Ch 12 - Genetics/Evolution

Question 1

Organization of genes

Answer 1

• genotype - genetic combination
• phenotype - observable trait
• allele - form of a gene
• homologues - 2 copies of each chromosome
  
  • locus - location of gene on a chromosome
• 2 alleles for each gene
  
  • dominant or recessive
  • homozygous, heterozygous, hemizygous (only 1 allele)

Question 2

Pattern of dominance

Answer 2

• complete dominance - only one dominant and one recessive allele for the gene. Phenotype dominant allele
• Codominance - more than one dominant allele
• Incomplete dominance - heterozygous expresses intermediate phenotype

Question 3

Penetrance and Expressivity

Answer 3

• penetrance - proportion of individuals in population carryign the allele that express the phenotype
  
  • full penetrance - 100 show phenotype
  • high penetrance - most but not all show phenotype
  • reduced penetrance, low penetrance
  • nonpenetrance
• expressivity - varying phenotypes from a given genotype, grey area in expression
  
  • constant - same phenotype
  • variable - different phenotypes

Question 4

Mendels First Law of Segregation

Answer 4

• genes exist in alternative forms (alleles)
• 2 alleles for each gene, one from each parent
• segregate for meiosis (anaphase I) and gametes have one allele
• expressed one allele is dominant, one silent allele is recessive
  
  • exception is codominance and incomplete dominance

Question 5

Mendel’s Second Law (Independent Assortment)

Answer 5

• inheritance of one gene does not affect the inheritance of another gene
  
  • due to recombination (swap material between chromatids in prophase I)
• exception is linked genes
• segregation of homologous chromosomes
• independent assortment of alleles

Question 6

Griffith Experiment

Answer 6

• showed transformation principle
• nonvirulent - survive
• virulent - dead
• heat killed virulent - survive
• nonvirulent and heat killed - dead
• other researchers did experiment with virulent bacteria
  
  • add enzyme to degrade protein - mice dies
  • add enzyme to degrade DNA - mice survive
• Both prove DNA is genetic material

Question 7

Gene pool

Answer 7

• all alleles that exist in a species
• new genes added due to mutation and genetic leakage

Question 8

What is a mutation

Answer 8

• change in DNA sequence
  
  • contrast the wild-type that is “normal”
• mutagens cause mutations
• transposon - insert and remove themselves from genome, can cause mutation
• nucleotide level mutations
• chromosomal mutations

Question 9

Nucleotide level mutations

Answer 9

• point mutations - one nucleotide swapped for another
  
  • silent - change has no effect, due to degeneracy (wobble) in genetic code
  • missense - substitute once amino acid for another
  • nonsense - substitute for a stop codon
• frameshift - insertion or deletion from genome
  
  • shifts reading frame of codons

Question 10

Chromosomal Mutations

Answer 10

• deletion - large segment lost
• duplication - segment copied multiple times
• insertion - segment moved from one chromosome to another
• inversion - segment is reversed
• translocation - segment from one chromosome swapped with segment from another

Question 11

Consequence of mutation

Answer 11

• advantageous - positive selective advantage
• deleterious - negative effect
  
  • inborn errors of metabolism - effect metabolic genes
    
    • ex. phenylketouria (PKU), toxic metabolites from phenylalanine accumulate

Question 12

Genetic leakage

Answer 12

• flow of genes between species
• mate to produce hybrid offspring

Question 13

Genetic Drift

Answer 13

• changes in composition of the gene pool due to chance
• more pronounced in smaller populations
• Founder effect - extreme case where small populations are in reproductive isolation and inbreeding occurs
  
  • less genetic diversity, reduced fitness
  • inbreeding depression
• outbreeding aka outcrossing - introduce unrelated individuals and increased variations and fitness

Question 14

Monohydrid cross

Answer 14

• parent or P generation - individuals being crossed
• filial or F generation - offspring
  
  • F₁ then F₂ is next generation
• Crossing 2 heterozygotes -
  
  • 1:2:1 genotype ratio
  • 3:1 phenotype ratio
• Test cross - used to determine unknown genotype
  
  • Px bred with pp
    
    • always breed with homo recessive
  • x = P then 100% dominant phenotype
  • x = p then 1:1 phenotypes
  • aka back cross
    *

Question 15

Dihybrid Cross

Answer 15

• inheritance of 2 genes
• independent assortment for unlinked genes
• Cross 2 plants that are hetero for both traits
  
  • phenotype 9:3:3:1

Question 16

Sex-linked crosses

Answer 16

• assume to be X linked recessive
• sex linked traits are more common in males
• carrier is written with subscript (X_h)
• man cannot pass sex linked trait to his son
• X_HX is carrier female
• X_HX_H is hemophiliac female
• XX is normal female
• XY_H is hemophiliac male
• XY is normal male

Question 17

Gene mapping

Answer 17

• Genes located close together are less likely to be seperated
• recombination frequency - roughly proportional to distance between genes on a chromosome
• tightly linked - close to 0% recomb freq
• 1 map unit = 1 centimorgan = 1 percent
• map units can be used to find order of genes on a chromosome

Question 18

Hardy-Weinberg Principle

Answer 18

• allele frequency - how often allele appears
  
  • a percent of all alleles for that gene
  • twice as many alleles as individuals
• 5 criteria for Hardy Weinberg Equilibrium
  
  • no genetic drift (large population)
  • no mutations
  • mating is random (no sexual selection)
  • no migration into or out of population
  • genes are equally fit for reproduction
• p + q = 1
• p² + 2pq + q² = 1
• use equations to determine
  
  • allele frequency
  • frequency of any genotype or phenotype

Question 19

Natural Selection

Answer 19

• survival of the fittest
• favorable characteristics help individuals have a greater reproductive success
• Basic theories:
  
  • organisms produce offspring
  • chance variations are heritable, if advantageous then favorable
  • fitness - level of reproductive success

Question 20

Modern synthesis model

Answer 20

• aka neo darwinism
• mutations and recombinations that are favorable to survival make the organism more likely to have reproductive success
• differential reproduction - disadvantageous mutations decrease reproductive success
• populations evolve, not individuals
• Inclusive fitness - measure organisms success in the population
  
  • includes altruistic behaviors that benefit the population

Question 21

Punctuated equilibrium

Answer 21

• changes in species occur in rapid bursts rather than evenly over time

Question 22

Modes of natural selection

Answer 22

• Stabilizing selection - keep in range and select against extremes. Ex. birth weight
• Directional selection - adaptive pressure to extreme phenotype
• Disruptive selection - 2 extreme phenotypes over the norm
  
  • due to polymorphisms - differences in form of members of a population
• adaptive radiation - rapid rise of different species from common ancestor
  
  • may be due to environmental changes
• Niche - specific environment

Question 23

Speciation

Answer 23

• species - largest group of organisms capable of breeding
• speciation - new species through evolution
• Reproductive isolation - progeny of the same species cannot breed due to evolutionary pressures. Ex. different environments
  
  • prezygotic - prevent formation of zygote
  • postzygotic - yield nonviable or sterile offspring

Question 24

Patterns of evolution

Answer 24

• Divergent - independent development of dissimilar characteristics in lineages that share a common ancestor
• Parallel - related species evolve in similar ways
• Convergent - independent development of similar characteristics in lineages NOT sharing a recent common ancestor. Ex. fish and dolphins

Question 25

Measure Evolutionary Time

Answer 25

* measured by rate of change of a genotype over time * molecular clock model * relate degree of genomic similarity with amount of time since species split from common ancestor