DNA (Deoxyribonucleic Acid) is the molecule that carries genetic instructions for building and maintaining living organisms. It’s composed of:

• Nucleotides: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G)

• Double helix structure: Two strands twisted around each other, held together by base pairing (A-T, C-G)

Each gene is a segment of DNA that codes for a specific protein, and these genes are located on chromosomes humans have 23 pairs.

Gregor Mendel’s experiments with pea plants laid the groundwork for classical genetics. His three laws explain how traits are inherited:

1. Law of Segregation

• Each individual has two alleles for each gene (one from each parent).

• These alleles separate during gamete formation, so each gamete carries only one allele.

• Example: A plant with genotype Aa will produce gametes with either A or a.

2. Law of Independent Assortment

• Genes for different traits are inherited independently of one another (if they’re on different chromosomes).

• This explains the genetic variation seen in offspring.

• Example: In a dihybrid cross (e.g., seed color and shape), the inheritance of one trait doesn’t affect the other.

3. Law of Dominance

• When two different alleles are present, one may mask the expression of the other.

• The dominant allele is expressed in the phenotype, while the recessive is hidden unless both alleles are recessive.

• Definition: Neither allele is completely dominant; the heterozygote shows a blend.

• Example: In snapdragons, red (RR) crossed with white (rr) yields pink (Rr) flowers.

• Genotype vs. Phenotype: Each genotype has a distinct phenotype no masking.

  Codominance

  • Definition: Both alleles are fully expressed in the heterozygote.

  • Example: Human ABO blood type type AB expresses both A and B antigens.

  • Key Feature: No blending; both traits appear simultaneously.

    Multiple Alleles

    • Definition: More than two alleles exist for a gene in the population.

    • Example: ABO blood group has three alleles IA, IB, and i.

    • Impact: Expands possible genotype and phenotype combinations.

      Polygenic Inheritance

      • Definition: Traits controlled by multiple genes, each contributing a small effect.

      • Example: Skin color, height, and intelligence.

      • Result: Continuous variation across a spectrum, often forming a bell curve.

        Pleiotropy

        • Definition: One gene influences multiple, seemingly unrelated traits.

        • Example: Marfan syndrome mutation in the FBN1 gene affects connective tissue, eyes, heart, and skeleton.

          Epistasis

          • Definition: One gene masks or modifies the expression of another gene.

          • Example: Coat color in Labrador retrievers one gene determines pigment, another controls deposition.

            Sex-Linked Inheritance

            • Definition: Genes located on sex chromosomes (usually X).

            • Example: Hemophilia and color blindness more common in males due to XY configuration.

      
      

A historical look at adaptation and evolution

• The idea that life has changed over Earth’s history was first proposed by early Greek philosophers (Xenophanes, Empedocles, and Aristotle). They recognized fossils as evidence for former life that was destroyed by a natural catastrophe.

• With the rise of the Catholic Church, this evolutionary thought declined among scholars who largely accepted the biblical account of earth’s creation which required no mechanistic explanation.

  • In the early 1600s A.D., Archbishop James Usher calculated based on the bible that the earth was formed on Sunday, October 23^rd 4004 B.C.

  • This became widely accepted amongst church officials and further declined evolutionary thought labeling it as heretical (which at the time carried a death sentence if found guilty).

• By the 1700s the church's influence had declined enough that naturalists could again speculate on the evolution of animals.

Patterns of evolution

• Evolution has caused large amounts of variation in form and function

• When two species evolve in a different direction from a common ancestor this is called Divergent evolution

• Other times independent species evolve similar phenotypes separately

  • Flight has evolved separately in insects, birds, and bats but there is no ancestor that had flight that unites these species. This is called convergent evolution

    • The wings of a bat and of a bird are called analogous structures

    • The wings of two bird’s species are called homologous structures

• Evolution is constantly-changing populations, the gradual change is referred to as microevolution, while the rise of new species and higher taxonomic groups is referred to as macroevolution

Population genetics

• A group of the same species of organisms interacting is a population

• Populations are what evolution act upon

  • Within a population, there are multiple alleles (all combined are called the gene pool) and the frequency that those alleles lead to reproduction is called fitness

  • Hardy-Weinberg Equilibrium

    • If there was no evolutionary pressure the frequency of alleles would remain constant

    • However, we now know that there are 4 important evolutionary forces that disrupt the equilibrium

Evidence of evolution

• The fossil record

  • A fossil is a remnant from past life uncovered from the earth.

  • The fossil record is the sequence in which fossils appear in rock strata.

  • Based on this sequence and the ages of rocks and fossils geologists have established a geologic record.

  • Ages of rocks or fossils can be determined using radiometric dating. This is a technique that looks at the ratio of an element’s stable form (ex. carbon-12) with its radioactive form (ex. carbon-14).

• Anatomy and Embryology

  • The presence of structures in organisms that share the same basic form

    • Appendages

    • Vestigial structures

  • Convergence of form in organisms that share a similar environment (e.g. artic animals)

  • Embryo development (e.g. Presence of gill slits and tails in vertebrates)

• Biogeography is the distribution of organisms being congruent with tectonic movement and evolution

• Molecular biology - molecular life reflects descent with modification

• Speciation - is the formation of 2 species from one original species.