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1C. Transmission of Heritable Information and the Processes That Increase Genetic Diversity

Meiosis, pedigrees, map distances and Hardy-Weinberg, plus the evolutionary forces that move allele frequencies once equilibrium breaks.

19 topics · 75 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

1C. Evidence That DNA Is the Genetic Material

  1. How it worksHow did Griffith's dead virulent bacteria turn living harmless ones into killers?
  2. Worked exampleFind which label stayed inside the cells when Hershey and Chase tagged sulfur and phosphorus separately.
  3. Common mistakeWhy isn't the Avery result alone enough to settle the argument for every skeptic?
  4. Going furtherWhat happens to transforming activity when DNase is added to a cell-free bacterial extract?
Your turneasy1 of 3
In a Griffith-style experiment, heat-killed virulent pneumococci are mixed with 957 living nonvirulent recipient cells, and 41 virulent colonies are recovered afterwards. What percentage of the recipient population was transformed?

1C. Genotype, Phenotype, Alleles and Loci

  1. How it worksHow does one locus hold a different allele on each of two homologous chromosomes?
  2. Common mistakeWhy isn't a gene the same thing as an allele?
  3. Going furtherWhat changes about the picture when one gene has not two alleles but dozens in a population?
Your turnmedium1 of 3
A single locus in a population carries 9 different alleles. How many different diploid genotypes are possible at that locus?

1C. Complete Dominance, Codominance and Incomplete Dominance

  1. How it worksHow does one working copy of an enzyme gene make a heterozygote look just like a homozygote?
  2. Worked exampleFind every blood type possible among children of an AB parent and an O parent.
  3. Going furtherWhat happens to the phenotype ratio when both alleles show up side by side instead of blending?
  4. Common mistakeWhy isn't a dominant allele automatically the most common one in a population?
Your turneasy1 of 3
Snapdragon flower color shows incomplete dominance: red crossed with white gives pink. Two pink plants are crossed and produce 957 offspring. How many pink offspring are expected?

1C. Penetrance, Expressivity and Multiple Alleles

  1. How it worksHow does incomplete penetrance let a genotype produce no phenotype at all in some carriers?
  2. How it worksWalk me through how modifier genes and environment stretch one genotype across a range of severities.
  3. Going furtherWhat changes in a family chart when a dominant condition is only 60 percent penetrant?
Your turnmedium1 of 3
An autosomal dominant condition is 75 percent penetrant. A heterozygous affected parent and a homozygous normal partner have 59 children. How many of those children are expected to show the condition?

1C. The Stages of Meiosis and How It Differs From Mitosis

  1. How it worksHow does prophase I pair homologs into tetrads that mitosis never forms at all?
  2. How it worksWalk me through the chromosome count and the DNA content at every stage of both meiotic divisions.
  3. Worked exampleFind how many chromosomes and how many chromatids a human cell has at metaphase I and metaphase II.
  4. Going furtherWhat happens to the gametes when a cell completes meiosis having skipped crossing over entirely?
  5. Common mistakeWhy isn't meiosis II simply mitosis running inside a haploid cell?
Your turneasy1 of 3
A diploid organism has 17 pairs of homologous chromosomes. How many chromatids are present in one of its cells at metaphase I of meiosis?

1C. Independent Assortment at the Metaphase Plate

  1. How it worksHow does each homologous pair line up at the plate without caring how the others lined up?
  2. Worked exampleFind how many genetically distinct gametes independent assortment alone makes from 23 pairs.
  3. Going furtherWhat happens to that number once crossing over is stacked on top of independent assortment?
Your turneasy1 of 3
A diploid organism has 10 pairs of homologous chromosomes. Counting independent assortment alone and ignoring crossing over, how many genetically distinct gametes can it produce?

1C. Crossing Over, the Tetrad and the Synaptonemal Complex

  1. How it worksHow does the synaptonemal complex zip homologs together so chromatids can swap matching segments?
  2. Common mistakeWhy doesn't a swap between sister chromatids create any new combination of alleles?
  3. Worked exampleFind the four gamete genotypes a single crossover makes in a double heterozygote for linked genes.
  4. Going furtherWhat changes in the gametes when two crossovers land between the same pair of genes?
Your turnmedium1 of 3
A single crossover falls between genes A and B in 59 percent of the meioses of a double heterozygote. What percentage of the gametes produced is recombinant for A and B?

1C. Linkage, Recombination Frequency and Gene Mapping

  1. How it worksHow does the distance between two genes set how often a crossover falls between them?
  2. Worked exampleFind the map distance between two genes when 180 of 1000 offspring come out recombinant.
  3. Worked exampleFind the gene order from three-point cross distances of 8, 12 and 20 map units.
  4. Common mistakeWhy isn't a recombination frequency above 50 percent ever observed between two genes?
  5. Going furtherWhat happens to a map distance when double crossovers go undetected across a long interval?
Your turneasy1 of 3
A testcross of a double heterozygote produces 1157 offspring, of which 170 are recombinant for the two genes. What is the map distance between the genes, in map units?

1C. Sex-Linked Characteristics and Sex Determination

  1. How it worksHow does having a single X turn every X-linked recessive allele into a visible trait?
  2. Going furtherWhat happens to X-linked gene dosage when one X is shut down into a Barr body?
  3. Worked exampleFind the chance that a carrier mother and unaffected father have a son with hemophilia.
  4. Common mistakeWhy doesn't an X-linked trait ever pass from a father straight to his son?
Your turnmedium1 of 3
An X-linked recessive allele has frequency 0.145 in a large population at equilibrium. How many times more common is the resulting condition in males than in females?

1C. Nondisjunction and Cytoplasmic Inheritance

  1. How it worksHow does nondisjunction in meiosis I give different gametes than nondisjunction in meiosis II?
  2. Common mistakeWhy doesn't a mitochondrial disease ever pass down through the father?
  3. Going furtherWhat happens to the karyotype when nondisjunction hits the sex chromosomes instead of an autosome?
Your turnmedium1 of 3
An organism has 18 pairs of homologous chromosomes. Nondisjunction in meiosis I produces a gamete that has gained a chromosome. How many chromosomes does that gamete contain?

1C. Mutation as a Source of Variation, Mutagens and Carcinogens

  1. How it worksHow do substitutions, insertions, inversions and translocations each damage a gene differently?
  2. Going furtherWhat changes about the mutation rate when a chemical mutagen or ionizing radiation is added?
  3. Common mistakeWhy isn't every mutation harmful to the organism that carries it?
  4. Ties togetherHow does an inborn error of metabolism trace one changed base all the way to a blocked pathway?
Your turnmedium1 of 3
The spontaneous mutation rate at one locus is 1 in 72678 gametes. If 1368189 gametes go on to form the next generation, how many newly mutant alleles at that locus appear in that generation?

1C. Probability, Punnett Squares and the Testcross

  1. How it worksHow does a Punnett square turn the segregation of alleles into a table of probabilities?
  2. How it worksWalk me through when to multiply probabilities and when to add them in a genetics problem.
  3. Worked exampleFind the fraction of offspring showing both recessive traits from a cross of two AaBb parents.
  4. Common mistakeWhy isn't a 3 to 1 ratio guaranteed in any single family of four children?
Your turneasy1 of 3
Two plants of genotype AaBb, with A and B on different chromosomes, are crossed and produce 766 offspring. How many of the offspring are expected to show both dominant phenotypes?

1C. Pedigree Analysis

  1. How it worksHow does a trait that skips generations mark itself out as recessive on a family chart?
  2. Common mistakeWhy isn't a trait appearing in every generation necessarily a dominant one?
  3. Going furtherWhat changes on a family chart when the trait turns out to be X-linked rather than autosomal?
  4. Ties togetherHow does a family chart let you assign carrier odds using the same multiplication rule as a square?
Your turnhard1 of 3
Two unaffected parents have a child with an autosomal recessive disease. An unaffected older sibling of that child marries someone drawn at random from a population in which 1 person in 145 is a carrier. What is the probability that the couple's first child is affected?

1C. The Hardy-Weinberg Principle and Its Conditions

  1. How it worksHow do p plus q equals one and the squared expansion describe the very same population?
  2. Worked exampleFind the carrier frequency of a recessive disease that affects 1 person in 2500.
  3. Common mistakeWhy doesn't a dominant allele climb in frequency just because it happens to be dominant?
  4. Going furtherWhat happens to the frequencies the moment one of the five equilibrium conditions breaks?
  5. Ties togetherHow does this equilibrium serve as the null model that natural selection gets measured against?
Your turnmedium1 of 3
A recessive disease affects 1 person in 28130 in a population at Hardy-Weinberg equilibrium. What fraction of the population is heterozygous for the disease allele?

1C. Biometry and Statistical Significance in Genetics

  1. How it worksHow does a chi-square test compare observed offspring counts against the ratio you predicted?
  2. How it worksWalk me through what a p value of 0.03 actually claims about a genetic cross.
  3. Going furtherWhat changes about the conclusion when the cross produces 40 offspring instead of 400?
Your turnhard1 of 3
A monohybrid cross predicts a 3 to 1 ratio of dominant to recessive offspring. The cross produces 339 offspring in all, of which 88 show the recessive phenotype. What is the chi-square statistic for this cross?

1C. Natural Selection, Fitness and Differential Reproduction

  1. How it worksHow does a tiny reproductive advantage compound into a fixed allele over many generations?
  2. How it worksWalk me through how variation, heritability and differential reproduction combine into selection.
  3. Common mistakeWhy isn't evolutionary fitness the same as being physically fit and healthy?
  4. Worked exampleFind the new recessive allele frequency after one generation when p equals q equals 0.5 and aa has fitness 0.5.
  5. Ties togetherHow does selection show up as a violation of one specific equilibrium condition?
Your turnhard1 of 3
In a population at Hardy-Weinberg proportions, p equals q equals 0.5, and the aa genotype has relative fitness 0.657 while AA and Aa both have fitness 1. What is the frequency of the a allele after one generation of selection?

1C. Stabilizing, Directional and Disruptive Selection

  1. How it worksHow does a bell curve of a trait shift, narrow or split under the three selection patterns?
  2. Worked exampleFind which selection pattern is at work when average human birth weight survives best.
  3. Going furtherWhat happens to a population's variance under disruptive selection over many generations?
Your turnmedium1 of 3
In a population of finches, the heritability of beak depth is 0.687 and a drought imposes a selection differential of 1.8 millimetres on the trait. What is the expected response to selection, in millimetres, in the next generation?

1C. Speciation, Polymorphism and Adaptive Radiation

  1. How it worksHow does reproductive isolation build up until two populations can no longer interbreed?
  2. Going furtherWhat happens when a single ancestor lands on an empty island chain full of open niches?
  3. Worked exampleFind the isolating mechanism at work when two frog species breed in one pond in different months.
  4. Ties togetherHow does polymorphism in a gene pool supply the raw material that speciation works from?
Your turnmedium1 of 3
Two populations were separated 21 million years ago. Neutral substitutions accumulate independently in each lineage at 0.0133 per site per million years. How many substitutions are expected to separate the two lineages at a single site?

1C. Genetic Drift, Bottlenecks, Founder Effects and Inbreeding

  1. How it worksHow does sampling only a few parents each generation make allele frequencies wander?
  2. Common mistakeWhy isn't genetic drift just a weaker version of natural selection?
  3. Going furtherWhat happens to diversity when a population crashes to a handful of survivors and rebounds?
  4. Worked exampleFind how much faster frequencies wander in a population of 20 than in one of 2000.
  5. Ties togetherHow do founder effects and inbreeding both raise the odds of a rare recessive disease appearing?
Your turnhard1 of 3
An allele is at frequency 0.657 in a population of 333 breeding adults. What is the variance in that allele's frequency after one generation of genetic drift?

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