Mendel's Abstract Concepts → Molecular Reality
| Mendel's Concept | Molecular Reality |
|---|---|
| "Allele" (factor) | Different DNA sequences at the same gene locus |
| "Dominant allele" | Allele whose protein product is sufficient to produce the full phenotype (or produces a gain-of-function product) |
| "Recessive allele" | Allele that produces non-functional/reduced protein; requires two copies for phenotype |
| "Segregation" | Physical separation of homologous chromosomes during Anaphase I of meiosis |
| "Independent Assortment" | Homologs from different chromosome pairs align independently at Meiosis I metaphase plate |
| "Crossing over" | Physical exchange of DNA segments between non-sister chromatids via Holliday junction resolution |
| "Gene locus" | Specific chromosomal position defined by flanking sequences |
| "Homozygous" | Both alleles at a locus have identical nucleotide sequences |
| "Heterozygous" | Two alleles at a locus have different nucleotide sequences |
Connections to Other Topics
- Chromosomal Theory (Sutton & Boveri) → connects Mendelian Laws to cytology (chromosome structure and meiosis)
- Morgan's linkage work → connects to recombination, crossing over, and chromosome mapping
- ABO blood group → connects to glycoprotein biochemistry (glycosyltransferase enzymes add different sugars to H antigen)
- HbS (sickle cell) → connects to molecular genetics (point mutation: GAG → GTG codon change) and protein structure (beta-globin amino acid change: Glu → Val)
- Polygenic traits → connects to quantitative genetics, normal distribution, and environmental effects on gene expression
Why Mendel's Laws "Work" — The Deeper Explanation
Mendel's Laws are mathematical summaries of meiotic chromosome behaviour. The reason gametes carry one allele (not two) is because meiosis is a reductive division. The reason different genes assort independently is because spindle fibers attach randomly to non-homologous chromosome pairs. Crossing over creates new allele combinations but preserves chromosome integrity.