Fundamentals and Concepts of Genetics
High-Yield Summary
- DNA suited as genetic material: stores information (nucleotide sequence), replicates with high fidelity (complementary base pairing — A-T via 2 H-bonds, G-C via 3 H-bonds), and can be expressed (transcribed/translated).
- Gene = DNA sequence coding for a functional product. Locus = gene's position on a chromosome. Allele = a version of a gene at a locus. Homologous chromosomes carry the same genes at the same loci (one maternal, one paternal).
- Genotype = allele combination carried; phenotype = observable trait (genotype × environment). Homozygous = two identical alleles at a locus; heterozygous = two different alleles.
- Dominance patterns: complete (heterozygote matches dominant homozygote, 3:1 phenotypic ratio), incomplete (heterozygote intermediate, 1:2:1 phenotypic ratio), codominance (both alleles fully expressed simultaneously, e.g. AB blood type).
- Penetrance = population-level, yes/no: % of a genotype's carriers who show the phenotype at all. Expressivity = individual-level, matter of degree: how severely the phenotype is expressed among those who have it.
- Epigenetics changes gene expression without changing DNA sequence: DNA methylation (usually silences), histone modification (packaging tightness), X-inactivation (Barr body, dosage compensation), genomic imprinting (expression depends on parent of origin).
Complete vs. Incomplete Dominance vs. Codominance
| Pattern | Heterozygote Phenotype |
|---|---|
| Complete dominance | Matches dominant homozygote exactly (recessive fully masked) — e.g. purple fully masks white |
| Incomplete dominance | Intermediate/blended between the two homozygotes — e.g. red + white → pink |
| Codominance | Both alleles fully and simultaneously expressed, not blended — e.g. type AB blood |
Key Terms
- Complementary base pairing
- A pairs with T (2 H-bonds), G pairs with C (3 H-bonds) — the basis for accurate DNA replication.
- Penetrance
- The proportion of individuals with a genotype who actually display the associated phenotype (population-level, yes/no).
- Expressivity
- The degree/severity of phenotype expression among individuals sharing the same genotype (individual-level, a matter of degree).
- Barr body
- The condensed, inactivated X chromosome in each female somatic cell, achieving dosage compensation.
- Genomic imprinting
- Epigenetic silencing of one allele based on parent of origin, regardless of dominance.
Epigenetic Mechanisms
- 1DNA methylation — methyl groups added to cytosines; methylation in promoters typically decreases transcription
- 2Histone modification — acetylation/methylation of histones loosens or tightens chromatin packing, changing transcription access
- 3X-chromosome inactivation — one of two X chromosomes randomly silenced per female somatic cell, forms a Barr body (dosage compensation, genetic mosaicism)
- 4Genomic imprinting — one allele silenced based on whether it came from mother or father, independent of dominance
Common MCAT Trap
- Penetrance ≠ expressivity: penetrance asks IF the phenotype appears at all (population, yes/no); expressivity asks HOW STRONGLY it appears (individual, degree). 80% penetrance means 20% of carriers show no phenotype at all, not a milder phenotype.
- Under complete dominance the genotypic ratio (1:2:1) and phenotypic ratio (3:1) differ; under incomplete dominance they're the SAME (1:2:1) because the heterozygote has its own distinct phenotype.
- Epigenetic changes alter gene expression, not the DNA sequence itself — don't call methylation or imprinting a mutation.
- X-inactivation is random per cell, making females genetic mosaics for X-linked traits — not every cell expresses the same X chromosome.
Quick Recall
What three properties make DNA suited as the genetic material?
A disease allele has 80% penetrance. What does that mean?
Why does X-inactivation happen, and what structure results?
What distinguishes codominance from incomplete dominance?