/Outcrossing refers to the introduction of unrelated individuals into a breeding group, which could increase variation within gene pool and increase fitness of the population.
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Monohybrid Cross (P to F₁)
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• Crossing homozygous dominant parent with homozygous recessive parent in P Generation results in 100% heterozygous offspring in F₁ Generation.
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Monohybrid Cross (F₁ to F₂)
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• Crossing heterozygotes with complete dominance from F₁ Generation results in 1:2:1 genotypic ratio (1HomoD, 2HeteroD, 1HomoR) and 3:1 phenotypic ratio (3DomTrait, 1RecTrait) of offspring in F₂ Generation.
10
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Test Cross
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- Used to determine an unknown genotype by crossing an organism with unknown genotype with an organism known to be homozygous recessive.
- If all offspring are of the dominant phenotype, the unknown genotype is likely to be homozygous dominant.
- If there is a 1:1 distribution of dominant or recessive phenotypes, the unknown genotype is likely to be heterozygous.
11
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Dihybrid Cross
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- Crossing parents that are both heterozygous for both traits with complete dominance results in 9:3:3:1 phenotypic ratio.
- 3:1 phenotypic ratio still holds for each trait, reflecting Mendel’s Second Law of Independent Assortment.
12
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Sex-Linked Cross
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- Sex-linked traits are X-linked recessive traits (unless told otherwise on MCAT). Sex-linked traits are much more common in males, since only having one recessive allele is sufficient for expression of the recessive phenotype.
- Father with a sex-linked trait will have daughters who are either carriers of the trait (if mother does not have the trait) or who express the trait (if mother carries or expresses the trait). This father can never pass down a sex-linked trait to his sons, but the mother can if she expresses the trait or is a carrier.
13
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Gene Mapping
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- Recombination Frequency: The likelihood that two alleles are separated from each other during crossing over; ranges from 0% (tightly linked genes) to 50% (weakly linked genes that independently assort).
- 1% recombination frequency is equal to 1 map unit or centiMorgan between two genes on Genetic Map of a chromosome.
- Order of genes on chromosome can be determined by adding recombination frequencies.
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Hardy-Weinberg Principle
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• A population in which evolution is not occurring (with constant allele frequencies) is said to be in Hardy-Weinberg Equilibrium if (1) the population is very large; no genetic drift, (2) there are no mutations that affect the gene pool, (3) meeting between individuals and the population is random; no sexual selection, (4) there is no migration of individuals into or out of the population, (5) the genes in the population are all equally successful at being reproduced.
- Given that a gene only has two alleles (one dominant and one recessive), two equations can be derived.
- p + q = 1, where “p” is the frequency of the dominant allele and “q” is the frequency of the recessive allele.
- p² + 2pq + q² = 1, where “p²” is the frequency of the homozygous dominant genotype, “2pq” is the frequency of the heterozygous dominant genotype, and “q²” is the frequency of the homozygous recessive genotype.
- Note that there will be twice as many alleles as individuals in a population.
15
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Modern Synthesis Model (Neo-Darwinism)
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- When mutation or recombination results in a change that is favorable to the organism’s reproductive success, that change is more likely to pass on to the next generation via Differential Reproduction. Populations evolve, not individuals, because the gene pool (allele frequency) changes over time.
- 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 then support others; takes into account the benefits of certain behaviors in the population at large, such as altruism (close relatives of an individual will share many of the same genes, so promoting the reproduction and survival of related or similar individuals can also lead to genetic success) or protecting the offspring of the group at large.
16
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Natural Selection
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• Natural Selection: Survival of the fittest; theory that certain characteristics or traits possessed by individuals within a species may help those individuals have greater reproductive success in passing on those traits to offspring. Natural selection is simply a mechanism for evolution. Three modes of natural selection.
- Stabilizing Selection: Phenotypes are kept within a specific range by selecting against extremes. Human birth weight is maintained within a narrow band by stabilizing selection; fetuses that way too little may not be healthy enough to survive, and fetuses that way too much can experience trauma during delivery through the relatively narrow birth canal.
- Directional Selection: An initially extreme phenotype is selected for by adaptive pressures. If a plate of heterogenous bacteria is treated with antibiotics, only those colonies that exhibit resistance to the antibiotics will survive; a new standard phenotype emerges as a result of differential survivorship.
- Disruptive Selection: Two extreme phenotypes are selected over the norm; facilitated by the existence of Polymorphisms (naturally occurring differences in form between members of the same population). Darwin’s finches had either slightly larger or smaller beaks, even though the original ancestor had a medium-sized beak.
- Adaptive Radiation: Rapid rise of a number of different species from a common ancestor that is favored by environmental changes or isolation of small groups of the ancestral species; allows for various species to occupy different Niches (specific environments, including habitat, available resources, and predators, for which a species is specifically adapted).
