Genetics Test 4

Describe how spontaneous mutations arise through replication and chemical changes. Know the examples described in class. What is a tautometric shift?

Spontaneous mutations occur routinely in all cells and come in the form of replication errors, errors during crossover, and due to chemical changes

Ex.)

• Base substitutions (represents incorporation errors)

  • Tautometric shift: rare tautometric forms of standard bases can pair w/ nonstandard partners. Creates a rare form of the base, such as the two NH’s instead of one NH2 bond in cytosine

  • Nonstandard base pairing occurs due to a wobble (C bonding with A, T bonding with G, etc)

• Insertions and deletions

  • Extra/less bases, occurs through slippage or unequal crossing over

• Base substitutions (again)

  • Depurination

    • Cleavage of purine (A or G aka pure As Gold) base from a 1’ carbon → can occur 5000x a day in a cell

    • Typically replaced by A → can replace a G:C pair with an A:T pair after replication

  • Deamination

    • Cleavage of amine (NH2) group from cytosine to U or T

    • Can replace G:C pair with A:T / A:U after replication

Describe how induced mutations occur via base analogs, base modification, oxidizers, and intercalating agents.

Induced mutations result from exposure to external factors such as chemicals or radiation

• Base analogs: may be incorporated into DNA and result in mispairing (inserts something random like bromine)

• Chemical modification of bases within DNA also may result in mispairing

• Oxidizers: type of reactive oxygen species (ROS) which may result in base mispairing (basically random addition of an O)

• Intercalating agents: Distort DNA leading to indels (addition of agents cause for DNA helix to have bigger gaps aka indels)

Distinguish between incorporated errors and replication errors.

Incorporated errors happen when DNA polymerase inserts the wrong nucleotide during DNA synthesis, usually because of temporary abnormal base pairing such as tautomeric shifts or wobble-like mispairing.

Replication errors are the mistakes that remain after proofreading/repair and then get copied in the next round of replication, making the change permanent.

Describe how radiation exposure induces mutations.

Ionizing radiation generates reactive ions and free radicals, which modify bases (through base sub) and/or break DNA (through rearrangements)

Lower energy leads to pyrimidine dimers, Covalent bond that occurs between two pyrimidine bases

Ex.)

• X-rays

• Gamma rays

• UV

Gene vs Chromosome mutations

Gene:

Small scale

Affects a single gene

Originally revealed through phenotypic effects

Chromosome mutations

Large scale (bc can be replicated on)

Affects chromosomal structure or number

Often can be visualized via microscopy

Both can be revealed by DNA sequencing

Define transposable elements and explain how they are able to move around in the genome.

Transposable elements are DNA sequences that move between genomic locations

• Look like multi colored corn kernels, with spots/stripes/veining appearance

• Present in some form in all genomes

  • Humans consist of 45% transposable DNA from all their DNA

• Such “insertional mutagenesis” may alter either coding sequence and protein function or the extent of gene expression

The general steps:

1.) Staggered cuts are made in the target DNA by transposase

2.) Transposon is joined to single stranded ends of target DNA

3.) DNA in single stranded gaps is replicated/filled in

• Relies on enzyme transposase, which is encoded by the transposon and components of cell’s own machinery

• Terminal inverted repeats are recognized by enzymes that catalyze transposition

Different mechanisms of transposition

Replicative: Copy + paste, generates copies, each new copy is inserted at a new site

• Increases number of transposons

Nonreplicative: Only cuts and pastes, a given transposon is cut and moved to a new site, relocates basically

• No increase in number of transposons

What are the different types of transposable elements

DNA transposons (Class II): moveable component is DNA

• Short terminal inverted repeats

• Encodes transposase gene

• Transposition through DNA

• PROK and EUK

Retrotransposons (Class I): Transpose through an RNA intermediate

• DNA is copied to RNA, which is then reverse transcribed to DNA

• Long terminal direct repeats

• Encodes reverse transcriptase gene

• Transposition through RNA intermediate

• EUK only

Explain the role of transposable elements in mutation.

• Allows for variation by the genes that are able to move locations, also called jumping genes

• Enzyme catalyzed insertion at the new sites is what leads to mutation or chromosomal rearrangements

Relationship between transposable elements and chromosomal rearrangements

Because replicative transposons are essentially identical → they can align and undergo recombination (the process where DNA strands break and rearrange, creating new allele combinations in offspring)

→ Recombination events can alter the structure of the chromosomes

Compare and contrast the different mechanisms for DNA repair and explain the importance of such mechanisms. (continues to next class)

Mismatch repair

• Repairs replication errors, including mispaired bases and strand slippage

1.) Mismatched nucleotides occasionally get incorporated, which causes kinks in the DNA

2.) Mismatch repair recognizes these kinks

3.) Exonucleases come to remove local stretch of new DNA strand including the mismatched nucleotide

4.) DNA polymerase replaces nucleotides and DNA ligase seals the nick

Direct repair

• Repairs pyrimidine dimers, and other specific types of alterations

1.) Works through enzymatic conversion of altered nucleotides back to their original

• Photolyase - cleaves pyrimidine dimers

• Methyltransferase - demethylates methyl-guanosine

Compare and contrast the different mechanisms for DNA repair and explain the importance of such mechanisms. (continues from last class)

Base excision repair - repairs abnormal bases, modified bases, and pyrimidine dimers

1.) DNA glycosylase removes damaged base

2.) AP endonuclease removes sugar (2) and phosphate (#)

3.) DNA polymerase fills in appropriate base

4.) DNA ligase seals the nick in the sugar phosphate backbone

Nucleotide excision repair - repairs DNA damage which distorts the double helix which includes the abnormal bases, modified bases, and pyrimidine dimers

1.) Some damage distorts the DNA, causing a lesion (disrupting normal genetic function)

2.) Recognized by repair enzymes that separates strands of damaged sequences, cleave damaged strands, fill in/seal gap

Summarize the importance of molecular techniques for our understanding of genetics and biology in general.

Transformed our understanding of genetics

• Shifted focus of genetics from observation of phenotypic effects to changes in nucleotide sequence

• Provided details of genetic processes including replication, transcription, translation, RNA processing, and regulation of gene expression

• Now applied to essentially all subdisciplines of biology

• Basis for commercial applications of biology-based products (biotechnology)

• Facilitated understanding of disease and advances in treatment of disease

Summarize the five key innovations in molecular techniques.

1.) Discovery of DNA structure

• Double helix composed of complementary strands assembled from four nucleotides

2.) Recombinant DNA technology

• DNA from different sources could be combined

3.) The polymerase chain reaction (PCR)

• Amplification of specific DNA fragments from a limited source

4.) Development of techniques to determine DNA sequences

• Continued improvements in speed, accuracy, and cost

5.) CRISPR-Cas systems

• Accurate and efficient editing of genomes

Summarize the purpose and techniques associated with recombinant DNA technology. What enzymes and DNA molecules are used in this approach?

Enables locating, isolating, altering, and expressing specific DNA segments (fragments) for genetic engineering

• Variation restriction enzymes (restriction endonucleases) 

  • The bacterial enzymes which defend against viruses usually, but in this case can create DNA fragments by cleaving DNA at specific sequences

  • More than 800 restriction enzymes exist to recognize the palindromic sequences, some will generate sticky ends (HINDIII) and others will generate blunt ends (Pvull)

• DNA ligase

  • Combines the fragments by sealing the “nicks” in the sugar-phosphate backbone

  • Allows for the development of gene cloning

  • Can combine DNA fragment of interest with a cloning vector (a stable, replicating DNA molecule that could be introduced to a bacterial cell)

  • Replication

Explain the purpose of gel electrophoresis and how this technique "works".

To determine the size (bp) of the fragments

1. DNA fragments of different sizes are placed in the wells at one end of the agarose gel

2. An electrical current is passed through the cell

3. All the DNA fragments will move toward the positive pole; smaller fragments will migrate/move faster than large fragments. After electrophoresis, the fragments of different sizes will have migrated different distances

DNA fragments (shown in bands) are visualized with a chemical that fluoresces under UV light

How do restriction enzymes and DNA ligase allow gene cloning?

DNA fragments cut by the same restriction enzyme have matching ends, so DNA ligase can join them together. A DNA fragment can be inserted into a cloning vector, which replicates in bacteria and makes many copies of the inserted DNA.

What must a useful vector look like?

1. First, must contain an origin of replication recognized in the host cell so that it is replicated along with the DNA it carries

2. Should carry selectable markers/ traits that enable cells containing the vector to be identified

3. A cloning vector needs a single cleavage site for each of the restriction enzymes used

Good example is a plasmid

Explain the purpose of the polymerase chain reaction. Be able to identify example uses of PCR.

Explain the purpose of the polymerase chain reaction. Be able to identify example uses of PCR.

PCR is a method to amplify specific DNA segments (fragment of interest) rather than the standard method of gene cloning which is slow, labor-intensive, and difficult to automate in comparison

Can be used in gene cloning, mutagenesis, forensics, infection testing, ancient DNA, genealogy, etc

Summarize how the polymerase chain reaction works and the purpose of each component in the reaction. Know the difference between "step" and "cycle" for PCR.

PCR Uses purified DNA pol to synthesize specific DNA segments in vitro

The amount of DNA produced doubles with each replication event (cycle)

• Can produce billions of copies in just a few hours

• Sensitive - can work with extremely small amounts of starting material

Components:

• DNA molecule of interest: used to serve as the template

• DNA polymerase: synthesizes the new DNA (taq poly)

• A pair of synthetic primers (each 17-25 nts with a 3’ -OH group): For DNA pol to extend

  • One is complimentary to a known sequence 

  • The other is complementary to a known sequence on other strand of template

• dNTPs: to be assembled into new daughter strands

Explain how dideox DNA sequencing "works" and how and why the original Sanger method was automated.

Uses purified DNA pol to synthesize (replicate) DNA strands in vitro

• Template to be sequenced (often produced by cloning or PCR)

• A primer complementary to one strand of template

• dNTPs to assemble

Identify the major improvements provided by next-generation sequencing technologies.

Faster and less expensive than traditional sanger sequencing

• Can be completed in days rather than years

• Approaching $100 instead of millions

Advances are important for genome sequencing and single cell sequencing studies

Summarize how the natural CRISPR system functions to protect many prokaryotic cells.

Through an array of palindromic repeats and unique DNA sequences (spacers) derived from phase and plasmids

Occurs in three stages:

• Adaptation: Stores memory of foreign DNA as spacers to create genetic memory of past infections

• Expression: Long precursor of crRNA transcribed and processed

  • The CRISPR region is transcribed into RNA and processed into short CRISPR RNAs (crRNAs).

  • Each crRNA contains a sequence matching a past invader and guides the defense system.

• Interference

  • If the same invader returns, the crRNA directs Cas proteins to the matching DNA.

  • The Cas enzyme cuts/cleaves and destroys the foreign DNA, preventing infection.

Summarize has the CRISPR system has been engineered for genome editing.

1. Researchers engineered a sgRNA (single guide RNA)  which is a 20nt sgRNA sequence which directs the effector to a DNA sequence of interest

2. Cas9 unwinds DNA so that sgRNA can pair with complementary sequences in DNA 

3. Cas9 cleaves the DNA 

4. Then cell will try to repair the break/cleave, based on the repair mechanism, it could either cause a frameshift mutation or insert a donor DNA sequence

What’s dideoxy/sanger sequencing and how can it work

DNA sequencing method that uses ddNTPs to stop DNA synthesis.

1. Four separate reactions are set up, each with the DNA template, primer, DNA polymerase, normal nucleotides, and one type of ddNTP.

2. When a ddNTP is added, DNA synthesis stops because ddNTPs lack a 3′-OH group.

3. This creates DNA fragments of different lengths.

4. The fragments are separated by gel electrophoresis.

5. The sequence is read from the bottom to the top of the gel.

6. The sequence obtained is the complement of the original target DNA.

What about automated dideoxy sequencing

Same purpose as sanger, but more modern/affective

1. The DNA template is mixed with a primer, DNA polymerase, normal dNTPs, and all four ddNTPs.

2. Each ddNTP has a different fluorescent dye.

3. All reactions happen in one tube and are run in one gel well.

4. When a ddNTP is added, DNA synthesis stops, creating fragments of different lengths.

5. During electrophoresis, a laser detector reads the fluorescent dye on each fragment.

6. The color tells the computer which base is present.

7. The sequence is sent directly to a computer.

Compare and contrast the forward genetics and reverse genetics approaches. What is the goal of these methods?

Goal of both forward and reverse methods are techniques to allow for analysis of gene function

Forward genetics: the traditional approach of “phenotype to gene”

1. Identify mutant organism (may first involve generating the mutation)

2. Map the mutation in the genome

3. Isolate and sequence the associated genes

4. Predict wild-type protein sequence and structure

5. Study location and function of protein

Reverse: the alternative approach of “gene to phenotype”

1. Start with gene of unknown function

2. Create mutations in gene (or add novel gene)

3. Observe effects on phenotype

What is a transgenic mouse and how may such organisms provide information on human disease?

Transgenic mice are mice affected by reverse genetics, where they engineer a gene into it to see the phenotype that results from it, allows for determination of human gene/protein function, therefore can create mouse models of human disease

Transgene = foreign DNA permanently added to genome

Can also produce knockout mice (knock out gene) and knock in mice (bring in a gene)

Explain how RNA interference may be used to study protein function.

RNA Interference (RNAi) can be used to temporarily turn off gene expression

• Can turn off gene expression and examine the effect on the phenotype

• Comes from/based on natural siRNA/miRNA control of gene expression

• Works by researcher designing short RNAs complementary to gene of interest and introduces it into the cell/organism

• These short RNA molecules complex with RISC and destroy or block translation of corresponding mRNA, therefore being expressed in the phenotype

List the primary goals of the field of genomics.

DEFINITION: Genomics is the field of genetics concerned with the content, organization, function, and evolution of the genetic information in whole genomes

Genomics is relevant for health, biotechnology, and agriculture

• Identifies important DNA sequences

• Facilitates comparative approach

• Allows for genome manipulation

• Provides potential for creating novel genomes

Compare and contrast the two major types of maps used in structural genomics.

Genetic (linkage) maps: rough location of genes relative to other genes based on frequency of recombination

• for linked genes, distance between loci is proportional to rate of recombination

• Measured in cM or m.u.

Physical maps: Based on direct analysis of DNA, may involve restriction mapping (enzyme digests), now commonly DNA sequencing

• Measured in base pairs (kilobases or megabases)

• Have better resolution and accuracy

Compare and contrast the two major approaches to genome sequencing.

Genome sequencing: determining the ordered nucleotide sequence of entire genome which allows researchers to answer other questions, the 2 diff approaches:  

Genome sequencing reveals sites where individuals of a species differ in their genome species

Map based sequencing

1. Partial digest genome into large overlapping fragments → insert each into vector

2. Use genetic markers to align large fragments in correct order → creates contig

3. A subset of overlapping clones is selected and cut into subclones

4. Subclones are sequenced and overlap used to assemble genome in correct order

Whole-genome shotgun sequencing

1. Small-insert clones are prepared directly from genomic DNA

2. Clones are sequenced 

3. Computer software assembles entire genome by looking for overlap among clones

• Genome must be sequenced multiple times to ensure overlap

• Sequencing coverage: Average number of times a nucleotide must be sequenced (move coverage = more accurate)

Explain the importance of SNPs for our understanding of the genome. How are SNPs related to haplotypes?

DEFINITION OF SNP: Example from genome sequencing and how it reveals the site where genome sequence differs. Useful markers to locate disease associated genes.

• Individuals differ at a single base pair

• Variant must reach at least 1% in population to be a SNP

• Differences arise through mutation and inherited as allele variants

• Mutations often do not cause phenotypic differences

• Numerous throughout genomes → average every 1000 bp in humans

If SNPs are physically close to disease associated genes, SNP tends to be inherited with the disease causing allele, thus people with disease often have different SNPs, and we can look for such SNPs to reveal location of the disease-associated gene

Haplotypes: Term for the specific set of SNPs/genetic variants on chromosome or part of a chromosome. SNPs within haplotype are physically linked, therefore tend to be inherited together.

• New haplotypes arise through mutations or crossing over

Describe the goal of genome-wide association studies and how this approach is related to SNPs.

GWAS is an approach that uses numerous SNPs across the genome to find genes of interest which contribute to traits through their complex interactions.

• Identifies genes associated with specific diseases, height, BMI, puberty age, etc

Any given gene through this approach provides only a modest proportion of genetic influence on the trait

Genes identified by GWAS often represent minority of total genetic variation in trait

Distinguish between bioinformatics, metagenomics, and synthetic biology.

Bioinformatics: Combines molecular biology and computer science

• Ex.) Databases, search algorithms, gene-prediction software, sequence analysis

Metagenomics: Samples/determines genomes of entire group of organisms in common environment

• Ex.) microbial community structure (microbiome) of the human gut

Synthetic biology: seeks to design genomes/organisms

• Ex.) Advances made with prokaryotes and eukaryotes

• Carries ethical and security concerns

Explain the difference between qualitative traits and quantitative traits and their relationship to the genotype.

Qualitative traits: Such as whether a plant is tall or dwarf, words

• Discontinuous (cant really continue/measure words) 

Quantitative: The actual number/range of heights, numbers

• Continuous

Describe the basis and key features of quantitative traits in terms of genotype and phenotype.

In quantitative traits, the relationship genotype and phenotype is more complex, 

Different things that affect it: 

• Polygenic: affected by more than one gene to the phenotype

• 3 genes could give 27 possible genotypes

  • Several genotypes could give the same phenotype

  • Further complicated if multiple alleles

• Environmental factors: Allow single genotype to produce range of phenotypes to adapt to environment

• Best understood for traits with additive alleles, (each additive allele contributes a small amount)

Explain the genetics of quantitative traits

As number of loci increases → number of potential phenotypes increases

• Differences between individual phenotypes become more difficult to distinguish

• Relationship between genotype and phenotype would become less obvious

Explain the various statistical methods used to analyze quantitative traits

Through frequency distributions, would summarize the numbers or proportions of all phenotypes of a quantitative characteristic, typically show a normal/bell shape distribution

• Mean: provides average/center of distribution

• Variance: Provides info about the spread of distribution, the greater the variance, the more it is spread out from the mean

  • Standard deviation is a square root of the variance

Explain how traits are additive and make predictions about genetic crosses based on additive effect.

Additive traits are quantitative traits where each “+” allele adds a small amount to the phenotype, so the effects of the allele “stack” together and are expressed more. In genetic crosses, this means F1 offspring are often intermediate between the parents, while F2 offspring show a wider range of phenotypes because they inherit different numbers of additive alleles.

More (+) = more of the trait

Less (+) = less of the trait

Increase of loci results in 2 more alleles, 2 more alleles expressed

Define 'heritability' in the context of quantitative traits.

Heritability is the proportion of total phenotypic variation due to genetic differences (not environmental). Total phenotypic variation depends on the environmental factors. To determine the extent of influence for each: must partition into components and have some quantitative measure of phenotype in question

Vp = Vg + Ve + Vge

For potential causes of differences across individual phenotypes: Vp = (Va+Vd+Vi) + Ve + Vge

Explain how heritability can be described with equations of different types of variance.

Genetic variance = VG

Additive variance (VA) - is where diff alleles contribute concrete effects to the phenotype, and the sum of allelic contributions determines the phenotype

Dominance variance (VD) - Allelic interactions show simple dominance so the presence of one masks the effect of the other

Gene interaction variance (VI) - when genes at different loci interact, makes the predictions of phenotype more difficult (like epistasis)

Distinguish between the two major types of heritability

Broad sense heritability (H2)

Proportion of phenotypic variance due to genetic variability (H2 = VG/VP)

• H2 = 0 means that it’s all environmental 

• H2 = 1 means its all genetic

Difficult to calculate because its calculation requires elimination of environmental component

Narrow sense heritability (h2) lowercase 

Proportion of phenotypic variance due to additive genetic variability (h2 = Va/Vp)

• Determines how likely offspring are to represent their parents

Knowing heritability of a trait allows statistical predication regarding the phenotypes of offspring to be made on the basis of the parents’ phenotypes

Correlation on h2

b=h2=0 

• Genetic differences have no contribution, mostly environmental

h2=1

• All differences are due to additive genetics

h2=0.5

• Both additive genetics and environment contribute

Summarize the limitations of heritability analysis.

Depends on a variance in a POPULATION, does not apply to INDIVIDUALS

Not universal but unique to the population under the study

• Bc diff populations can have different allele distributions which affects heritability

Environment can still influence a trait even when heritability is high (human height)

Describe how allelic frequencies represent or describe the gene pool.

Gene pool - The collection of genotypes/alleles that is present in a population and contributes to the phenotype

Allelic frequency tells you the proportion of each allele within that pool.

Therefore allelic frequency helps up describe the alleles in the gene pool

Be able to calculate genotypic frequencies and allelic frequencies in normal traits.

Genotypic frequencies

fAA = #AA/Ntotal  faa = #aa/Ntotal   fAa=#Aa/Ntotal

Frequencies must add up to 1

Ex.) population = 500: if 130 were AA, and 100 were aa, what are the frequencies of each genotype

AA = 130/500 = 0.26

aa = 100/500 = 0.2

Aa = ?

Aa must be 0.54 because they all have to add up to 1

Allelic frequencies

p = f(AA) + ½ f(Aa)

q = f(a) = faa + ½ f(Aa)

p + q always = 1

Explain the Hardy-Weinberg law and its assumptions.

Hardy-Weinberg law = mathematical model that evaluates the effect of reproduction on genotypic and allelic frequencies of a population

If one assumes a population is infinitely large, is randomly mating, and is not affected by mutation, migration, or natural selection, then:

• The allelic frequencies of a population will not change

• The genotypic frequencies of the population would stabilize/not change after one generation

When Hardy - Weinberg assumptions are met → reproduction ALONE will not alter allelic or genotypic frequencies

If all HW assumptions are met, a population which is not at equilibrium will reach equilibrium after one generation of random mating

Use the Hardy-Weinberg equilibrium to describe the effects of mating on genotypic and allelic frequencies

Principle of segregation: frequencies of alleles in gametes = frequencies of alleles in parents

Random mating means the gametes will combine in random combinations though

Calculation of genetic frequencies of next generation according to HW

AA = p2

Aa = 2pq

aa = q2

Describe the effect of nonrandom mating on genotypic and allelic frequencies.  Discuss the concept of inbreeding and its consequences for genetic diversity.

Assortative mating: kind of nonrandom mating which individuals show distinct choice of partner

Positive - select partners that look like themselves

Negative - select partners that look different from themselves

Inbreeding - form of positive assortative mating, increase homozygotes 

• The higher the inbreeding coefficient (F), the faster the population will reach homozygosity (can be harmful)

Higher genotypic frequency in AA and aa, showing representing those in the phenotype

Explain the effects of evolutionary forces (mutation, migration, genetic drift, selection) on allele frequencies.

Mutation: All genetic variations will ultimately arise through mutation

• If 25 individuals with 2 alleles: p = 0.9  q = 0.1     (G1 = 45 copies, G2 5 copies)

• If one G1 mutates to G2, then p = 0.88  and q = 0.12

• But there's also the possibility that G2 can mutate back to G1

• AT EQUILIBRIUM: the number of forward mutations = number of reverse mutations, meaning there’s no net change in allele frequency

Migration: movement of individuals from one population to the other will move the alleles from one population to the other, which changes frequencies of alles in the resulting populations

Consequences: 

Genetic drift - Deviation from expected ratio due to limiting sample size is “sampling error”

Sampling error can result from:

• Limiting resources for several generations

• Founder effect

• Bottleneck effect

Effects of this genetic drift being

• Changes to allelic frequencies, indicating no equilibrium

• Leads to loss of variation within population

• Different populations will diverge genetically from each other

Selection - Alleles that confer traits that improve the ability of an individual to produce offspring that will be selected for and this increase in frequency of those alleles

• Fitness (W) can be expressed as percentage of offspring produced by a genotype relative to the most prolific genotypic

Differential fitness among genotypes leads to changes in genotypic frequencies, thus allelic frequency over time.

Analyze the role of population size and sampling error in shaping genetic variation and the strength of genetic drift. What events may produce a reduction in population size?

Small population size increases the impact of sampling error, meaning allele frequencies can change just by chance because only a limited number of individuals reproduce. This strengthens genetic drift, causing random shifts in allele frequencies, loss of genetic variation, and possible fixation of alleles. In contrast, large populations reduce sampling error, so allele frequencies stay more stable.

Events that reduce population size include bottleneck events (e.g., disasters, disease, habitat loss) and the founder effect (a small group starting a new population)

Define fitness and describe how fitness may be calculated for a given genotype.  Using fitness, be able to predict the effect of natural selection on allelic frequency.

A1A1 = 10(W1110/10 = 1)   A1A2 = 5 (W125/10 = 0.5) A2A2 = 3(W22 3/10 =0.3)

Predicting natural selection effects:

Genotypes with higher fitness increase in frequency

Their alleles become more common over time

Genotypes with lower fitness decrease, so their alleles become less common

Differentiate between genetic drift and natural selection in terms of their impact on population variation.

Genetic drift: random change in allele frequencies due to chance (sampling error), strongest in small populations. It typically reduces genetic variation within a population (can lead to fixation) and increases differences between populations over time

Natural selection: non-random change driven by differences in fitness. It can either reduce variation (e.g., selecting one optimal allele) or maintain variation (e.g., heterozygote advantage), depending on the selection type

Drift = random, chance-driven loss of variation

Selection = fitness-driven, directional change in variation

Compare and contrast the effects of evolutionary forces within and between populations.

Within populations (genetic variation inside one group):

Mutation: introduces new alleles → increases variation

Gene flow (migration): brings in new alleles → increases variation

Genetic drift: random loss/fixation of alleles → decreases variation, especially in small populations

Natural selection: usually reduces variation by favoring certain alleles (but can maintain it in some cases, e.g., heterozygote advantage)

Nonrandom mating (inbreeding): increases homozygosity (reduces genotype variation, not allele frequencies)

Between populations (differences across groups):

Gene flow: makes populations more similar (reduces differences)

Genetic drift: makes populations more different over time (divergence)

Natural selection: can increase or decrease differences depending on whether environments are different or similar

Mutation: can contribute to differences slowly over time