Genetics & Evolution: Quick Reference

Genotype: the genetic makeup; the alleles you carry for a trait.
Phenotype: the observable traits, results of genotype in a given environment.
Alleles: different versions of a gene.
Dominant vs Recessive:
- Dominant alleles are expressed in the phenotype, even in heterozygotes.
- Recessive alleles are expressed only in the homozygous recessive state.
Homozygous vs Heterozygous:
- Homozygous: two identical alleles (AA or aa).
- Heterozygous: two different alleles (Aa).
Punnett square basics: outside of square shows parental alleles; inside shows possible offspring genotypes.
Example (eye color placeholder): Brown (B) dominant to blue (b).
- Bb × Bb → Genotype ratio: $1:2:1$ (BB:Bb:bb).
- Phenotype ratio: $3:1$ (dominant vs recessive).

Sickle cell anemia: caused by a single base change in the beta-globin gene.
Molecular change: Glutamic acid (Glu) to Valine (Val) at a specific position; HbS forms long fibers that distort red blood cells.
Conceptual takeaway: a single mutation can have big physiological effects.
Evolutionary angle: sickle cell trait provides malaria resistance; beneficial in malaria-endemic regions but not elsewhere.
Terminology:
- Sickle cell trait = heterozygous (carrying one mutant allele).
- Sickle cell disease = homozygous recessive.

Darwinian idea: organisms better suited to their environment survive and reproduce more.
Examples discussed: changes in teeth, walking upright, potential reduction of pinky toe over long timescales, taller height in certain populations, high-altitude lung adaptations.
Tailbone persistence: vestigial structures illustrate remnants of evolution.
Rapid vs gradual change: some traits may shift over millions of years; others may appear or disappear depending on environment.

Evolutionary psychology: looks for universal patterns in behavior and cognition shaped by evolution.
Behavioral genetics: studies how genes and environment contribute to individual differences (e.g., twin/adoption studies).
Nature vs Nurture: both genes and environment shape outcomes; twin/adoption studies help disentangle them.

Twin studies show how similar/different twins are due to genes vs environment.
Historical twin studies sometimes separated multiples at birth for research.
Ethics: separation for studies raised concerns about consent and welfare; now generally deemed unethical and often illegal.
Legal changes: since 1985, adoption agencies in many places must keep siblings together.
Modern context: such studies are retrospective or observational; archival data may be used with strict ethics.

DNA: double helix; base pairs encode genes.
Chromosomes: long DNA molecules carrying many genes.
Gene and allele terminology:
- Gene: unit of heredity.
- Allele: variant form of a gene.
Punnett square recap:
- Outside letters: parental alleles.
- Inside squares: possible offspring genotypes.
- Example: Dominant/recessive cross across a single trait.
More terms:
- Polygenic trait: multiple genes contribute to a single trait.
- Mutation: a sudden permanent change in a gene (e.g., PKU example).

PKU: classic example of an autosomal recessive disorder.
Cross guidance: with two carriers (Aa × Aa), offspring probabilities typically include 1/4 affected, 1/2 carriers, 1/4 unaffected.

Range of reaction: genotype sets potential bounds; environment helps determine where within that range an individual ends up.
- Example: athletic ability may be bounded by genetics but expressed differently depending on environment.
Gene-Environment correlation: genes influence the kind of environment a person experiences, and those environments influence gene expression.
- Example: intelligence and stimulation in the environment can affect realized outcomes despite genetic potential.
Epigenetics: study of how gene expression is modified by environment without changing the DNA sequence itself; helps explain why individuals with the same genotype can have different phenotypes.

Define phenotype vs genotype.
What does a Punnett square show? Distinguish homozygous vs heterozygous.
Why is sickle cell advantageous in some populations but not others?
What is the difference between evolutionary psychology and behavioral genetics?
What is a range of reaction and what does it imply about nature vs nurture?
How can environment influence gene expression (epigenetics) and vice versa (gene-environment correlation)?

Ethics in genetics and twin studies are crucial; consider consent, welfare, and potential for harm.
Legislation now often protects siblings and restricts ethically dubious experiments.