MCAT Bio Chapter12

• Chromosomes contain genes in a linear sequence.
• Alleles are alternative forms of a gene.
• A dominant allele requires only one copy to be expressed.
• A recessive allele requires two copies to be expressed.
• A genotype is the combination of alleles one has at a given genetic locus.
• Having two of the same allele is termed homozygous.
• Having two different alleles is termed heterozygous.
• Having only one allele is termed hemizygous, such as in male sex chromosomes.
• A phenotype is the observable manifestation of a genotype.
• There are different patterns of dominance: complete dominance when the effect of one allele completely masks the effect of another, codominance has more than one dominant allele, and incomplete dominance has no dominant alleles; heterozygotes have intermediate phenotypes.
• Penetrance is the proportion of a population with a given genotype who express the phenotype.
• Expressivity refers to the varying phenotypic manifestations of a given genotype.
• The modern interpretations of Mendel’s laws help explain the inheritance of genes from parent to offspring.
• Mendel’s first law (of segregation) states that an organism has two alleles for each gene, which segregate during meiosis, resulting in gametes carrying only one allele for a trait.
• Mendel’s second law (of independent assortment) states that the inheritance of one allele does not influence the probability of inheriting an allele for a different trait.
• Support for DNA as genetic material came through a number of experiments.
• The Griffith experiment demonstrated the transforming principle, converting nonvirulent live bacteria into virulent bacteria by exposure to heat-killed virulent bacteria.
• The Avery–MacLeod–McCarty experiment demonstrated that DNA is the genetic material because degradation of DNA led to a cessation of bacterial transformation.
• The Hershey–Chase experiment confirmed that DNA is the genetic material because only radiolabeled DNA could be found in bacteriophage-infected bacteria.
• All of the alleles in a given population constitute the gene pool.
• Mutations are changes in DNA sequence.
• Nucleotide mutations include point mutations (the substituting of one nucleotide for another) and frameshift mutations (moving the three-letter transcriptional reading frame).
• A silent mutation has no effect on the protein.
• A missense mutation results in the substitution of one amino acid for another.
• A nonsense mutation results in the substitution of a stop codon for an amino acid.
• Insertions and deletions result in a shift in the reading frame, leading to changes for all downstream amino acids.
• The founder effect results from bottlenecks that suddenly isolate a small population, leading to inbreeding and increased prevalence of certain homozygous genotypes.
• Inclusive fitness considers an organism’s success to be based on the number of offspring, success in supporting offspring, and the ability of the offspring to then support others; survival of offspring or relatives ensures appearance of genes in subsequent generations.
• Punctuated equilibrium considers evolution to be a very slow process with intermittent rapid bursts of evolutionary activity.
• According to the molecular clock model, the degree of difference in the genome between two species is related to the amount of time since the two species broke off from a common ancestor.
• Different types of selection lead to changes in phenotypes.
• The recombination frequency ( θ ) is the likelihood of two alleles being separated during crossing over in meiosis.
• The Hardy–Weinberg principle states that if a population meets certain criteria, then the allele frequencies will remain constant ( Hardy–Weinberg equilibrium ).
• Adaptive radiation is the rapid emergence of multiple species from a common ancestor, each of which occupies its own ecological niche.
• Disruptive selection moves the population toward two different phenotypes at the extremes and can lead to speciation.
• Stabilizing selection keeps phenotypes in a narrow range, excluding extremes.
• A monohybrid cross accounts for one gene; a dihybrid cross accounts for two genes.
• Punnett squares visually represent the crossing of gametes from parents to show relative genotypic and phenotypic frequencies.
• Genetic maps can be made using recombination frequency as the scale in centimorgans.
• Parallel evolution occurs when two species sharing a common ancestor evolve in similar ways due to analogous selection pressures.