Understanding Human genetic diversity

• A. Chromosome
• B. Gene ✓
• C. Nucleotide
• D. Protein

Why: A gene is a segment of DNA that codes for a functional product, usually a protein or RNA molecule, and is the basic unit of heredity passed from parents to offspring. Chromosomes are structures containing many genes, and nucleotides are the building blocks of DNA.

• A. 23
• B. 46 ✓
• C. 64
• D. 92

Why: Human somatic cells are diploid, containing 46 chromosomes arranged in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes. Each parent contributes one set of 23 chromosomes to the offspring.

• A. RNA
• B. Lipid
• C. DNA ✓
• D. Carbohydrate

Why: Deoxyribonucleic acid (DNA) carries the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms. Its sequence of four bases (A, T, C, G) encodes the information in genes.

• A. Genotype
• B. Phenotype
• C. Allele ✓
• D. Mutation

Why: Alleles are alternative versions of a gene found at the same position (locus) on homologous chromosomes. Variation among alleles across individuals is a major source of human genetic diversity. Genotype refers to the genetic makeup, and phenotype refers to observable traits.

• A. About 50%
• B. About 75%
• C. About 85%
• D. About 99.9% ✓

Why: Any two unrelated humans share about 99.9% of their DNA sequence. The remaining roughly 0.1% of variation accounts for the genetic differences among individuals and populations, which is why human genetic diversity is relatively limited compared to many other species.

• A. Natural selection
• B. Mutation ✓
• C. Genetic drift
• D. Gene flow

Why: Mutations are changes in the DNA sequence and are the ultimate source of new alleles. Natural selection, genetic drift, and gene flow act on existing variation rather than creating new variation from scratch.

• A. Prophase I ✓
• B. Metaphase II
• C. Anaphase I
• D. Telophase II

Why: Crossing over happens during prophase I, when homologous chromosomes pair up in synapsis and exchange segments. This shuffles alleles and creates new combinations on each chromosome.

• A. Independent assortment
• B. Random fertilization ✓
• C. Crossing over
• D. Genetic drift

Why: Random fertilization means any sperm can fuse with any egg, so each offspring receives a unique combination of parental genetic material. Independent assortment refers to how chromosome pairs separate during meiosis, not fertilization.

• A. Chromosomal inversion
• B. Point mutation ✓
• C. Gene duplication
• D. Aneuploidy

Why: A point mutation alters one nucleotide base, such as a substitution. Chromosomal inversions and duplications involve larger segments, and aneuploidy refers to an abnormal number of whole chromosomes.

• A. Gene flow ✓
• B. Genetic drift
• C. Bottleneck effect
• D. Founder effect

Why: Gene flow is the transfer of alleles between populations when individuals migrate and reproduce. Genetic drift, the bottleneck effect, and the founder effect are changes in allele frequencies driven by chance, not by migration.

• A. Most variation exists between continental populations
• B. Most variation exists within populations rather than between them ✓
• C. Most variation is found in the Y chromosome
• D. Most variation is concentrated in a single gene family

Why: Analyses such as Lewontin's 1972 study estimated that roughly 85-90% of human genetic variation occurs within populations, with only about 10-15% lying between major groups. This is why genetic differences between individuals from the same population are usually larger than average differences between populations.

• A. The complete isolation of a population over many generations
• B. The mixing of genetic material from previously separate populations ✓
• C. The random loss of alleles due to small population size
• D. The process by which new mutations arise in germline cells

Why: Admixture is the interbreeding between groups that were previously separated, producing individuals whose genomes contain ancestry from multiple source populations. Examples include the genomes of many people in the Americas, which reflect African, European, and Indigenous American ancestry.

• A. Directional selection favoring one beneficial allele
• B. Random changes in allele frequencies from one generation to the next, especially pronounced in small populations ✓
• C. Movement of individuals between populations that introduces new genes
• D. Mutations that occur in response to environmental stress

Why: Genetic drift is the chance fluctuation of allele frequencies due to sampling error when passing genes to the next generation. Its effects are strongest in small populations, which can lead to alleles being lost or becoming fixed by chance alone.

• A. Karyotype analysis
• B. Principal component analysis (PCA) of genotype data ✓
• C. Gel electrophoresis of protein samples
• D. Phylogenetic tree of bacterial species

Why: PCA applied to genome-wide SNP data reduces many genetic variables to a few axes, revealing clusters that correspond to geographic ancestry. Plots of the first two components often show individuals grouping by continental or regional origin, making population structure visible.

• A. They have no genetic variation at all
• B. Founder effects and reduced gene flow can increase the frequency of rare recessive alleles ✓
• C. They always have higher mutation rates than other populations
• D. They are immune to genetic drift

Why: When a small group founds a new population or remains isolated, rare alleles can rise to unusually high frequencies by chance, a founder effect. This makes recessive disorders more common in those groups, which can help researchers identify causative genes more easily.

• A. Individuals deliberately change their traits to survive better in their environment
• B. Individuals with heritable traits that improve survival and reproduction tend to leave more offspring ✓
• C. All members of a population acquire the same traits over a single generation
• D. Random mutations are always harmful and are eliminated immediately

Why: Natural selection occurs when individuals with heritable traits that aid survival and reproduction leave more offspring, gradually shifting the population's trait frequencies over generations. Individuals do not choose to change, and most mutations are neutral or harmful rather than uniformly eliminated instantly.

• A. Natural selection
• B. Genetic drift
• C. Mutation ✓
• D. Gene flow between distant species

Why: Mutations create new alleles, introducing genetic variation. Natural selection, genetic drift, and gene flow change the frequencies of existing variants rather than creating new ones.

• A. Everyone is born with the ability and some people lose it through disuse
• B. Dairy farming created a selective advantage for people who could digest lactose as adults ✓
• C. It is caused by exposure to sunlight in northern regions
• D. It spread through random chance alone, with no selective pressure

Why: Most humans lose lactase production after childhood, but in populations with a history of dairy herding, mutations allowing lactase production into adulthood provided a nutritional advantage and were selected for, increasing their frequency.

• A. Directional selection
• B. Stabilizing selection
• C. Balancing selection ✓
• D. Sexual selection

Why: Balancing selection maintains multiple alleles in a population. Heterozygotes (carriers) have an advantage over both homozygous normal individuals (vulnerable to malaria) and homozygous sickle cell individuals (severe anemia), keeping the allele from being eliminated.

• A. Mutation
• B. Adaptation ✓
• C. Genetic drift
• D. Migration

Why: An adaptation is a heritable trait that improves fitness in a particular environment. Mutation is a source of change, genetic drift is random fluctuation in allele frequencies, and migration is movement of individuals between populations.

• A. Natural selection
• B. Genetic drift ✓
• C. Gene flow
• D. Mutation pressure

Why: Genetic drift is the random sampling of alleles across generations. Its effects are strongest in small populations because chance events can easily eliminate or fix alleles.

• A. A rapid increase in allele diversity after a population expands
• B. Reduced genetic diversity when a new population is established by a small number of individuals ✓
• C. The spread of beneficial alleles through natural selection
• D. Gene flow between two neighboring populations

Why: The founder effect occurs when a small group colonizes a new area. The new population carries only a subset of the original population's alleles, so its genetic diversity is reduced.

• A. A sudden, sharp reduction in population size that reduces genetic diversity ✓
• B. A barrier that prevents migration between two regions
• C. A period of rapid population growth that increases mutation rates
• D. The fixation of a single beneficial allele

Why: A bottleneck is a sharp population decline caused by events such as disease, famine, or disaster. Surviving individuals carry only a fraction of the original genetic variation.

• A. Small populations have higher mutation rates
• B. Random sampling effects are larger when fewer individuals contribute to the next generation ✓
• C. Natural selection is absent in small populations
• D. Small populations always have more genetic variation

Why: With fewer individuals, chance fluctuations in which alleles get passed on have a much larger proportional impact on allele frequencies.

• A. Genetic diversity highest outside Africa and lowest within Africa
• B. Genetic diversity decreasing with distance from Africa ✓
• C. Equal genetic diversity across all continents
• D. Genetic diversity highest in populations farthest from Africa

Why: Each successive founding event carried only a subset of variation from the previous group, so diversity declines as populations are located farther from Africa, the source region of modern human ancestry.

• A. Allele frequency
• B. Heterozygosity (gene diversity) ✓
• C. Linkage disequilibrium
• D. Hardy-Weinberg equilibrium index

Why: Heterozygosity, also called gene diversity, gives the probability that two alleles drawn at random from a population are different. Allele frequency describes a single allele's prevalence, while linkage disequilibrium and Hardy-Weinberg measures address associations and departures from expected genotype proportions rather than overall variability.

• A. The number of distinct genotypes observed in a sample
• B. The average number of nucleotide differences per site between two randomly chosen sequences ✓
• C. The total number of single nucleotide polymorphisms in a genome
• D. The percentage of individuals carrying a disease allele

Why: Nucleotide diversity (π) is the average proportion of nucleotide sites at which two randomly sampled sequences differ. It captures the level of sequence variation within a population rather than simply counting variants or genotypes.

• A. Fst ✓
• B. Tajima's D
• C. Watterson's theta
• D. Ne (effective population size)

Why: Fst, the fixation index, measures the proportion of genetic variance found among populations relative to the total variance, and is widely used to quantify population differentiation. Tajima's D and Watterson's theta relate to neutrality tests and within-population diversity, while Ne describes the size of an idealized population.

• A. It directly gives the frequency of the most common allele
• B. It counts positions in a sequence alignment where variation occurs, providing a simple diversity estimate ✓
• C. It measures recombination rates across chromosomes
• D. It indicates the proportion of individuals who are homozygous

Why: The number of segregating sites counts the polymorphic positions in an aligned set of DNA sequences. It is a straightforward way to summarize sequence variation and forms the basis of Watterson's estimator of theta.

• A. Different statistics capture different aspects of variation, such as the number of variants versus their frequencies ✓
• B. Only one statistic is mathematically valid for human populations
• C. Multiple statistics always give identical results, so repeating them confirms accuracy
• D. Using several statistics eliminates the need for sampling

Why: Statistics like segregating sites, nucleotide diversity, and heterozygosity emphasize different features of genetic variation, such as how many variants exist versus how evenly they are distributed in frequency. Using several gives a fuller picture of diversity than any one measure alone.

• A. Genetic drift
• B. Genetic diversity ✓
• C. Gene flow
• D. Natural selection

Why: Genetic diversity refers to the total variation in DNA sequences among individuals within a population or species. Genetic drift, gene flow, and natural selection are mechanisms that can change diversity, but they are not the term for the variation itself.

• A. It makes all individuals equally susceptible to every disease
• B. Different genetic variants can affect how people respond to infections, medications, and environmental exposures ✓
• C. It only matters for inherited eye colour and hair texture
• D. It eliminates the need for medical research across populations

Why: Genetic variants can influence susceptibility to diseases, the effectiveness of drugs, and responses to environmental factors. Recognizing this diversity helps explain why health outcomes and treatment responses differ between individuals and groups.

• A. A large deletion of an entire chromosome
• B. A variation in which a single DNA base differs between individuals at a specific position ✓
• C. A protein that repairs damaged DNA
• D. A type of chromosome found only in mitochondria

Why: A SNP is a change in one nucleotide (A, T, C, or G) at a specific location in the genome. SNPs are the most common form of human genetic variation and are widely used in genetic association studies.

• A. European genomes contain no genetic variation
• B. Research funding and sample collection have been concentrated in certain regions and populations ✓
• C. Europeans are the only population with a genome
• D. Scientists have proven that only European DNA is medically relevant

Why: Historically, research funding, sample collection, and access to participants have been concentrated in certain regions, leading to underrepresentation of many populations. This can reduce the accuracy and benefit of genetic tests and treatments for people from other ancestries.

• A. A mutation that always causes disease in every carrier
• B. How genetic variation can be shaped by environmental pressures such as infectious disease ✓
• C. That malaria has no effect on human populations
• D. That genetic traits are unrelated to geography

Why: The sickle cell allele offers heterozygous carriers resistance to severe malaria, so natural selection has increased its frequency in malaria-affected regions. It shows how environmental pressures can shape genetic diversity and health outcomes, while homozygous individuals still face the disease.