MCAT Basics Podcast

MCAT Basics: Population Genetics

Sam reviews genotypes/phenotypes, inheritance patterns, the Hardy-Weinberg equation, and the four ways a population’s genetics change over time.

  • [01:09] The Difference Between Population Genetics and Molecular Genetics
  • [01:54] Defining Genotypes and Phenotypes
  • [06:24] The Six Inheritance Patterns
  • [13:27] The Hardy-Weinberg Equation
  • [26:17] Natural Selection
  • [32:22] Mutation
  • [36:47] Genetic Drift
  • [40:04] Gene Flow

Population genetics is the macro-level study of genetic variation within a population, i.e., how the frequencies of alleles change over time. It is different from molecular genetics which looks at genetics from the perspective of a microscope, studying micro-level concepts like meiosis, mitosis, transcription, etc.

A person’s genotype is the set of genes or alleles that they carry at a specific locus. For a given locus, a person might have a homozygous dominant genotype (two similar, dominant alleles), a homozygous recessive genotype (two similar, recessive alleles), or a heterozygous genotype (two different alleles). Consider the BRCA1 mutation for breast cancer. The relevant gene consists of two alleles, one inherited from the mother and the other from the father. And each of these two alleles might manifest a BRCA1 mutation. If either allele is mutated, then chances of breast cancer increase. However, having the BRCA1 mutation does not guarantee breast cancer. Instead genotype and environment (carcinogens) interact to determine the breast cancer phenotype.

Inheritance patterns explain how the genotype relates to the phenotype. There are six common inheritance patterns namely, the autosomal dominant pattern, the autosomal recessive pattern, patterns linked to the X-chromosome, mitochondrial inheritance, genomic imprinting, and complex, multifactorial inheritance:

  1. Autosomal dominant pattern, only one “dominant” allele is needed to display a phenotype.
  2. Autosomal recessive pattern, both alleles are needed to display a particular phenotype.
  3. X-chromosome linked inheritance works similar to autosomal inheritance for females since they inherit an X chromosome from their father and an X chromosome from their mother. However, since males only have one X chromosome, inherited from their mother, X-linked inheritance is different for them. In short, for an X-linked trait, males express the phenotype of whatever allele they receive.
  4. Mitochondrial inheritance pattern, each person simply expresses the phenotype of the mother.
  5. Genomic imprinting is an epigenetic phenomenon which occurs when the allele of one parent is silenced, causing the child to express only the other parent’s genes.
  6. In complex, multifactorial inheritance, multiple genes, as well as environmental factors interact to determine the phenotype.

For more details on inheritance patterns, listen to MCAT Basics 9: Genetics: Gene Expression.

The Hardy-Weinberg equation describes the allele and genotype frequencies for a given population:

p² + 2pq + q² = 1

p refers to the frequency of the dominant allele at a locus and q refers to the frequency of the recessive allele at a locus. p + q = 1, since the allele frequencies must add to 100%. refers to the frequency of the homozygous dominant genotype, 2pq refers to the heterozygous genotype, and refers to the homozygous recessive genotype. To illustrate how to use the Hardy-Weinberg equation, imagine that black or blonde hair color was determined by a single gene (this is not true, and hair color falls under complex, multifactorial inheritance). Further assume that black hair is the dominant allele, and blonde hair is the recessive allele. We can count the number of blonde people in the population to determine the value of (homozygous recessive). From this, it is straightforward to determine the value of q (simply take the square root), and to further determine the value of p by subtracting q from 1. Then, determining and 2pq simply becomes an issue of imputing the values of p and q.

The Hardy-Weinberg equation assumes that we are dealing with genetic variation in a population at equilibrium. “Equilibrium” is constrained by six assumptions:

  1. The organisms under investigation are diploid (two sets of chromosomes);
  2. Mating is random;
  3. The population size is very large;
  4. There is neither emigration nor immigration;
  5. There is no natural selection;
  6. There is no genetic mutation.

While these are very strict and unrealistic constraints, the Hardy-Weinberg equation still helps us to obtain useful estimates of genetic variation in a population.

Finally, Sam talks about the four ways in which a population’s genetics might change over time, through natural selection, mutation, genetic drift and gene flow.

Natural selection refers to the differential reproduction and survival of organisms due to differences in phenotype. A famous example of natural selection occurred in the peppered moths in England during the Industrial Revolution. The factories were producing lots of soot which would sit on trees, rendering them black. Peppered moths can be either black or white. During this time, the black peppered moths were able to camouflage themselves better in the soot-covered trees, protecting themselves from predators, and thereby increasing their chances of reproduction and survival.

Mutation is the next way in which there might be a change in a population’s genetics. There are two types of mutation i.e. recurrent and non-recurrent mutations. A recurrent mutation occurs in genetic “hot spots.” For example, over time, the size of the Y-chromosome has decreased, due to a recurrent mutation. Recurrent mutations tend to cause more genetic change over time, because non-recurrent mutations might just die out. Mutations introduce new alleles into populations, which through the application of other selective pressures, can become commonplace.

Genetic drift is a change in allele frequency due to random chance. This can occur because of a population bottleneck or the Founder Effect, among other things. In a population bottleneck, a random sample of the population is eliminated. For example, during the fictious ‘Great Zebra Fire of 215 BC,’ many zebras were killed. Initially, the zebras had various colors in their stripes, but later, only black and white stripes remained. The Founder effect occurs when a population is descended from a small number of colonizing ancestors. This might lead to inbreeding and reduced genetic fitness.

Gene flow refers to the transfer of genetic variation from one population to another. For example, during the Great Mating Frenzy of 57 BC, striped zebras introduced the allele for stripes to the solid-colored zebra population. Similarly, there is something called genetic leakage, when genetic variation is transferred from one species to another. For example, genetically-modified organism crops (GMO crops) are known to pass genes to other plant species nearby. All these four phenomena — natural selection, mutation, genetic drift and gene flow – interact to produce genetic changes in a population over time.

Check out the paper Natural Selection, Genetic Drift, and Gene Flow Do Not Act in Isolation in Natural Populations.

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Sam Smith

Sam completed his Bachelors of Science in Chemical and Biological Engineering at the University of Colorado Boulder. Following his graduation, he worked at the National Institutes of Health Vaccine Research Center studying HIV. Meanwhile, with a microphone in his garage, Sam founded the MCAT Basics podcast. The podcast has grown to become the top rated MCAT podcast on iTunes. In addition to podcasting, Sam enjoys the outdoors, sports, and his friends and family.

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