Human Genetics - Exam 3

Question 1

Describe natural selection.

Answer 1

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.

Question 2

Describe genetic drift.

Answer 2

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.

Question 3

Define overdominance.

Answer 3

Overdominance (Heterozygote Advantage

• Heterozygotes are favored over homozygotes and have a reproductive advantage which maintains both alleles in the population.

Question 4

Define underdominance.

Answer 4

Underdominance (heterozygotes selected against) - The heterozygote has a lower fitness than both
homozygotes. This leads to an unstable equilibrium.

Question 5

Describe heterzygote advantage.

Answer 5

Heterzygote Advantage

Question 6

Describe the effects of different evolutionary forces on allelic frequencies within populations.

Answer 6

Effects of evolutionary forces on allelic frequencies within populations.

Question 7

Describe biological evolution.

Answer 7

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.

Question 8

What are the levels of genetic variation?

Answer 8

Genetic Variation - Levels

• Molecular
• Protein
• DNA sequence

Question 9

Describe molecular variation.

Answer 9

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.

Question 10

Describe protein variation.

Answer 10

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.

Question 11

Describe DNA sequence variation.

Answer 11

Genetic Variation - DNA Sequence Variation

• Restriction-site variation.
• Microsatellite variation.
• Variation detected by DNA sequencing.

Question 12

Answer 12

Question 13

Describe reproductive isolating mechanisms.

Answer 13

Reproductive Isolating Mechanisms

• Prezygotic reproductive isolating mechanisms:
  
  • Gametic
  • Mechanical
  • Temporal
  • Behavioral
  • Ecological
• Postzygotic reproductive isolating mechanisms:
  
  • Hybrid inviability
  • Hybrid sterility
  • Hybrid breakdown

Question 14

Describe the modes of speciation.

Answer 14

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.

Question 15

Describe genetic differentiation associated with speciation.

Answer 15

Genetic Differentiation Associated with Speciation

• How much genetic differentiation is required for reproductive isolation to take place?

Question 16

Describe phylogeny.

Answer 16

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

Question 17

Describe the theories on the role of the environment in human evolution.

Answer 17

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?

Question 18

Describe the possible outcomes of population evolution in environmental dynamics.

Answer 18

Population Evolution in Environmental Dynamics

Question 19

How can molecular changes reveal patterns of evolution?

Answer 19

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.

Question 20

Describe quantitative genetics.

Answer 20

Quantitative Genetics

• Deals with phenotypes that vary continuously (e.g. characters such as height or mass) - as opposed to discretely identifiable phenotypes and gene-products (such as bristle number in flies, or the presence of a particular biochemical).
• Used to identify a quantitative trait that determines oil content in corn.

Question 21

Describe discontinuous (qualitative) traits.

Answer 21

Discontinuous (Qualitative) Traits

• Traits possess only a few phenotypes (e.g., red or white petals).
• All of the traits Mendel studied were discontinuous.

Question 22

Describe continuous (quantitative) traits.

Answer 22

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.

Question 23

What is a GWAS?

Answer 23

Genome-wide association study is an examination of many common genetic variants in different individuals to see if any variant is associated (co-segregates) with a trait.

Question 24

Describe polygenic inheritance.

Answer 24

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.

Question 25

Describe the 2 types of quantitative characteristics.

Answer 25

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.

Question 26

How are quantitative characteristics analyzed?

Answer 26

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.

Question 27

Answer 27

Question 28

Answer 28

Question 29

Answer 29

Question 30

Describe heritability.

Answer 30

Heritability

• Heritability: The proportion of the total phenotypic variation that is due to genetic difference.
• Phenotypic Variance: V_p
  
  • Components of phenotypic variance V_p=V_G+V_E+V_GE
    
    • Genetic variance: V_G
    • Environmental variance: V_E
    • Genetic x environmental Interaction VGE
  • Components of genetic variance: V_G=V_A+V_D+V_I
    
    • Additive genetic variance: V_A
    • Dominance genetic variance: V_D
    • Genic interaction variance: V_I
• Summary: V_p=V_A+V_D+V_I+V_E+V_GE

Question 31

Describe the types of heritability.

Answer 31

Heritability - Types

• Broad-Sense Heritability (H² = V_G/V_P; all genetic modifiers).
  
  • The ratio of total genetic variance to total phenotypic variance.
• Narrow-Sense Heritability (h² = V_A/V_P; additive effects only).
  
  • The ratio of additive genetic variance to the total phenotypic variance.
• Calculating Heritability:
  
  • Heritability by elimination of variance components:
    
    • (V_P-V_E=V_G)
  • 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(r_MZ-r_PZ) correlation coefficient of mono vs dizygotic twins.

Question 32

Describe genetic-environmental interaction variance.

Answer 32

Genetic-Environmental Interaction Variance

Question 33

What are the important points about heredity?

Answer 33

Heredity - Important Points

• The heritability estimate is specific to the population and environment you are analyzing.
• The estimate is for a population, not for an individual parameter - and therefore is specific to that population.
• Heritability does not indicate the degree to which a trait is genetic, it measures the proportion of the phenotypic variance that is the result of genetic factors for a population in a given environment.

Question 34

What are the limitations of heritability?

Answer 34

Heritability - Limitations

• Heritability does not indicate the degree to which a characteristic is genetically determined.
• An individual does not have heritability.
• There is no universal heritability for a characteristic.
• Even when heritability is high, environmental factors still influence a characteristic.
• Heritability indicates nothing about the nature of population differences in a characteristic.

Question 35

Describe a quantitative trait locus (QTL).

Answer 35

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).

Question 36

Describe how genetically variable traits change in response to selection.

Answer 36

Genetically Variable Traits Change in Response to Selection

• Natural selection arises through the differential reproduction of individuals with different genotypes.
• Artificial selection: selection by promoting the reproduction of organisms with traits perceived as desirable.
• Both are a consequence of successful reproduction that increases the frequency of certain alleles.
• Predicting the Response to Selection:
  
  • = The extent to which a characteristic, subject to selection, changes in one generation.
• Factors influencing response to selection:
  
  • S = Selection differential = [Mean phenotype of population] - [Mean phenotype of the parents selected for breeding].
• Calculation of Response to selection = R:
  
  • R=h² x S
  • h² = narrow sense heritability (variation in a phenotypic trait in a population that is due to genetic variation between individuals in that population).