Understanding variation: Mendelian genetics (study notes)

Understanding variation: Mendelian genetics

• I. Darwin's problems with variation
  • Darwin proposed evolution by natural selection as a gradual process; he argued that “Natura non facit saltus” = nature does not make jumps.
  • Problems with gradualism and variation:
  • Postulate 2: Organisms within populations vary.
  • Blending inheritance would erase variation over generations, leaving little to fuel evolution.
• II. Mendel's breeding experiments
  • Gregor Mendel studied inheritance in pea plants (Pisum sativum).
  • Pea plants have seven chromosome pairs; the seven studied traits were located on four different chromosomes.
  • F2 from parental crosses showed discrete phenotypes (no blending): e.g., for color, the F1 all yellow when crossing yellow x green; F2 yielded 3 yellow : 1 green.
  • Mendel conducted thousands of crosses between 1856–1863; results published in 1865; Darwin’s Origin of Species published in 1859.
  • Why peas?
  • Many characteristics had only two variants, making inheritance easy to trace across generations; parental traits did not blend when passed to offspring.
  • Key observation: some traits do not blend, yet variation persists across generations in offspring combinations.
• III. The rediscovery of Mendel's laws
  • Mendel's work was rediscovered in the early 1900s; its significance linked to discoveries about chromosomes and cell division.
  • A. Chromosomes
  • B. Mitosis and meiosis
  • C. Chromosomal basis of inheritance
  • D. Mendel’s Laws of Inheritance
• IV. What does it all mean?
  • The lecture promises further exploration over the next two weeks and encourages questions.

Darwin & Natural Selection (conceptual foundation)

• Natural selection defined: the process by which heritable variation that affects survival and reproduction in a given environment becomes more or less common across generations.
• Darwin’s initial view: evolution is gradual; variation within populations is essential for adaptive change.
• Key implication: without heritable variation, there can be no evolution by natural selection.

Pangenesis, blending, and early explanations for variation loss

• Blending inheritance (historical concept): offspring are an average of parental traits, which would gradually erase variation.
  • Example: Blending inheritance would imply eventual loss of variation after several generations, hindering evolution.
• Frances Galton and regression toward the mean:
  • Anecdote: Tall fathers tend to have shorter sons; short fathers tend to have taller sons.
  • This suggested a tendency toward the population mean rather than persistence of extreme traits.
• Darwin's proposed solution: Pangenesis
  • Gemmules: hypothetical particles shed by organs throughout life; environment can alter gemmules; gemmules spread to egg and sperm and are passed to offspring.
  • Claim: environmentally induced variation can be inherited, enabling gradual or non-gradual changes depending on how gemmules transmit to gametes.
• Galton's test of pangenesis:
  • Transfused blood from wild brown rabbits into purebred white rabbits to test whether offspring would carry traits from the wild type.
  • If pangenesis were correct, offspring of purebreds would show part of the wild-type phenotype.
  • Conclusion (as presented): Natura non facit saltus; Darwin was wrong about gradualism; evolution could not occur gradually through pangenesis alone; large abrupt changes (sports) were favored by this view.
• Saltationists and mutations:
  • Galton and colleagues were called “Saltationists,” proposing large, abrupt changes as drivers of evolution.
  • Hugo de Vries (1900) mutation theory: new species arise in jumps through mutations.
  • Evening primrose crosses reportedly produced discrete forms supporting jumps in variation.

Mendel's breeding experiments (detailed)

• Mendel examined inheritance in peas to test how traits are transmitted across generations.
• F1 generation from purebred yellow × purebred green:
  • F1 were all yellow (dominant phenotype).
  • F2 showed the classic 3:1 yellow:green phenotypic ratio.
• Summary of the Mendelian puzzle:
  • Some traits do not blend, yet variation persists across generations; Mendel’s results offered a particulate mechanism for inheritance that preserves variation.

Cell division, chromosomes, and inheritance (historical context)

• Mendel’s work was rediscovered in the context of two major discoveries:
  • Chromosomes were identified in the late 1800s.
  • Their linkage to inheritance was clarified in the early 1900s.
• Key numeric facts:
  • Humans typically have 23 pairs of chromosomes, i.e., 46 individual chromosomes (diploid).
  • In peas, the study noted 7 chromosome pairs (7 pairs; 14 chromosomes total).
• Conceptual framing:
  • Chromosomes are the carriers of genetic material; inheritance follows from their behavior during cell division.

Chromosomes, Cells, and Cell Division

• Nucleus houses genetic material (chromosomes).
• Two main types of cell division:
  • Mitosis — non-reductive division; diploid to diploid (2n → 2n).
  • Meiosis — reductive division; diploid to haploid (2n → n); produces gametes.
• Meiosis overview:
  • A diploid somatic cell becomes four haploid gametes after meiosis.
  • Gametes carry one chromosome from each homologous pair, ensuring diverse gametes.
• Humans and cells:
  • The human body contains roughly on the order of 37 trillion diploid cells (2n).

Meiosis vs. Mitosis (mechanisms and outcomes)

• Mitosis (non-reductive): 1 diploid cell → 2 diploid daughter cells; preserves chromosome number.
• Meiosis (reductive): 1 diploid cell → 4 haploid gametes; halves chromosome number and increases genetic diversity through recombination.
• Gamete haploidy explains fertilization:
  • 1 haploid egg + 1 haploid sperm = a diploid fertilized egg.
• Result: a fertilized egg begins mitotic divisions to develop into a multicellular diploid organism.
• Human context: ~37 trillion diploid cells in the body; diversity arises from meiosis and recombination.

Mendel: Particulate inheritance and basic terminology (definitions)

• Alleles: alternative forms of a gene at the same locus on a chromosome; arise by mutation.
• Genotype: the genetic makeup; e.g., AA, Aa, or aa.
• Phenotype: the observable trait; e.g., yellow or green.
• Dominant allele: one that determines the phenotype when present (Yellow in this example).
• Recessive allele: trait expressed only when the dominant allele is not present (Green in this example).
• Heterozygote: Aa.
• Homozygote: AA or aa.
• The view today is that inheritance is particulate, and alleles segregate and assort independently under appropriate conditions.

Mendel's Crosses: Punnett square and two key laws

• Punnett square concept (illustrated in the slides):
  • If a cross is Aa × Aa, the offspring genotype proportions are:
  • $ext{Pr}(AA) = frac{1}{4}, \ ext{Pr}(Aa) = frac{1}{4}, \ ext{Pr}(aA) = frac{1}{4}, \ ext{Pr}(aa) = frac{1}{4}$
  • Phenotype ratio for a single gene with complete dominance: $3:1$ (Yellow:Green).
• Mendel's Law of Segregation:
  • Offspring inherit one allele from each parent.
• Mendel's Law of Independent Assortment:
  • Alleles for different genes on different chromosomes assort independently during gamete formation.
• Example context (Two genes on different chromosomes):
  • Start with purebred for color and texture: AABB (yellow, smooth) × aabb (green, wrinkled).
  • Meiosis yields gametes: from AABB → AB; from aabb → ab.
  • Offspring in the F1 generation are AaBb (heterozygous for both traits) and display yellow and smooth phenotype (dominant for both traits).
• F1 × F1 cross (AaBb × AaBb):
  • Phenotype ratio: $9:3:3:1$ (yellow-smooth : yellow-wrinkled : green-smooth : green-wrinkled).
  • This is the classic dihybrid cross illustrating independent assortment when loci are on different chromosomes.
• Terms in the example:
  • Alleles on different chromosomes assort independently.
  • The four possible phenotypic classes occur in the 9:3:3:1 ratio when both genes show complete dominance and assort independently.

Linkage and Recombination: Crossing Over

• Genes on the same chromosome are linked and do not assort independently unless they are separated by crossing over.
• Crossing over (recombination):
  • Occurs during meiosis after DNA duplication but before cell division.
  • Exchange of chromosome segments between homologous chromosomes.
  • Happens at various locations on the chromosome in each meiotic event.
• Consequences of recombination:
  • Each gamete is genetically unique; each fertilized egg is genetically unique.
  • This genetic shuffling contributes to familial diversity among siblings.
• Frequency and sex differences (illustrated):
  • About ~50 events per meiosis in males; ~70 events per meiosis in females (estimates given in the slides).

Relevance to Darwin’s problem of loss of variation

• Particulate inheritance ensures that variation is not irretrievably lost across generations.
• Alleles from each parent are transmitted to offspring and can recombine, preserving variation even under selection.
• Two-part takeaway:
  • Part 1: Inheritance is particulate; alleles are transmitted to offspring.
  • Part 2: Alleles on different genes assort independently during gamete formation; overall variation persists through generations.
• Practical implication: Under random mating, genotype and phenotype frequencies stabilize in predictable ways, keeping variation in the population.
• Example (single-gene case under random mating):
  • From Aa × Aa crosses, offspring frequencies stabilize at:
  • Genotypes: $ext{Pr}(AA) = frac{1}{4}, \ ext{Pr}(Aa) = frac{1}{2}, \ ext{Pr}(aa) = frac{1}{4}$
  • Phenotypes: $ext{Yellow} ext{ (AA or Aa)}: frac{3}{4}, \ ext{Green} ext{ (aa)}: frac{1}{4}$
• When considering two genes (dihybrid cross) with independent assortment, the classic phenotypic ratio is $9:3:3:1$, illustrating how combinations arise and persist rather than being lost.

Quick summary: What does it all mean for evolution?

• Darwin’s problem of variation via blending was resolved by Mendel’s particulate inheritance, which preserves variation across generations.
• Mendel’s laws explain how traits are transmitted:
  • Law of Segregation: offspring inherit one allele from each parent.
  • Law of Independent Assortment: alleles of unlinked genes assort independently during gamete formation.
• Crosstalk with chromosomes, meiosis, and crossing over explains how genetic variation is generated in gametes and zygotes.
• The combination of segregation, independent assortment, and recombination ensures that variation is maintained and reshaped across generations—providing the raw material for natural selection to act upon.

References to upcoming topics

• Part 2 on Tuesday: deeper exploration of the chromosomal basis of inheritance and the implications for evolution and genetics.