MCAT Biology Review

Chapter 12: Genetics and Evolution

Conclusion

Genetics and the mechanisms of evolution are becoming increasingly important in medicine, as we unintentionally breed strains of highly resistant bacteria. Antibiotic stewardship, or the use of the appropriate antibiotics only as necessary, is very important as the medical community seeks to preserve the effectiveness of antibiotics. In order to understand and apply the concepts of antibiotic stewardship, one must be aware of how creating environmental pressures leads to directional selection in microorganisms and can increase the frequency of the resistant phenotype. In this chapter, we covered genetics and mutations, as well as evolution. We also gave you some tools to analyze the mathematical (statistical) side of genetics through the use of Punnett squares, recombinant frequencies, and the Hardy–Weinberg equations.

It seems fitting to complete this book with a discussion of evolution. You’ve spent hundreds of pages (and hours!) preparing for the MCAT, learning the basics of cell biology, embryogenesis and development, anatomy and physiology, genetics, and evolution. Our understanding of these topics relies on generations and generations of scientists who came before us, and who passed down their knowledge through books, letters, articles, lectures, and—more recently—television, film, and popular media. But science is a field that is constantly evolving, itself. At the beginning of medical school, students are often told that no more than 25 percent of what they learn during the first year will remain true by the time they enter practice. We’re not sure if this statistic actually holds, but it does bespeak the importance of staying on top of the latest research—not only as a medical student, but also once in practice. Every day, new discoveries about the human body and the practice of medicine are being made—and soon, you’ll be one of those making these very discoveries and bringing them to patients, improving their lives. And at the end of it all, as a provider, an attending physician, a researcher, you too will pass your knowledge to future generations of physicians, who will also help medical science to evolve and improve. The human body is astoundingly complex. Take a moment to genuinely think about that—the human body is astoundingly complex. There’s so much more to learn; medical school and your future awaits!

Concept Summary

Fundamental Concepts of Genetics

· Chromosomes contain genes in a linear sequence.

· Alleles are alternative forms of a gene.

o A dominant allele requires only one copy to be expressed.

o A recessive allele requires two copies to be expressed.

· A genotype is the combination of alleles one has at a given genetic locus.

o Having two of the same allele is termed homozygous.

o Having two different alleles is termed heterozygous.

o Having only one allele is termed hemizygous (such as male sex chromosomes).

o A phenotype is the observable manifestation of a genotype.

· There are different patterns of dominance.

o Complete dominance has one dominant allele and one recessive allele.

o Codominance has more than one dominant allele.

o 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.

o 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.

o Mendel’s second law (of independent assortment) states that the inheritance of one allele does not influence the probability of inheriting a given allele for a different trait.

· Support for DNA as genetic material came through a number of experiments.

o The Griffith experiment demonstrated the transforming principle, converting non-virulent bacteria into virulent bacteria by exposure to heat-killed virulent bacteria.

o The Avery–MacLeod–McCarty experiment demonstrated that DNA is the genetic material because degradation of DNA led to a cessation of bacterial transformation.

o The Hershey–Chase experiment confirmed that DNA is the genetic material because only radiolabeled DNA could be found in bacteriophage-infected bacteria.

Changes in the Gene Pool

· 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).

o A silent mutation has no effect on the protein.

o A missense mutation results in the substitution of one amino acid for another.

o A nonsense mutation results in the substitution of a stop codon for an amino acid.

o Insertions and deletions result in a shift in the reading frame, leading to changes for all downstream amino acids.

· Chromosomal mutations include much larger-scale mutations affecting whole segments of DNA.

o Deletion mutations occur when a large segment of DNA is lost.

o Duplication mutations occur when a segment of DNA is copied multiple times.

o Inversion mutations occur when a segment of DNA is reversed.

o Insertion mutations occur when a segment of DNA is moved from one chromosome to another.

o Translocation mutations occur when a segment of DNA is swapped with a segment of DNA from another chromosome.

· Genetic leakage is a flow of genes between species through hybrid offspring.

· Genetic drift occurs when the composition of the gene pool changes as a result of chance.

· The founder effect results from bottlenecks that suddenly isolate a small population, leading to inbreeding and increased prevalence of certain homozygous genotypes.

Analytical Approaches in Genetics

· Punnett squares visually represent the crossing of gametes from parents to show relative genotypic and phenotypic frequencies.

o The parent generation is represented by P; filial (offspring) generations are represented by F₁, F₂, and so on in sequence.

o A monohybrid cross accounts for one gene; a dihybrid cross accounts for two genes.

o In sex-linked crosses, sex chromosomes are usually used to indicate sex as well as genotype.

· The recombination frequency (θ) is the likelihood of two alleles being separated during crossing over in meiosis. Genetic maps can be made using recombination frequency as the scale, in centimorgans.

· The Hardy–Weinberg principle states that if a population meets certain criteria (aimed at a lack of evolution), then the allele frequencies will remain constant (Hardy–Weinberg equilibrium).

Evolution

· Natural selection states that chance variations exist between individuals, and that advantageous variations—those that increase an individual’s fitness for the environment—afford the most opportunity for reproductive success.

· The modern synthesis model (neo-Darwinism) accounts for mutation and recombination as mechanisms of variation and considers differential reproduction to be the mechanism of reproductive success.

· 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 continuation of genes in subsequent generations.

· Punctuated equilibrium considers evolution to be a very slow process with intermittent rapid bursts of evolutionary activity.

· Different types of selection lead to changes in phenotypes.

o Stabilizing selection keeps phenotypes in a narrow range, excluding extremes.

o Directional selection moves the average phenotype toward one extreme.

o Disruptive selection moves toward two different phenotypes at the extremes and can lead to speciation.

o Adaptive radiation is the rapid emergence of multiple species from a common ancestor, each of which occupies its own ecological niche.

· A species is the largest group of organisms capable of breeding to form fertile offspring. Species are reproductively isolated from each other by pre- or postzygotic mechanisms.

· Two species can evolve with different relationship patterns.

o Divergent evolution occurs when two species sharing a common ancestor become more different.

o Parallel evolution occurs when two species sharing a common ancestor evolve in similar ways due to analogous selection pressures.

o Convergent evolution occurs when two species not sharing a recent ancestor evolve to become more similar due to analogous selection pressures.

· 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.

Answers to Concept Checks

· 12.1

1. A dominant allele is one that requires only one copy for expression. A recessive allele requires two copies for expression.

2. A homozygous genotype is one in which the two alleles are the same. A heterozygous genotype is one in which the two alleles are different. A hemizygous genotype is one in which only one allele is present for a given gene (such as parts of the X chromosome in males).

3. Complete dominance occurs when a gene has only one dominant and one recessive allele. Codominance occurs when a gene has more than one dominant allele, and two different dominant alleles can be expressed simultaneously. Incomplete dominance occurs when a gene has no dominant alleles, and heterozygotes have phenotypes that are intermediate between homozygotes.

4. Penetrance describes the proportion of the population that expresses a phenotype, given a particular genotype. Expressivity describes the differences in expression (severity, location, and so on) of a phenotype across affected members of a population.

5. Mendel’s first law (of segregation) most aligns with anaphase I of meiosis. Mendel’s second law (of independent assortment) most aligns with prophase I of meiosis.

· 12.2

1. Silent point mutations occur when one nucleotide is changed for another, but there is no change in the protein coded for by this DNA sequence (due to redundancy in the genetic code). Missense mutations occur when one nucleotide is changed for another, and one amino acid is substituted for another in the final protein. Nonsense mutations occur when one nucleotide is changed for another, and a stop codon substitutes for an amino acid in the final protein.

2. The two types of frameshift mutations are insertion and deletion mutations.

3. Duplication mutations occur when a segment of DNA is copied multiple times in the genome. Inversion mutations occur when a segment of DNA is reversed in the genome. Translocation mutations occur when a segment of DNA from one chromosome is swapped with a segment of DNA from another chromosome.

4. Genetic leakage requires the formation of a hybrid organism that can then mate with members of one or the other parent species. While hybrids existed historically (especially mules), fertile hybrids were certainly rare before a more modern understanding of genetics (and a commercial, financial, or academic impetus to create these organisms).

5. Genetic drift occurs due to chance, so its effects will be more pronounced with a smaller sample size (in smaller populations). The founder effect occurs when a small group is reproductively isolated from the larger population, allowing certain alleles to take on a higher prevalence in the group than the rest of the population.

· 12.3

1. 

2. The genes must be in the order SQRT:

3. The criteria for the Hardy–Weinberg principle all imply that the study population is not undergoing evolution; thus, the allele frequencies will remain stable over time.

4. The frequency of the dominant allele (p) is 0.3. The frequency of the recessive allele (q) is 0.7. The fraction of the population with a heterozygous genotype (2pq) is 2 × 0.3 × 0.7 = 0.42 (42%). The fraction of the population with a homozygous recessive genotype (q²) is (0.7)²= 0.49 (49%). The fraction of the population with a dominant phenotype (p² + 2pq) is 0.09 + 0.42 = 0.51 = 51%.

· 12.4

1. Natural selection states that certain traits that arise from chance are more favorable for reproductive success in a given environment, and that those traits will be passed on to future generations. The modern synthesis model takes natural selection and explains that selection is for specific alleles, which are passed to future generations through formation of gametes, and that these favorable traits arise from mutations. Inclusive fitness explains that the reproductive success of an organism is not only due to the number of offspring it creates, but also the ability to care for young (that can then care for others); it explains changes not only at the individual level, but based on the survival of the species (and that individual’s alleles within the species, including in other related individuals). Punctuated equilibrium states that for some species, little evolution occurs for a long period, which is interrupted by rapid bursts of evolutionary change.

2. 

3. 

4. A species is defined as the largest group of organisms capable of breeding to form fertile offspring.

Equations to Remember

Shared Concepts

1. Behavioral Sciences Chapter 10

o Social Thinking

2. Biochemistry Chapter 6

o DNA and Biotechnology

3. Biochemistry Chapter 7

o RNA and the Genetic Code

4. Biology Chapter 1

o The Cell

5. Physics and Math Chapter 11

o Reasoning About the Design and Execution of Research

6. Physics and Math Chapter 12

o Data-Based and Statistical Reasoning