Human Genetics - Exam 3 Flashcards
1
Q
Describe natural selection.
A
Natural Selection
- The differential survival and reproduction of individuals due to differences in phenotype.
- Fitness:
- Is the relative reproductive success of a genotype compared to other genotypes in the population.
- Fitness ranges from 0 to 1.
- To calculate fitness, take the average number of offspring produced by a genotype and divide it by the mean number of offspring produced by the most prolific genotype.
- Selection coefficient:
- Is the relative intensity of selection against a genotype.
- Selection coefficient equals 1 – the fitness for a particular genotype.
- Directional selection:
- A type of selection in which one allele or trait is favored over another.
- The frequency of a recessive allele at equilibrium is equal to the square root of the mutation rate divided by the selection coefficient.
- The frequency of a dominant allele at equilibrium is equal to the mutation rate divided by the selection coefficient.
- Example:
- Lactose-tolerant allele spread from very low frequencies to high frequencies in less than 9000 years after dairy farming produced ample supplies of milk. The estimated selection coefficient was 0.09-0.19 for a Scandinavian population. Though this selection coefficient might seem like a very small number, over evolutionary time, the favored alleles accumulate in the population and become more and more common, potentially reaching fixation.
2
Q
Describe genetic drift.
A
Genetic Drift
- Variation in the relative frequency of different genotypes in a small population, owing to the chance disappearance of particular genes as individuals die or do not reproduce.
- Causes of genetic drift:
- Founder effect: the reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors (e.g., Hutterites).
- Genetic bottleneck - is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide).
- Populations diverge at random in allelic frequency and can become fixed for one allele as a result of genetic drift – especially when the population is small.
3
Q
Define overdominance.
A
Overdominance (Heterozygote Advantage
- Heterozygotes are favored over homozygotes and have a reproductive advantage which maintains both alleles in the population.
4
Q
Define underdominance.
A
Underdominance (heterozygotes selected against) - The heterozygote has a lower fitness than both
homozygotes. This leads to an unstable equilibrium.
5
Q
Describe heterzygote advantage.
A
Heterzygote Advantage
6
Q
Describe the effects of different evolutionary forces on allelic frequencies within populations.
A
Effects of evolutionary forces on allelic frequencies within populations.
7
Q
Describe biological evolution.
A
Biological Evolution
- Genetic change in a group of organisms (Change in gene frequency in a population).
- Two-step process.
- Types of evolution:
- Anagenesis - evolution taking place in a single group (a lineage) with the passage of time.
- Cladogenesis - splitting of one lineage into two; new species arise.
8
Q
What are the levels of genetic variation?
A
Genetic Variation - Levels
- Molecular
- Protein
- DNA sequence
9
Q
Describe molecular variation.
A
Genetic Variation - Molecular Variation
- Molecular data are genetic.
- Molecular methods can be used with all organisms.
- Molecular methods can be applied to a huge amount of genetic variation.
- All organisms can be compared with the use of some molecular data.
- Molecular data are quantifiable.
- Molecular data often provide information about the process of evolution.
- The database of molecular information is large and growing.
10
Q
Describe protein variation.
A
Genetic Variation - Protein Variation
- Protein variation: analyze proteins in a population to identify genotype.
- Measures of genetic variation:
- Proportion of polymorphic loci.
- Expected heterozygosity.
- Explanation for protein variation:
- Neutral-mutation hypothesis: individuals with different molecular variants have equal fitness at realistic population size.
- Balance hypothesis: genetic variation in natural populations is maintained by selection that favors variation.
11
Q
Describe DNA sequence variation.
A
Genetic Variation - DNA Sequence Variation
- Restriction-site variation.
- Microsatellite variation.
- Variation detected by DNA sequencing.
12
Q
A
13
Q
Describe reproductive isolating mechanisms.
A
Reproductive Isolating Mechanisms
- Prezygotic reproductive isolating mechanisms:
- Gametic
- Mechanical
- Temporal
- Behavioral
- Ecological
- Postzygotic reproductive isolating mechanisms:
- Hybrid inviability
- Hybrid sterility
- Hybrid breakdown
14
Q
Describe the modes of speciation.
A
Modes of Speciation
- Speciation: process by which new species arise.
- Allopatric speciation:
- When a geographic barrier splits a population into two or more groups and prevents gene flow between the isolated groups.
- Sympatric speciation:
- Arises in the absence of any geographic barrier to gene flow; reproductive isolation mechanisms evolve within a single interbreeding population.
- Speciation through polyploidy.
15
Q
Describe genetic differentiation associated with speciation.
A
Genetic Differentiation Associated with Speciation
- How much genetic differentiation is required for reproductive isolation to take place?
16
Q
Describe phylogeny.
A
Phylogeny
- The evolutionary relationships among a group of organisms are termed a phylogeny.
- Phylogenetic tree.
- Phylogenetic trees are often constructed from DNA sequence data.
- Two approaches:
- Distance approach
- Parsimony approach
- Two approaches:
17
Q
Describe the theories on the role of the environment in human evolution.
A
Theories on the Role of the Environment in Human Evolution
- Adaptation to Change:
- Assume that certain adaptations, such as upright walking or tool-making, were associated with drier habitat and the spread of grasslands, an idea often known as the savanna hypothesis.
- According to this long-held view, many important human adaptations arose in the African savanna or were influenced by the environmental pressure of an expanding dry grassland.
- If key human adaptations evolved in response to selection pressure by a specific environment, we would expect those adaptations to be especially suited to that habitat. Hominin fossils would be found in those environments and not present in diverse types of habitat.
- Variability Selection Hypothesis:
- The key events in human evolution were shaped not by any single type of habitat (e.g., grassland) or environmental trend (e.g., drying) but rather by environmental instability.
- This hypothesis calls attention to the variability observed in all environmental records and to the fact that the genus Homo was not limited to a single type of environment.
- Over the course of human evolution, human ancestors increased their ability to cope with changing habitats rather than specializing on a single type of environment.
- How did hominins evolve the ability to respond to shifting surroundings and new environmental conditions?
18
Q
Describe the possible outcomes of population evolution in environmental dynamics.
A
Population Evolution in Environmental Dynamics
19
Q
How can molecular changes reveal patterns of evolution?
A
Patterns of Evolution - Revealed by Molecular Changes
- Rates of molecular evolution:
- Rates of nucleotide substitution.
- Nonsynonymous and synonymous rates of substitution.
- Substitution rates for different parts of a gene.
- The molecular clock:
- The rate at which a protein evolves is roughly constant over time.
- Therefore, the amount of molecular change that a protein has undergone can be used as a clock.
- Evolution through changes in gene regulation:
- Genome evolution:
- Exon shuffling.
- Gene duplication.
- Multigene family concept.
- Genome evolution:
21
Q
Describe discontinuous (qualitative) traits.
A
Discontinuous (Qualitative) Traits
- Traits possess only a few phenotypes (e.g., red or white petals).
- All of the traits Mendel studied were discontinuous.
22
Q
Describe continuous (quantitative) traits.
A
Continuous (Quantitative) Traits
- Characteristics vary along a scale of measurement with many overlapping phenotypes.
- For a quantitative characteristic, each genotype may produce a range of possible phenotypes. In this hypothetical example, the phenotypes produced by genotypes AA, Aa, and aa overlap.
- 2 types:
- Meristic characteristics.
- Threshold characteristics.
- Exhibit complex relationship between genotype and phenotype.
- Are likely polygenic.
- May have environmental influences.
- Phenotypic ranges may overlap.
- Cannot use standard methods to analyze.
24
Q
Describe polygenic inheritance.
A
Polygenic Inheritance
- Refers to quantitative characteristics controlled by cumulative effects of many genes.
- Often the genes are large in quantity but small in effect.
- Each character still follows Mendel’s rules.
- May be influenced by environmental factors.
- Examples of human polygenic inheritance are height, skin color, and weight.
25
Q
Describe the 2 types of quantitative characteristics.
A
Quantitative Characteristics - Types
- Meristic characteristics:
- Determined by multiple genetic and environmental factors, and can be measured in whole numbers.
- Animal litter size.
- Threshold characteristics:
- Display only two possible phenotypes - the trait is either present or absent.
- Quantitative because the underlying susceptibility to the characteristic varies continuously.
- When the susceptibility exceeds a threshold value, the characteristic is expressed.
26
Q
How are quantitative characteristics analyzed?
A
Quantitative Characteristics - Analysis
- Statistical methods are required.
- Distribution:
- Frequency distribution.
- Normal distribution: a symmetrical (bell-shaped) curve.
- Samples and populations:
- Population: group of individuals of interest.
- Sample: small collection of individuals from the population.
- The Mean: the average of a set of values.
- The Variation and Standard Deviation:
- Variance (sd2) - the variability within a group of measurements.
- Standard deviation: the square root of the variance.
- The proportions of a normal distribution occupied by plus or minus one, two, and three standard deviations from the mean.
- Correlation: when two characteristics are correlated, a change in one characteristic is likely to be associated with a change in the other.
- Correlation coefficient (r): a statistical measure of the strength of the association.
- Correlation does not demonstrate a cause-and- effect relation. It simply means that a change in a variable is associated with a proportional change in the other variable.
- Regression: predicting the value of one variable, if the value of the other is given.
- Regression coefficient: represents the slope of the regression line, indicating how much one value changes on average per increase in the value of another variable.
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31
Q
Describe the types of heritability.
A
Heritability - Types
- Broad-Sense Heritability (H2 = VG/VP; all genetic modifiers).
- The ratio of total genetic variance to total phenotypic variance.
- Narrow-Sense Heritability (h2 = VA/VP; additive effects only).
- The ratio of additive genetic variance to the total phenotypic variance.
- Calculating Heritability:
- Heritability by elimination of variance components:
- (VP-VE=VG)
- Heritability by parent-offspring regression:
- (h2=b or h2=2b) regression against the mean of both parents [b] or 1 parent [2b].
- Heritability and degrees of relatedness:
- H2 = 2(rMZ-rPZ) correlation coefficient of mono vs dizygotic twins.
- Heritability by elimination of variance components:
32
Q
Describe genetic-environmental interaction variance.
A
Genetic-Environmental Interaction Variance
35
Q
Describe a quantitative trait locus (QTL).
A
Quantitative Trait Locus (QTL)
- A section of DNA that correlates with variation in a quantitative trait.
- Is typically linked to genes that control that phenotype.
- Are mapped by identifying which molecular markers (such as SNPs or microsatellites) correlate with an observed trait.
- This is often an early step in identifying and sequencing the actual genes that cause the trait variation (e.g., oil content in corn or muscle mass in pigs).
